Measurement of viscoelastic properties of homogeneous soft solid using transient elastography: An inverse problem approach Catheline, S., J.L. Gennisson, G. Delon, M. Fink, R. Sinkus, S. Abouelkaram, and J. Culioli Journal of the Acoustical Society of America 116, no. 6, 37343741 (2004)
Résumé: Two main questions are at the center of this paper. The first one concerns the choice of a rheological model in the frequency range of transient elastography, sonoelasticity or NMR elastography for soft solids (201000 Hz). Transient elastography experiments based on plane shear waves that propagate in an Agargelatin phantom or in bovine muscles enable one to quantify their viscoelastic properties. The comparison of these experimental results to the prediction of the two simplest rheological models indicate clearly that Voigt's model is the better. The second question studied in the paper deals with the feasibility of quantitative viscosity mapping using inverse problem algorithm. In the ideal situation where plane shear waves propagate in a sample, a simple inverse problem based on the Helmholtz equation correctly retrieves both elasticity and viscosity. In a more realistic situation with nonplane shear waves, this simple approach fails. Nevertheless, it is shown that quantitative viscosity mapping is still possible if one uses an appropriate inverse problem that fully takes into account diffraction in solids. © 2004 Acoustical Society of America.
Motsclés: Frequencies; Mapping; Nuclear magnetic resonance spectroscopy; Problem solving; Rheology; Shear waves; Elastography; Soft solids; Viscoelastic properties; Viscosity mapping; Viscoelasticity; agar; gelatin; algorithm; article; cattle; elastography; feasibility study; flow kinetics; measurement; muscle; nuclear magnetic resonance; phantom; priority journal; solid; viscoelasticity; viscosity; echography; elasticity; human; image processing; image quality; instrumentation; mathematical computing; sh


Phononic crystals Page, J. H., A. Sukhovich, S. Yang, M. L. Cowan, F. Van Der Biest, A. Tourin, M. Fink, Z. Liu, C. T. Chan, and P. Sheng Physica Status Solidi (B) Basic Research 241, no. 15, 34543462 (2004)
Résumé: Phononic crystals are periodic composite materials with lattice spacings comparable to the acoustic wavelength. They are of interest not only because of the profound effects of their periodic structure on wave propagation (e.g., the existence of acoustic band gaps), but also because of potential applications (e.g., their possible role in sound filters, transducer design and acoustic mirrors). In this paper, we summarize recent progress using ultrasonic experiments to investigate both two and threedimensional phononic crystals. By measuring the ultrasonic wave field transmitted through slabshaped samples of different thicknesses, both the dispersion curves and amplitude transmission coefficient can be determined. Because the field is pulsed, the dynamics of the wave fields can also be investigated; this has allowed us to make a systematic study of ultrasonic wave tunneling in phononic crystals. New results on resonant tunneling, focussing and negative refraction phenomena in phononic crystals are also presented. Our data are well explained using Multiple Scattering Theory, giving additional insight into the physical properties and potential applications of these novel materials. © 2004 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.


Sound field modeling in a street canyon with partially diffusely reflecting boundaries by the transport theory Le Pollès, T., J. Picaut, M. Bérengier, and C. Bardos Journal of the Acoustical Society of America 116, no. 5, 29692983 (2004)
Résumé: The transport theory, applied to the concept of sound particles, seems to be well adapted to predict the sound propagation in urban areas, including most complex effects, like diffuse scattering by building facades, atmospheric attenuation, scattering by urban objects in streets, etc. In this paper, the transport theory is then applied to the sound field modeling in an empty street canyon with partially diffusely building facades. In this case, the temporal and spatial distribution of sound energy in a street is the solution of a transport equation, with mixed speculardiffuse boundary conditions. Using an asymptotic approach, the transport equation may be reduced to a diffusion equation for the sound energy, where the diffusion coefficient depends only on the building facades properties. Comparisons with experiments show that the diffusion model gives consistent results with experimental data, both for the sound attenuation level and the reverberation time. © 2004 Acoustical Society of America.
Motsclés: Attenuation; Boundary conditions; Diffusion; Mathematical models; Transport properties; Urban planning; Reverberation time; Spatial distribution; Transport theory; Acoustics; article; diffusion; energy; model; priority journal; sound; theory


Timereversal breaking of acoustic waves in a cavity Bertaix, V., J. Garson, N. Quieffin, S. Catheline, J. Derosny, and M. Fink American Journal of Physics 72, no. 10, 13081311 (2004)
Résumé: Acoustic timereversal is a wellestablished technique that focuses an ultrasonic wave on the location of its source. It is based on the timereversal invariance of the wave equation and is usually implemented using timereversal mirrors made up of a hundred of piezoelectric transducers. However, a timereversal experiment can be performed in a closed cavity (a waterfilled beaker) with only one transducer as a pulseecho system. This easytobuild and low cost experiment involves students in the general concept of the timereversal invariance of the wave equation. We show that it also can be adapted to become an ultrasonic timereversal thermometer. A careful study of the focal point shows a dependence of its position as a function of temperature variations. © 2004 American Association of Physics Teachers.


Relation between time reversal focusing and coherent backscattering in multiple scattering media: A diagrammatic approach De Rosny, J., A. Tourin, A. Derode, B. Van Tiggelen, and M. Fink Physical Review E  Statistical, Nonlinear, and Soft Matter Physics 70, no. 4 2, 046601104660112 (2004)
Résumé: Onechannel time reversal (TR) experiments through multiple scattering media were discussed. The hyperresolution and the selfaveraging property was described. It was found that the developed formalism leads to a deeper understanding of the role of the ladder and mostcrossed diagrams in a TR experiment. Results show that when the initial source and the time reversal point are at the same location, the timereversed amplitude is twice higher.
Motsclés: Approximation theory; Diffusion; Green's function; Mathematical models; Random processes; Spectroscopy; Statistical methods; Ultrasonic propagation; Paraxial approximation; Spatial focusing; Spatiotemporal focusing; Time reversal (TR); Acoustic wave backscattering


Investigation of surface acoustic wave propagation on a sphere using laser ultrasonics Clorennec, D., and D. Royer Applied Physics Letters 85, no. 12, 24352437 (2004)
Résumé: A laser ultrasonic technique was used for the study of the propagation of surface acoustic waves on a sphere (SSAW). The combined focusing and reversal effects for the SSAW generated by a thermoelastic line source were also investigated. It was observed that the mechanical displacement was periodically reversed and focused. It was also observed that the focusing effect that was due to the curvature of the surface, was dependent on the angular aperture of the source.
Motsclés: Acoustic wave diffraction; Acoustic wave scattering; Computer simulation; Finite element method; Heterodyning; Interferometers; Laser pulses; Q switched lasers; Surface waves; Thermoelasticity; Transducers; Ultrasonics; Beam expanders; Energy distribution; Heterodyne interferometers; Rayleigh waves; Spherical surface acoustic waves (SSAW); Acoustic wave propagation


3D realtime motion correction in highintensity focused ultrasound therapy Pernot, M., M. Tanter, and M. Fink Ultrasound in Medicine and Biology 30, no. 9, 12391249 (2004)
Résumé: A method for tracking the 3D motion of tissues in realtime is combined with a 2D highintensity focused ultrasound (US), or HIFU, multichannel system to correct for respiratory motion during HIFU therapy. Motion estimation is based on an accurate ultrasonic speckletracking method. A pulseecho sequence is performed for a subset of the transducers of the phased array. For each of these subapertures, the displacement is estimated by computing the 1D crosscorrelation of the backscattered signals acquired at two different times. The 3D motion vector is then computed by a triangulation algorithm. This technique is experimentally validated in phantoms moving as fast as 40 mm s 1, and combined with HIFU sequences. A realtime feedback correction of the HIFU beam is achieved by adjusting the delays of each channel. The sonications "locked on target" are interleaved with very short motionestimation sequences. Finally, in vitro experiments of "locked on target" HIFU therapy are performed in fresh moving tissues. © 2004 World Federation for Ultrasound in Medicine & Biology.
Motsclés: Focused ultrasound; HIFU; Motion tracking; Therapy; Highintensity focused ultrasound (HIFU) therapy; Pulseecho sequences; Respiratory motion; Ultrasonic speckletracking method; Acoustic wave backscattering; Algorithms; Correlation methods; Motion estimation; Respiratory system; Tissue; Transducers; Vectors; Ultrasonics; algorithm; animal tissue; article; breathing mechanics; correlation analysis; feedback system; high intensity focused ultrasound; motion; nonhuman; priority journal; signal tr


Comparison between an experimental turbulent vortex and the Lundgren vortex Cuypers, Y., A. Maurel, and P. Petitjeans Journal of Turbulence 5 (2004)
Résumé: In a recent letter (Cuypers Y et al 2003 Phys. Rev. Lett. 91 194502), the authors presented experimental results on a structure resulting from a vortex burst. The temporal evolution of this structure results in the k5/3 Kolmogorov spectrum and some common features with the Lundgren theoretical vortex have been shown. The purpose of the present paper is to go further in the comparison with the Lundgren model by a parallel analysis of the experimental structure and of a Lundgren single spiral vortex, whose evolution is numerically obtained based on the calculations of Pullin et al (1993 Phys. Fluids A 5 126; 1994 Phys. Fluids 6 3010). ©2004 IOP Publishing Ltd.
Motsclés: mathematical modeling; turbulence; vortex


Quantum dots in biology: Recent applications Dubertret, B. Medecine/Sciences 20, no. 89, 737740 (2004)
Motsclés: antinuclear antibody; beta cyclodextrin; maltose binding protein; mercaptoethanol; protein tyrosine kinase; quantum dot; antigen detection; autofluorescence; energy absorption; Escherichia coli; fluorescence resonance energy transfer; microtubule assembly; molecular biology; note; quantum chemistry; Biology; Quantum Dots; Escherichia coli


Elastic wave propagation in a threedimensional periodic granular medium Anfosso, J., and V. Gibiat Europhysics Letters 67, no. 3, 376382 (2004)
Résumé: We present an experimental study of acoustic wave propagation in granular media made of monodisperse spherical beads under stress and periodically arranged. A previous work presented the predominance of multiplescattered acoustic waves for random closedpacking granular media, whereas our experimental results lead to the conclusion that the acoustical propagation in our arrangement of spherical beads proceeds essentially from surface waves. This provides another mechanism for the understanding of acoustical wave propagation in granular media, that may be useful at least to analyse weak disorder ones.


Telecommunication in a disordered environment with iterative time reversal Montaldo, G., G. Lerosey, A. Derode, A. Tourin, J. De Rosny, and M. Fink Waves Random Media 14, no. 3, 287302 (2004)
Résumé: We present a method to transmit digital information through a highly scattering medium in a MIMOMU (multiple input multiple output multiple users) context. It is based on iterations of a timereversal process, and permits us to focus short pulses, both spatially and temporally, from a base antenna to different users. This iterative technique is shown to be more efficient (lower intersymbol interference and lower error rate) than classical timereversal communication, while being computationally light and stable. Experiments are presented: digital information is conveyed from 15 transmitters to 15 receivers by ultrasonic waves propagating through a highly scattering slab. From a theoretical point of view, the iterative technique achieves the inverse filter of propagation in the subspace of nonnull singular values of the timereversal operator. We also investigate the influence of external additive noise, and show that the number of iterations can be optimized to give the lowest error rate. © 2004 IOP Publishing Ltd.
Motsclés: Antenna lobes; Antennas; Digital communication systems; Eigenvalues and eigenfunctions; Iterative methods; Matrix algebra; Signal receivers; Transmitters; Ultrasonic propagation; Multiple input multiple output multiple users; Short pulses; Time reversal method; Electromagnetic wave scattering


Imaging from onebit correlations of wideband diffuse wave fields Larose, E., A. Derode, M. Campillo, and M. Fink Journal of Applied Physics 95, no. 12, 83938399 (2004)
Résumé: An imaging technique based on onebit correlations of wideband diffuse wave fields was discussed. The first step of the imaging process was to retrive the Green's function between two passive sensors from the correlation of the scattered wave fields generated by distant source. It was found that a short ultrasonic pulse, sent through a highly scattering slab, generates a randomly scattered field. The results show that the soundspeed profile of the layered medium can be precisely imaged.
Motsclés: Acoustic waves; Electromagnetic wave scattering; Green's function; Quantum theory; Seismology; Spectroscopic analysis; Spurious signal noise; Ultrasonic waves; Wave propagation; Waveform analysis; Radiative transfer; Random noise; Thermal fluctuations; Timereversal invariance; Imaging techniques


Time reversal of electromagnetic waves Lerosey, G., J. De Rosny, A. Tourin, A. Derode, G. Montaldo, and M. Fink Physical Review Letters 92, no. 19, 1939041 (2004)
Résumé: A onechannel electromagnetic timereversal mirror (TRM) was used for investigating the feasibility of time reversal focusing with electromagnetic waves in the GHz range. Two omnidirectional antennas with a frequency of 2.45 GHz and two transceiver circuit boards were also used for the investigations. The baseband signals were time reversed and the wave carriers were phase conjugated in order to avoid digitizing the radio signals at GHz frequencies. The circuit boards demodulated the radio frequency signal back to the baseband. The frequency bandwidth and the spectral correlations determined the quality of focusing.
Motsclés: Analog to digital conversion; Bandwidth; Correlation methods; Demodulation; Light scattering; Light transmission; Low pass filters; Mirrors; Monochromators; Signal receivers; Signal to noise ratio; Transceivers; Ultrasonic effects; Wireless telecommunication systems; Phase conjugation; Quasimonochromatic signals; Spectral correlations; Time reversal mirrors (TRM); Antenna radiation


Realtime focusing using an ultrasonic one channel timereversal mirror coupled to a solid cavity Quieffin, N., S. Catheline, R. K. Ing, and M. Fink Journal of the Acoustical Society of America 115, no. 5 I, 19551960 (2004)
Résumé: Focusing and beam steering is achieved by using a timereversal process and a single transducer coupled to a solid cavity that is immersed in water. This lowcost technique makes it possible to focus acoustic energy anywhere on a 3D domain with a spatiotemporal resolution comparable to that of multiple transducers array. A short pulse is emitted from a transducer stuck at the surface of the solid cavity. The multiplescattered field is measured in front of the solid cavity using a hydrophone needle at a reference point. This signal is then time reversed and remitted by the transducer. Around the reference point, one can observe a spatiotemporal recompression. The sidelobe level as well as the focal width no longer depend on the transducer aperture but on the dimensions of the solid cavity and the multiple paths covered by the acoustic waves in the solid. Moreover, it is shown how the experimental impulse responses on the front face of the cavity can be used to control the emitting ultrasonic field. This "synthetic timereversal" technique is shown to be as powerful as a real timereversal process. © 2004 Acoustical Society of America.
Motsclés: Arrays; Focusing; Hydrophones; Mirrors; Ultrasonic transducers; Spatiotemporal recompression; Time reversal mirrors; Ultrasonic testing; acoustics; acoustics; article; diffraction; mathematical analysis; pressure; priority journal; solid; time; ultrasound; ultrasound transducer


Temperature estimation using ultrasonic spatial compound imaging Pernot, M., M. Tanter, J. Bercoff, K. R. Waters, and M. Fink IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 51, no. 5, 606615 (2004)
Résumé: The feasibility of temperature estimation during highintensity focused ultrasound therapy using pulseecho diagnostic ultrasound data has been demonstrated. This method is based upon the measurement of thermallyinduced modifications in backscattered RF echoes due to thermal expansion and local changes in the speed of sound. It has been shown that strong ripple artifacts due to the thermoacoustic lens effect severely corrupt the temperature estimates behind the heated region. We propose here a new imaging technique that improves the temperature estimation behind the heated region and reduces the variance of the temperature estimates in the entire image. We replaced the conventional beamforming on transmit with multiple steered plane wave insonifications using several subapertures. A twodimensional temperature map is estimated from axial displacement maps between consecutive RF images of identically steered plane wave insonifications. Temperature estimation is then improved by averaging the twodimensional maps from the multiple steered plane wave insonincations. Experiments were conducted in a tissuemimicking gelatinbased phantom and in fresh bovine liver.
Motsclés: Backscattered signals; Plane wave insonification; Ultrasound therapy; Backscattering; Extrapolation; Image quality; Medical applications; Medical imaging; Temperature distribution; Thermal expansion; Vectors; Ultrasonic imaging


Imaging subwavelength holes in chromium films in scanning nearfield optical microscopy. Comparison between experiments and calculation Ducourtieux, S., S. Grésillon, J. C. Rivoal, C. Vannier, C. Bainier, D. Courjon, and H. Cory EPJ Applied Physics 26, no. 1, 3543 (2004)
Résumé: Nearfield optical signals are imaged in the vicinity of nanoholes using two different nearfield optical microscopes. The experimental results are compared with electromagnetic field calculations based on a modal approximation. It turns out that an optical fibre detects the Poynting vector whereas the apertureless tip is sensitive to the field amplitude. © EDP Sciences.
Motsclés: Approximation theory; Electromagnetic field effects; Optical fibers; Optical microscopy; Surfaces; Thin films; Vectors; Apertureless tip; Field amplitude; Imaging subwavelength holes; Poynting vector; Scanning near field optical microscopy; Chromium


Sonic boom in soft materials: The elastic Cerenkov effect Bercoff, J., M. Tanter, and M. Fink Applied Physics Letters 84, no. 12, 22022204 (2004)
Résumé: An experimental evidence of an elastic sonic boom in soft materials was reported. The experimental method was based on the ultrasonic remote generation, inside soft media, of a supersonic moving source radiating shear waves in a Mach cone. The sonic boom in soft materials was found to be analogus to the Cerenkov electromagnetic radiation emitted by a beam of charged particles moving at a speed greater than the speed of light. It was suggested that intense shear wave radiated in soft materials are sensitive to medium elasticity inhomogeneities.
Motsclés: Backscattering; Charged particles; Elasticity; Electromagnetic wave propagation; Electromagnetic waves; Piezoelectric transducers; Shear waves; Ultrasonic waves; Cerenkov effects; Mach waves; Sonic booms; Soft magnetic materials


A highresolution algorithm for wave number estimation using holographic array processing Roux, P., D. Cassereau, and A. Roux Journal of the Acoustical Society of America 115, no. 3, 10591067 (2004)
Résumé: This paper presents an original way to perform wave number inversion from simulated data obtained in a noisy shallowwater environment. In the studied configuration an acoustic source is horizontally towed with respect to a vertical hydrophone array. The inversion is achieved from the combination of three ingredients. First, a modified version of the Prony algorithm is presented and numerical comparison is made to another highresolution wave number inversion algorithm based on the matrixpencil technique. Second, knowing that these highresolution algorithms are classically sensitive to noise, the use of a holographic array processing enables improvement of the signaltonoise ratio before the inversion is performed. Last, particular care is taken in the representations of the solutions in the wave number space to improve resolution without suffering from aliasing. The dependence of this wave number inversion algorithm on the relevant parameters of the problem is discussed. © 2004 Acoustical Society of America.
Motsclés: Acoustic holography; Algorithms; Computer simulation; Hydrophones; Signal to noise ratio; Matrixpencil techniques; Wave numbers; Acoustics; sea water; acoustics; holography; acoustics; algorithm; article; calculation; holography; mathematical analysis; nonlinear system; oceanic regions; priority journal; signal noise ratio; signal processing; sound; sound transmission; theory; underwater sound; velocity


Measurement of the scattering and absorption cross sections of the human body Conti, S. G., P. Roux, D. A. Demer, and J. De Rosny Applied Physics Letters 84, no. 5, 819821 (2004)
Résumé: The measurements of the acoustic intensity scattered and absorbed by humans were presented. The study used longduration acoustic reverberation to obtain the total scattering and absorption crosssections for individual humans walking randomly in a room. Within the audible limit, the human body's sound scattering spectra was similar to that of a hard ellipsoid with same volume. The increase in the clothing amount was found to increase the absorption parameter but show little effect on the scattering factor.
Motsclés: Absorption crosssections; Coda wave interferometry (CWI); Scattering crosssections; Acoustic impedance; Acoustic intensity measurement; Acoustic signal processing; Acoustic wave absorption; Bandwidth compression; Fabrics; Human engineering; Interferometry; Mathematical models; Microphones; Reverberation; Signal filtering and prediction; Spectrum analysis; Time series analysis; Acoustic wave scattering


Revisiting iterative time reversal processing: Application to detection of multiple targets Montaldo, G., M. Tanter, and M. Fink Journal of the Acoustical Society of America 115, no. 2, 776784 (2004)
Résumé: The iterative time reversal processing represents a high speed and easy way to selffocus on the strongest scatterer in a multitarget medium. However, finding weaker scatterers is a more difficult task that can be solved by computing the eigenvalue and eigenvector decomposition of the time reversal operator, the socalled DORT method. Nevertheless, as it requires the measurement of the complete interelements response matrix and timeconsuming computation, the separation of multiple targets may not be achieved in real time. In this study, a new real time technique is proposed for multitarget selective focusing that does not require the experimental acquisition of the time reversal operator. This technique achieves the operator decomposition using a particular sequence of filtered waves propagation instead of computational power. Due to its simplicity of implementation, this iterative process can be achieved in real time. This high speed selective focusing is experimentally demonstrated by detecting targets through a heterogeneous medium and in a speckle environment. A theoretical analysis compares this technique to the DORT formalism. © 2004 Acoustical Society of America.
Motsclés: Acoustic wave propagation; Eigenvalues and eigenfunctions; Iterative methods; Matrix algebra; Problem solving; Multiple target detection; Time reversal processing; Acoustic wave scattering; acoustics; algorithm; article; calculation; comparative study; filter; priority journal; sound detection; technique; theory; velocity


Real time inverse filter focusing through iterative time reversal Montaldo, G., M. Tanter, and M. Fink Journal of the Acoustical Society of America 115, no. 2, 768775 (2004)
Résumé: In order to achieve an optimal focusing through heterogeneous media we need to build the inverse filter of the propagation operator. Time reversal is an easy and robust way to achieve such an inverse filter in nondissipative media. However, as soon as losses appear in the medium, time reversal is not equivalent to the inverse filter anymore. Consequently, it does not produce the optimal focusing and beam degradations may appear. In such cases, we showed in previous works that the optimal focusing can be recovered by using the socalled spatiotemporal inverse filter technique. This process requires the presence of a complete set of receivers inside the medium. It allows one to reach the optimal focusing even in extreme situations such as ultrasonic focusing through human skull or audible sound focusing in strongly reverberant rooms. But, this technique is time consuming and implied fastidious numerical calculations. In this paper we propose a new way to process this inverse filter focusing technique in real time and without any calculation. The new process is based on iterative time reversal process. Contrary to the classical inverse filter technique, this iteration does not require any computation and achieves the inverse filter in an experimental way using wave propagation instead of computational power. The convergence from time reversal to inverse filter during the iterative process is theoretically explained. Finally, the feasibility of this iterative technique is experimentally demonstrated for ultrasound applications. © 2004 Acoustical Society of America.
Motsclés: Acoustic wave propagation; Focusing; Iterative methods; Ultrasonics; Beam degradations; Real time inverse filters; Acoustic devices; acoustics; absorption; article; calculation; feasibility study; filter; inverse filter; priority journal; sound; sound transmission; ultrasound


Reply to the Comment on "Multiple scattering in a reflecting cavity: Application to fish scattering" [J. Acoust. Soc. Am. 113, 29782979 (2003)] (L) De Rosny, J., and P. Roux Journal of the Acoustical Society of America 115, no. 1, 3134 (2004)
Résumé: In a recent letter, Ye and Chu comment on a previously published paper by the present authors ["Multiple scattering in a reflecting cavity: Application to fish counting in a tank," J. Acoust. Soc. Am. 109, 25872597 (2001)]. This reply answers the questions asked in that letter and seeks to clarify some of the details in the original paper that may have led to a misunderstanding. © 2004 Acoustical Society of America.
Motsclés: Acoustic wave reflection; Fisheries; Water tanks; Fish counting; Acoustic wave scattering; acoustics; acoustics; fish; mathematical analysis; nonhuman; priority journal; review; theory


The role of the coupling term in transient elastography Sandrin, L., D. Cassereau, and M. Fink Journal of the Acoustical Society of America 115, no. 1, 7383 (2004)
Résumé: The transient radiation of lowfrequency elastic waves through isotropic and homogeneous soft media is investigated using the Green's function approach. A careful analysis of the coupling term is performed and yields the introduction of a very near field region in which its amplitude behaves as 1/r. To address the calculation of impulse responses, a simplified Green's function is proposed for semiinfinite media and compared to exact solutions. Impulse response calculations are successfully compared with experimental measurements obtained for circular radiators of different diameters using transient elastography. Results presented in this paper provide a better understanding of the role of the coupling term in elastography and should be used to compensate diffraction and coupling effects observed in transient elastography. © 2004 Acoustical Society of America.
Motsclés: Acoustic wave diffraction; Green's function; Coupling effects; Elastography; Elastic waves; acoustics; article; calculation; elastography; mathematical model; measurement; priority journal; radiation; Elastic Tissue; Elasticity; Humans; Models, Theoretical; Phantoms, Imaging; Transducers; Ultrasonography


Measurement of elastic nonlinearity of soft solid with transient elastography Catheline, S., J.L. Gennisson, and M. Fink Journal of the Acoustical Society of America 114, no. 6 I, 30873091 (2003)
Résumé: Transient elastography is a powerful tool to measure the speed of lowfrequency shear waves in soft tissues and thus to determine the secondorder elastic modulus μ (or the Young's modulus E). In this paper, it is shown how transient elastography can also achieve the measurement of the nonlinear thirdorder elastic moduli of an Agargelatinbased phantom. This method requires speed measurements of polarized elastic waves measured in a statically stressed isotropic medium. A static uniaxial stress induces a hexagonal anisotropy (transverse isotropy) in solids. In the special case of uniaxially stressed isotropic media, the anisotropy is not caused by linear elastic coefficients but by the thirdorder nonlinear elastic constants, and the medium recovers its isotropic properties as soon as the uniaxial stress disappears. It has already been shown how transient elastography can measure the elastic (secondorder) moduli in a media with transverse isotropy such as muscles. Consequently this method, based on the measurement of the speed variations of a lowfrequency (50Hz) polarized shear strain waves as a function of the applied stress, allows one to measure the Landau moduli A, B, C that completely describe the thirdorder nonlinearity. The several orders of magnitude found among these three constants can be justified from the theoretical expression of the internal energy. © 2003 Acoustical Society of America.
Motsclés: Anisotropy; Elastic moduli; Elasticity; Stress analysis; Tissue; Soft solids; Speed measurement; Thirdorder nonlinear elastic constants; Transient elastography; Shear waves; agar; gelatin; acoustics; anisotropy; article; calculation; elasticity; elastography; energy; mathematical analysis; phantom; priority journal; soft tissue; solid; theory; Elasticity; Humans; Mathematical Computing; Nonlinear Dynamics; Phantoms, Imaging; Shear Strength; Transducers; Ultrasonography; Vibration


Sound focusing in rooms. II. The spatiotemporal inverse filter Yon, S., M. Tanter, and M. Fink Journal of the Acoustical Society of America 114, no. 6 I, 30443052 (2003)
Résumé: The potential of time reversal processing for room acoustics has been extensively investigated in the companion of this paper [J. Acoust. Soc. Am. 113(3), 15331543 (2003)]. In particular, a simple implementation of a loudspeaker time reversal antenna able to take advantage of the multiple reflections in reverberating rooms demonstrates its potential for audible range acoustics while improving focusing both in space and time. However, loss of information (e.g., sound absorption in walls or nonequalized bandwidths of the loudspeakers) during a time reversal experiment degrades the quality of time reversal focusing. In this paper, a more sophisticated technique called spatiotemporal inverse filtering is investigated that achieves time and space deconvolution of the propagation operator between the loudspeakers antenna and a set of microphones embedded inside the insonified volume. Theoretical and experimental comparisons between time reversal and inverse filter focusing are presented. Finally, advantages and limitations of both focusing approaches are highlighted. © 2003 Acoustical Society of America.
Motsclés: Acoustic wave propagation; Acoustic wave reflection; Antennas; Bandwidth; Loudspeakers; Microphones; Inverse filters; Spatiotemporal inverse filtering; Acoustic waves; acoustics; acoustics; antenna; article; comparative study; controlled study; filter; frequency modulation; microphone; priority journal; sound; sound transmission; space; theory


Virtual gravitational wave interferometers with actual mirrors Brillet, A., J.Y. Vinet, V. Loriette, J.M. Mackowski, L. Pinard, and A. Remillieux Physical Review D 68, no. 10 (2003)
Résumé: Long base interferometers for gravitational wave detection involve a number of mirrors having outstanding levels of quality. These large mirrors are now currently being produced. Each mirror has its own signature in reflected wave fronts due to its particular roughness pattern. Direct wave front measurements, carried out after the coating process, provide numerical maps from which very useful information can be extracted, either directly or by insertion in a numerical model of the interferometer. In particular, we show how it is possible to test the performances of the instrument for various attitudes of the mirrors before installing them in the vacuum chambers, a heavy and dangerous task. ©2003 The American Physical Society.


Vortex burst as a source of turbulence Cuypers, Y., A. Maurel, and P. Petitjeans Physical Review Letters 91, no. 19, 194502/1194502/4 (2003)
Résumé: The vortex burst in a laminarflow environment was shown to be responsible for the build up of the energy cascade. It was believed to be the first time that a single isolated flow structure was identified to produce the part of the turbulent energy spectrum described by Kolmogorov's k5/3 law. In addition, this structure seems to have some common feature with the theoretical one of Lundgren.
Motsclés: Imaging techniques; Laminar flow; Mathematical models; Numerical methods; Turbulence; Turbulent flow; Velocity measurement; Energy spectrum; Flow structure; Particle imaging velocimetry; Vortex flow


Experimental Simulation of Supersonic Superboom in a Water Tank: Nonlinear Focusing of Weak Shock Waves at a Fold Caustic Marchiano, R., J.L. Thomas, and F. Coulouvrat Physical Review Letters 91, no. 18, 18430111843014 (2003)
Résumé: The supersonic boom produced by an accelerating supersonic aircraft was investigated. The sonic boom was produced due to the nonlinear focusing of acoustical shock waves at a fold caustic. The nonlinear Tricomi equation was used to model the phenomenon. A nonlinear numerical simulation of the sonic boom in a water tank was also carried out. It was observed that the nonlinear weak shock waves limited the field amplitude, distorted the sonic line and strongly altered the phase of the signal.
Motsclés: Acoustic noise; Acoustic wave propagation; Algorithms; Computer simulation; Shock waves; Supersonic aircraft; Water tanks; Wave filters; Supersonic superbooms; Tricomi equation; Supersonic aerodynamics


Observation of Shock Transverse Waves in Elastic Media Catheline, S., J.L. Gennisson, M. Tanter, and M. Fink Physical Review Letters 91, no. 16, 16430111643014 (2003)
Résumé: A shock transverse wave propagation in an elastic medium was studied by using Burger's equation. The shock formation was observed over a distance of less than a few wavelengths. The results showed that the shock distance increases when the wave amplitude is decreased.
Motsclés: Algorithms; Boundary conditions; Computer simulation; Equations of motion; Mathematical models; Shear waves; Wave propagation; Shock transverse wave propagation; Shock waves


High power transcranial beam steering for ultrasonic brain therapy Pernot, M., J.F. Aubry, M. Tanter, J.L. Thomas, and M. Fink Physics in Medicine and Biology 48, no. 16, 25772589 (2003)
Résumé: A sparse phased array is specially designed for noninvasive ultrasound transskull brain therapy. The array is made of 200 single elements corresponding to a new generation of high power transducers developed in collaboration with Imasonic (Besançon, France). Each element has a surface of 0.5 cm2 and works at 0.9 MHz central frequency with a maximum 20 W cm2 intensity on the transducer surface. In order to optimize the steering capabilities of the array, several transducer distributions on a spherical surface are simulated: hexagonal, annular and quasirandom distributions. Using a quasirandom distribution significantly reduces the grating lobes. Furthermore, the simulations show the capability of the quasirandom array to electronically move the focal spot in the vicinity of the geometrical focus (up to ±15 mm). Based on the simulation study, the array is constructed and tested. The skull aberrations are corrected by using a time reversal mirror with amplitude correction achieved thanks to an implantable hydrophone, and a sharp focus is obtained through a human skull. Several lesions are induced in fresh liver and brain samples through human skulls, demonstrating the accuracy and the steering capabilities of the systems.
Motsclés: Computer simulation; Hydrophones; Transducers; Ultrasonic applications; High power transducers; Brain; accuracy; article; brain injury; controlled study; echoencephalography; electronics; equipment design; human; nonhuman; priority journal; process model; process optimization; skull; surface property; ultrasound therapy; ultrasound transducer; Brain; Brain Diseases; Computer Simulation; ComputerAided Design; Equipment Design; Equipment Failure Analysis; Heat; Humans; Liver; Models, Biological;


A simple model for the prediction of thermal conductivity in pure and doped insulating crystals Gaumé, R., B. Viana, D. Vivien, J.P. Roger, and D. Fournier Applied Physics Letters 83, no. 7, 13551357 (2003)
Résumé: A model was proposed to predict the thermal conductivity in pure and doped insulating crystals. The thermal conductivity evaluation of the point defects encountered in luminescent materials was also carried out. The thermal conductivity coefficients were determined by photothermal experiments and were in agreement with the predicted values in solid state lasers.
Motsclés: Crystal defects; Insulating materials; Semiconductor doping; Solid state lasers; Thermal conductivity; Insulating crystals; Crystals


Ultrasonic speckle correlation imaging of 2D particle velocity profiles in multiphase flows Carlson, J., and R. K. Ing Flow Measurement and Instrumentation 14, no. 45, 193200 (2003)
Résumé: Twodimensional ultrasonic speckle correlation velocimetry (USV) is a new technique that allows imaging of moving scattering media, at a high framerate. In this paper we apply the technique to determine twodimensional particle velocity profiles of multiphase flows. Experiments are realized with suspensions of Sonazoid (medical contrast agent) and Magnetite (Fe3O4) in water. All measurements are performed in a vertical pipe with the flow moving downwards. The twodimensional particle velocity profiles are then compared with a reference liquid volume flow velocity. As expected from theory, the heavier Magnetite particles have slightly higher velocity than the liquid, whereas the contrast agent simply follows the liquid motion. The proposed technique can be used in combination with other techniques to measure the mass flow of the solid phase, in solid/liquid multiphase flow. This is generally more interesting than measuring the bulk mass or volume flow. © 2003 Elsevier Ltd. All rights reserved.
Motsclés: Multiphase flow; Speckle correlation imaging; Ultrasound; Velocity measurements; Correlation methods; Speckle; Ultrasonic velocity measurement; Particle velocity profiles; Flow measurement; flow measurement; flow velocity; multiphase flow; speckle photography; ultrasonic flowmeter; velocity profile


Analysis of surface acoustic wave propagation on a cylinder using laser ultrasonics Clorennec, D., and D. Royer Applied Physics Letters 82, no. 25, 46084610 (2003)
Résumé: The surface acoustic wave propagation on a cylinder was analyzed using laser ultrasonics. A short Rayleigh wave pulse was reversed during its propagation. The radial component of the displacement was measured by a heterodyne optical interferometer. A onedimensional analytical calculation taking into account the dispersion effect was developed.
Motsclés: Acoustic wave propagation; Heterodyning; Interferometers; Laser beam effects; Ultrasonics; Laser ultrasonics; Acoustic surface wave devices


Acoustical propagation in a prefractal waveguide Gibiat, V., A. Barjau, K. Castor, and E. B. Du Chazaud Physical Review E  Statistical, Nonlinear, and Soft Matter Physics 67, no. 6 2, 066609/1066609/9 (2003)
Résumé: A prefractal acoustical system based on the geometry of the Cantor set was built and studied from both a theoretical and an experimental point of view. The results confirm in a totally different acoustical system those obtained by Alippi et al. As such, trapped or localized modes were shown in the central part of the system for waveguides of order higher than 2.
Motsclés: Acoustic impedance; Acoustic wave reflection; Acoustic wave scattering; Acoustic wave transmission; Bessel functions; Computer simulation; Fractals; Frequency domain analysis; Iterative methods; Mathematical models; Tubes (components); Waveguides; Acoustical waveguide; Cantor like waveguide; Prefactal waveguide; Acoustic wave propagation


Resolution enhancement and separation of reverberation from target echo with the time reversal operator decomposition Folégot, T., C. Prada, and M. Fink Journal of the Acoustical Society of America 113, no. 6, 31553160 (2003)
Résumé: Time reversal operator (TRO) decompositions are performed in a model of an ocean wave guide containing a target and having different kinds of bottom. The objective is to study the effects of bottom reverberation and absorption by means of ultrasonic experiments. It is shown experimentally that the echo from a target can be separated from the bottom reverberation. Reverberation eigenvectors are back propagated in the wave guide leading to focus on the bottom. An amplitude correction is applied to both reverberation and signal eigenvectors to compensate for bottom absorption and thus to improve target resolution. © 2003 Acoustical Society of America.
Motsclés: Acoustic wave absorption; Backpropagation; Eigenvalues and eigenfunctions; Mathematical operators; Ultrasonic applications; Water waves; Target resolution; Reverberation; poly(methyl methacrylate); water; acoustics; absorption; acoustics; article; Fourier transformation; priority journal; sea; signal noise ratio; time; ultrasound


Photoreflectance studies of optical transitions in type II (GaAs)m(AlAs)n superlattices Wang, G., P. Tronc, Y. E. Kitaev, and R. Planel Journal of Physics Condensed Matter 15, no. 19, 33433354 (2003)
Résumé: Photoreflectance (PR) spectra of two type II [001](GaAs)m (AlAs)n superlattices (SLs) have been measured at 77 K. In the conventional picture of the envelopefunction approximation, the lowest conduction state originates, in the first sample, from the Xz point of the AlAs Brillouin zone (z being the growth direction) whereas it originates from the Xx,y point in the second sample. Our spectra exhibit FranzKeldysh oscillation (FKO) features and interband transition lines. The origin of the builtin electric field within the samples is discussed and its strength calculated from FKOs. For interpreting our spectra of interband optical transitions, a leastsquares fit of the data to the Aspnes thirdderivative functional form has been performed as well as computation of the optical transition energies. From the energy and amplitude of the interband transition lines in PR spectra, we showed that the two SLs are pseudodirect, i.e. the ground optical transition in any of them is direct in the k space and takes place at the Γ point of the SL Brillouin zone. All the other interband transitions appearing in the SL spectra are also direct in k space.
Motsclés: Approximation theory; Electric field effects; Light reflection; Oscillations; Phase transitions; Semiconducting aluminum compounds; Semiconducting gallium arsenide; Strength of materials; Aluminum arsenide; Envelopefunction approximation; FranzKeldysh oscillation; Optical transition; Photoreflectance; Semiconductor superlattices


The radial velocities and physical parameters of ER Vul Duemmler, R., C. Doucet, F. Formanek, I. Ilyin, and I. Tuominen Astronomy and Astrophysics 402, no. 2, 745754 (2003)
Résumé: ER Vul is an eclipsing binary consisting of two solar type stars in a very close orbit with a period of 0 d.7. Accordingly, the two stars rotate very fast, leading to a blending of many spectral features at all phases. Therefore, measuring the radial velocity curve without systematic errors is not trivial. Here, we use a twodimensional crosscorrelation method applied to 137 high quality spectra, collected over 3 years, in order to obtain the radial velocity curve and determine the orbital and some physical parameters of the system from it. Primarily, we improve the binary period to 0 d.69809458 ± 0 d.00000014, and find that the two amplitudes are slightly smaller than those measured by others, while the mass ratio is still similar. While at least the primary almost fills its Roche lobe, the system is still detached, i.e. not yet a fully fledged W UMasystem. The behaviour of the Ca H IRT line at 8662 Å confirms that the secondary is the more active component, and that the chromospheric emission is not symmetrically distributed over the surfaces of either star.
Motsclés: Binaries: eclipsing; Binaries: spectroscopic; Stars: individual: ER Vul; Stars: latetype; Techniques: radial velocities; Atmospheric spectra; Doppler effect; Imaging techniques; Interpolation; Orbits; Planetary surface analysis; Signal to noise ratio; Space research; Spectrographs; Spectroscopic analysis; Temperature distribution; Velocity; Radial velocities; Astrophysics


Field fluctuation spectroscopy in a reverberant cavity with moving scatterers De Rosny, J., P. Roux, M. Fink, and J. H. Page Physical Review Letters 90, no. 9, 094302/1094302/4 (2003)
Résumé: Ultrasonic waves were used to investigate the fluctuations of the wave field due to the motion of scatterers inside a static reverberant cavity. A theory, in the limit of large scattering mean free path compared to the largest cavity dimension, that gives an excellent description of the experimental results was developed. By showing how the field autocorrelation function depends on the scattering mean free path of the waves and motion of the scatterers, the principles of a new technique in field fluctuation spectroscopy, diffusing reverberant acoustic wave spectroscopy (DRAWS) was demonstrated.
Motsclés: Calculations; Correlation methods; Diffusion; Spectroscopic analysis; Stainless steel; Stepping motors; Ultrasonic reflection; Ultrasonic scattering; Water tanks; Acoustic wave field; Acoustic wave spectroscopy; Elastic mean free path; Field fluctuation spectroscopy; Fish monitoring; Stainless steel ball; Ultrasonic propagation


Sound focusing in rooms: The timereversal approach Yon, S., M. Tanter, and M. Fink Journal of the Acoustical Society of America 113, no. 3, 15331543 (2003)
Résumé: New perspectives in audible range acoustics, such as virtual sound space creation and active noise control, rely on the ability of the rendering system to recreate precisely a desired sound field. This ability to control sound in a given volume of a room is directly linked to the capacity to focus acoustical energy both in space and time. However, sound focusing in rooms remains a complicated problem, essentially because of the multiple reflections on obstacles and walls occurring during propagation. In this paper, the technique of timereversal focusing, well known in ultrasound, is experimentally applied to audible range acoustics. Compared to classical focusing techniques such as delay law focusing, time reversal appears to considerably improve quality of both temporal and spatial focusing. This socalled superresolution phenomenon is due to the ability of time reversal to take into account all of the different sound paths between the emitting antenna and the focal point, thus creating an adaptive spatial and temporal matched filter for the considered propagation medium. Experiments emphasize the strong robustness of timereversal focusing towards small modifications in the medium, such as people in motion or temperature variations. Sound focusing through walls using the timereversal approach is also experimentally demonstrated. © 2003 Acoustical Society of America.
Motsclés: Acoustic variables control; Acoustic wave propagation; Acoustics; Timereversal focusion; Acoustic noise measurement; acoustics; acoustics; article; energy; motion; noise reduction; priority journal; sound; space; technique; temperature; time; Acoustics; Auditory Perception; Humans; Noise; Social Environment; Sound Spectrography


Green's function estimation using secondary sources in a shallow water environment Roux, P., and M. Fink Journal of the Acoustical Society of America 113, no. 3, 14061416 (2003)
Résumé: This work provides a new way to measure the Green's function between two points in an acoustic channel without emitting a pulse by any of the two points. The Green's function between A and B is obtained from a set of secondary sources in the guide by averaging either the correlation or the convolution of the signals received in A and B. A theoretical approach based on mode propagation in a monochromatic regime is presented. Results are then extended to the time domain. Estimation of the Green's function is performed numerically in a rangeindependent and a rangedependent environment. Application to discreet acoustic communications is discussed. © 2003 Acoustical Society of America.
Motsclés: Correlation methods; Green's function; Time domain analysis; Water; Acoustic channels; Acoustic wave propagation; water; acoustics; acoustics; analytic method; article; correlation analysis; mathematical analysis; priority journal; sound transmission; theory; time


Acoustic nonlinearity parameter measurements in solids using the collinear mixing of elastic waves Jacob, X., C. Barrière, and D. Royer Applied Physics Letters 82, no. 6, 886888 (2003)
Résumé: An alternative method was presented for measuring the nonlinearity parameters β of longitudinal acoustic waves propagating in solids. The method was based on the detection of the phase modulation resulting from the parametric interaction between a lower frequency acoustic pulse and a high frequency acoustic waves. It was found that the measurement was absolute and the sign of the parameter β could be determined.
Motsclés: Acoustic wave propagation; Phase modulation; Polystyrenes; Titanium; Collinear mixing; Acoustic waves


Taking advantage of multiple scattering to communicate with timereversal antennas Derode, A., A. Tourin, J. De Rosny, M. Tanter, S. Yon, and M. Fink Physical Review Letters 90, no. 1, 014301/1014301/4 (2003)
Résumé: An overview is given of the experimental demonstration that highorder scattering in a disordered medium can help by increasing the information transfer rate, especially if the timereversal technique is used to naturally focus the different bistreams onto the receivers. The first key parameter in the experiment is the number of independent focal spots that can be created by the transmitting array in the receiving plane. The second key parameter is the number of uncorrelated frequencies within the bandwidth, which governs the peaktonoise ratio on each receiver.
Motsclés: Antenna arrays; Communication channels (information theory); Data communication systems; Decoding; Eigenvalues and eigenfunctions; Electromagnetic wave propagation; Fourier transforms; Matrix algebra; Signal receivers; Ultrasonic devices; Vectors; Wireless telecommunication systems; Time reversal antennas; Ultrasonic antennas; Electromagnetic wave scattering


Experimental demonstration of noninvasive transskull adaptive focusing based on prior computed tomography scans Aubry, J.F., M. Tanter, M. Pernot, J.L. Thomas, and M. Fink Journal of the Acoustical Society of America 113, no. 1, 8493 (2003)
Résumé: Developing minimally invasive brain surgery by highintensity focused ultrasound beams is of great interest in cancer therapy. However, the skull induces strong aberrations both in phase and amplitude, resulting in a severe degradation of the beam shape. Thus, an efficient brain tumor therapy would require an adaptive focusing, taking into account the effects of the skull. In this paper, we will show that the acoustic properties of the skull can be deduced from high resolution CT scans and used to achieve a noninvasive adaptive focusing. Simulations have been performed with a full 3D finite differences code, taking into account all the heterogeneities inside the skull. The set of signals to be emitted in order to focus through the skull can thus be computed. The complete adaptive focusing procedure based on prior CT scans has been experimentally validated. This could have promising applications in brain tumor hyperthermia but also in transcranial ultrasonic imaging. © 2003 Acoustical Society of America.
Motsclés: Aberrations; Computer simulation; Computerized tomography; Neurosurgery; Oncology; Brain tumor; Ultrasonic imaging; tomography; acoustics; adaptation; article; brain tumor; calculation; computer assisted tomography; Doppler echography; human; human tissue; hyperthermia; image analysis; priority journal; simulation; skull; sound detection; ultrasound; Acoustics; Brain Neoplasms; Computer Simulation; Echoencephalography; Humans; Image Processing, ComputerAssisted; Imaging, ThreeDimensional; Math


Combined optical/mcd/odmr investigations of photochromism in doublydoped Bi12GeO20 Briat, B., M. T. Borowiec, H. B. Rjeily, F. Ramaz, A. Hamri, and H. Szymczak Radiation Effects and Defects in Solids 157, no. 612, 989993 (2002)
Résumé: Electron paramagnetic resonance is detected optically via the change of magnetic circular dichroism under microwaves at 35 GHz. The technique is applied to Bi12GeO20 samples codoped with vanadium and a second transition metal (Cr, Mn, Co, Cu). The optical and magnetic properties of several paramagnetic defects (VGe 4+ and Cr Ge 4+) are directly correlated. The basic photochromic processes occuring in samples doped with V, Mn, and Mn + V are explained. The VGe 4+/5+level is positioned roughly 2.2 eV above the valence band.
Motsclés: Bismuth germanate; EPR; Magnetic circular dichroism; Photochromic; Photorefractive; Sillenite; Bismuth compounds; Doping (additives); Microwaves; Optical devices; Paramagnetic resonance; Photorefractive materials; Transition metals; Bismuth germanate; Magnetic circular dichroism; Paramagnetic defects; Sillenite; Photochromism


Time reversal techniques in ultrasonic nondestructive testing of scattering media Prada, C., E. Kerbrat, D. Cassereau, and M. Fink Inverse Problems 18, no. 6, 17611773 (2002)
Résumé: Time reversal techniques are adaptive methods that can be used in nondestructive evaluation to improve flaw detection through inhomogeneous and scattering media. Two techniques are presented: the iterative time reversal process and the DORT (French acronym for decomposition of the time reversal operator) method. In pulse echo mode, iterative time reversal mirrors allow one to accurately control wave propagation and focus selectively on a defect reducing the speckle noise due to the microstructure contribution. The DORT method derives from the mathematical analysis of the iterative time reversal process. Unlike time reversal mirrors, it does not require programmable generators and allows the simultaneous detection and separation of several defects. These two procedures are presented and applied to detection in titanium billets where the grain structure renders detection difficult. Then, they are combined with the simulation code PASS (phased array simulation software) to form images of the samples.
Motsclés: Adaptive systems; Computer simulation; Computer software; Iterative methods; Microstructure; Mirrors; Nondestructive examination; Speckle; Ultrasonic testing; Time reversal techniques; Inverse problems


Spectroscopic ellipsometry studies of index profile of indium tin oxide films prepared by spray pyrolysis El Rhaleb, H., E. Benamar, M. Rami, J. P. Roger, A. Hakam, and A. Ennaoui Applied Surface Science 201, no. 14, 138145 (2002)
Résumé: Spectroscopic ellipsometry (SE) has proven to be a very powerful diagnostic for thin film characterisation. It was used to determine thin film parameters such as film thickness and optical functions of polycrystalline tindoped indium oxide (ITO) films deposited by spray pyrolysis onto Pyrex substrates. Dielectric ITO films often present microstructures which give rise to a variation of the refractive index with the distance from substrate. In this work, it was found that the fit between ellipsometric data and optical models results could be significantly improved when it was assumed that the refractive index of ITO films varied across the upper 60 nm near the film surface. Also, the surface roughness was modelled and compared with that given by the atomic force microscope (AFM). © 2002 Elsevier Science B.V. All rights reserved.
Motsclés: Graded structure; ITO; Optical properties; Spectroscopic ellipsometry; Spray pyrolysis; Atomic force microscopy; Ellipsometry; Indium compounds; Polycrystalline materials; Pyrolysis; Refractive index; Spectroscopic analysis; Thin films; Spray pyrolysis; Surface phenomena


In vivo imaging of quantum dots encapsulated in phospholipid micelles Dubertret, B., P. Skourides, D. J. Norris, V. Noireaux, A. H. Brivanlou, and A. Libchaber Science 298, no. 5599, 17591762 (2002)
Résumé: Fluorescent semiconductor nanocrystals (quantum dots) have the potential to revolutionize biological imaging, but their use has been limited by difficulties in obtaining nanocrystals that are biocompatible. To address this problem, we encapsulated individual nanocrystals in phospholipid blockcopolymer micelles and demonstrated both in vitro and in vivo imaging. When conjugated to DNA, the nanocrystalmicelles acted as in vitro fluorescent probes to hybridize to specific complementary sequences. Moreover, when injected into Xenopus embryos, the nanocrystalmicelles were stable, nontoxic (<5 × 109 nanocrystals per cell), cell autonomous, and slow to photobleach. Nanocrystal fluorescence could be followed to the tadpole stage, allowing lineagetracing experiments in embryogenesis.
Motsclés: Biocompatibility; Biological organs; Copolymers; DNA; Encapsulation; Nanostructured materials; Phospholipids; Semiconductor materials; Complementary sequences; Micelles; complementary DNA; copolymer; DNA; fluorescent dye; nanoparticle; phospholipid; DNA; imaging method; article; biocompatibility; cell interaction; conjugation; diagnostic imaging; DNA hybridization; DNA sequence; DNA structure; embryo development; encapsulation; fluorescence; micelle; molecular stability; nanotechnology; nonhuman


Charge states and optical transitions of vanadium in Bi4Ge3O12 identified by MCD and ODMR Briat, B., A. Watterich, F. Ramaz, L. Kovács, B. C. Forget, and N. Romanov Optical Materials 20, no. 4, 253262 (2002)
Résumé: The spectroscopic properties of Vdoped Bi4Ge3O12 have been investigated in detail via several complementary techniques bringing a series of consistent results. The optical absorption spectrum of annealed samples is strongly modified under UVillumination and the initial state can be restored optically with visible light. Optical absorption and magnetic circular dichroism (MCD) demonstrate that a diamagnetic defect is partly destroyed during UVillumination while a paramagnetic one is created. The latter shows a very characteristic Sshaped MCD pattern in the nearIR, which is readily assigned to the 2E → 2T2 internal transition of tetragonal V4+ centers at the Ge sites. This assignment was further confirmed by optically detected magnetic resonance (ODMR), via the change of the MCD under microwaves at 35 GHz. The g tensor of V4+ was found to be anisotropic with principal values g∥ = 1.81±0.03 and g⊥ = 1.94±0.02. Two additional MCD bands in the visible spectral region could be attributed to V4+ via ODMR measurements. Ionization thresholds for holes (V5+) and electrons (V4+) were determined by optical absorption experiments and the V4+/5+ donor level was positioned 1.9 eV below the conduction band. The presence of the V4+/3+ acceptor state in the forbidden band is also suggested. © 2002 Elsevier Science B.V. All rights reserved.
Motsclés: Bismuth germanate; EPR; Eulytite; Magnetic circular dichroism; Photochromic; Photorefractive; Absorption spectroscopy; Anisotropy; Annealing; Bismuth compounds; Doping (additives); Light absorption; Magnetic resonance; Microwaves; Photochromism; Tensors; Magnetic circular dichroism (MCD); Optical transitions; Photorefractive materials


Instability of a confined jet impinging on a water/air free surface Bouchet, E., G. Climent, and A. Maurel Europhysics Letters 59, no. 6, 827833 (2002)
Résumé: Selfsustained oscillations in sinuous mode occur when a water jet impinges from below on a water/air free surface. Confined jet instability is experimentally investigated by image processing and velocity measurements. Despite small deformations of the surface, dynamic response of the jet provides unusual behaviour with comparable configurations (holetone, jet edge ⋯). The central feature is a bounded evolution of the oscillation frequency. Modal transitions are observed when physical parameters are varied. Each frequency jump is related to wavelength modification of the spatial pattern. Atypical evolution of the predominant length scale has to be connected to strong coupling with the weak deformations induced by the impinging jet on the free surface.


Ultrasonic nondestructive testing of scattering media using the decomposition of the timereversal operator Kerbrat, E., C. Prada, D. Cassereau, and M. Fink IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 49, no. 8, 11031113 (2002)
Résumé: In ultrasonic nondestructive testing, the iterative timereversal process is an adaptive technique that can be used to detect flaws in complex samples with a large array of transducers. The decomposition of the timereversal operator (D.O.R.T.) method is a detection technique that is derived from the mathematical analysis of the iterative timereversal process. Contrary to timereversal techniques, the D.O.R.T. method does not require programmable generators, and it allows the simultaneous detection and separation of several defects. In this paper, the method is applied to a Ti64 titanium cylindrical sample to separate the echo of a defect from the speckle due to microstructure contribution. The grain structure of this titanium alloy makes detection very difficult and, for large depths, conventional techniques do not allow the detection of small flaws with a satisfactory signaltonoise ratio. The efficiency of the D.O.R.T. method to detect a flat bottom hole with a diameter of 0.4 mm located at a depth of 140 mm in a titanium alloy sample is shown.
Motsclés: Acoustic wave scattering; Mathematical operators; Microstructure; Signal to noise ratio; Titanium alloys; Ultrasonic transducers; Timereversal operators; Acoustic emission testing


Depth and range shifting of a focal spot using a timereversal mirror in an acoustic waveguide Conti, S., P. Roux, and M. Fink Applied Physics Letters 80, no. 19, 36473649 (2002)
Résumé: We present a technique based on time reversal to focus an acoustic field at any depth and range in a waveguide. We take advantage of the signal received from an acoustic beacon in the guide to build a timereversed modified version of the field that refocuses at a different point in the waveguide. This method is based on the application of the images theorem in the guide and, as in the classical timereversal experiment, it does not require a priori knowledge of the waveguide characteristics. Ultrasonic laboratory experiments and underwater acoustic simulations are presented and compared to classical timereversal focusing results. © 2002 American Institute of Physics.
Motsclés: Acoustic simulations; Acoustic waveguides; Focal spot; Laboratory experiments; Priori knowledge; Timereversal; Timereversal focusing; Timereversed; Waveguide characteristic; Acoustic fields; Experiments; Ultrasonic applications; Waveguides; Underwater acoustics


Detection of cracks in a thin airfilled hollow cylinder by application of the DORT method to elastic components of the echo Kerbrat, E., D. Clorennec, C. Prada, D. Royer, D. Cassereau, and M. Fink Ultrasonics 40, no. 18, 715720 (2002)
Résumé: The DORT method (Décomposition de l'Opérateur de Retournement Temporel in French) is a scattering analysis technique which uses arrays of transducers. This method is efficient for detection of selective focusing on pointlike scatterers. It has been also applied to analyze the scattering by an airfilled cylindrical steel shell immersed in water. It was shown that the diagonalization of the time reversal operator allows us to separate the different elastic components of the scattered field. Here, we apply the method to detect flaws in hollow cylinders. In this case, the dominant components are the three circumferential waves (A0, A1 and S0 Lamb modes). Each Lamb mode corresponds to an invariant of the time reversal operator. The dispersion curves of these waves are calculated from these invariants. Resonance frequencies of the shell are deduced from the frequency dependence of the eigenvalues of the time reversal operator. It is shown that the presence of a crack (0.2 mm in depth) affects significantly the eigenvalue distribution of the time reversal operator. Thus, the DORT method offers a new means for detecting defects in a shell. © 2002 Elsevier Science B.V. All rights reserved.
Motsclés: Crack; DORT method; Hollow cylinder; Lamb waves; Nondestructive evaluation; Acoustic wave scattering; Cracks; Cylinders (shapes); Eigenvalues and eigenfunctions; Natural frequencies; Nondestructive examination; Transducers; Lamb waves; Acoustic wave propagation


Nondestructive evaluation of cylindrical parts using laser ultrasonics Clorennec, D., D. Royer, and H. Walaszek Ultrasonics 40, no. 18, 783789 (2002)
Résumé: We applied the laser ultrasonic technique for detecting surface breaking slots in steel cylinders (25 mm in diameter). The observation of the detected signal over a long time (500 μs), shows that the interaction of the two contrapropagating incident Rayleigh waves reinforce the echoes coming from the defect. These echoes are slowly growing with time whereas the main signals decrease. This energy transfer occurring at each revolution of the waves around the cylinder allows the detection of cracks having a depth (h ≈ 80 μm), very small compared to the Rayleigh wavelength (λ ≅ 2 mm). The evaluation of the material was performed by processing the detected signal in a sliding time window. A crosscorrelation is made either between a reference signal and the signal from the tested sample or between two signals probed for two different positions of the sample. In both cases, the slope of the crosscorrelation coefficient versus the number of turns is proportional to the depth of the slot. © 2002 Elsevier Science B.V. All rights reserved.
Motsclés: Cylinder; Laser ultrasonics; Nondestructive evaluation; Surface acoustic waves; Cracks; Cylinders (shapes); Laser applications; Nondestructive examination; Signal detection; Slotting; Steel; Surface waves; Ultrasonic waves; Laser ultrasonics; Ultrasonic testing


Time reversal versus phase conjugation in a multiple scattering environment Derode, A., A. Tourin, and M. Fink Ultrasonics 40, no. 18, 275280 (2002)
Résumé: We present experimental results on the reversibility of ultrasound in a multiple scattering medium. An ultrasonic pulsed wave is transmitted from a point source to a 128element receiving array through 2D samples with various thickness. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed and sent back into the medium. The timereversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the secondorder moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation of the correlations on a single realisation of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by diffraction. Moreover, successful timereversal focusing using a single element instead of an array is possible, whereas a onechannel monochromatic phase conjugation fails. The efficiency of broadband time reversal compared to monochromatic phase conjugation lies in the number of 'information grains' in the frequency bandwidth. © 2002 Elsevier Science B.V. All rights reserved.
Motsclés: Multiple scattering; Phase conjugation; Time reversal; Bandwidth; Focusing; Monochromators; Natural frequencies; Optical correlation; Optical phase conjugation; Optical resolving power; Robustness (control systems); Ultrasonic waves; Multiple scattering; Ultrasonic scattering


Boosting sonoluminescence with a highintensity ultrasonic pulse focused on the bubble by an adaptive array Thomas, J.L., Y. Forterre, and M. Fink Physical Review Letters 88, no. 7, 743021743024 (2002)
Résumé: A new experimental approach for boosting sonoluminescence was presented, where an high intensity ultrasonic pulse of high frequency was adaptively focused on the bubble during the collapse. A pressure pulse of 0.7 MPa doubled the flash intensity using an array of eight transmitters. A brightness gain of a factor of 2 was obtained, which was limited only by the power of the transducers.
Motsclés: Bubbles (in fluids); Focusing; Fusion reactions; Photons; Pressure effects; Transmitters; Ultrasonic waves; Singlebubble sonoluminescence (SBSL); Ultrasonic pulses; Sonoluminescence


Ultrasound shock wave generator with onebit time reversal in a dispersive medium, application to lithotripsy Montaldo, G., P. Roux, A. Derode, C. Negreira, and M. Fink Applied Physics Letters 80, no. 5, 897899 (2002)
Résumé: The building of highpower ultrasonic sources from piezoelectric ceramics is limited by the maximum voltage that the ceramics can endure. We have conceived a device that uses a small number of piezoelectric transducers fastened to a cylindrical metallic waveguide. A onebit time reversal operation transforms the longlasting lowlevel dispersed wave forms into a sharp pulse, thus taking advantage of dispersion to generate highpower ultrasound. The pressure amplitude that is generated at the focus is found to be 15 times greater than that achieved with comparable standard techniques. Applications to lithotripsy are discussed and the destructive efficiency of the system is demonstrated on pieces of chalk. © 2002 American Institute of Physics.
Motsclés: Dispersive medium; High power ultrasound; Highpower ultrasonics; Lithotripsy; Long lasting; Metallic waveguide; Pressure amplitudes; Shock wave generator; Time reversal; Wave forms; Building materials; Ceramic materials; Piezoelectric ceramics; Piezoelectricity; Ultrasonics; Dispersion (waves)


Mathematical foundations of the time reversal mirror Bardos, C., and M. Fink Asymptotic Analysis 29, no. 2, 157182 (2002)
Résumé: In the present paper a mathematical analysis of the "time reversal mirror" (cf. [4,9,10]) is given. As a first step of a more detailed program, the emphasis is put on phenomena which are described by the genuine acoustic equation with Dirichlet or "impedance" boundary conditions. An ideal situation is first considered then relation between the question of the local decay of energy and the accuracy of the method are exploited. The positive effect of the ergodicity is explained and eventually comparison with control theory approach is considered.


Sensitivity to perturbations of a timereversed acoustic wave in a multiple scattering medium Tourin, A., A. Derode, and M. Fink Physical Review Letters 87, no. 27 I, 27430112743014 (2001)
Résumé: Analysis of experimental results on the robustness of acoustic focusing in a multiple scattering medium undergoing perturbations was presented. A time reversal experiment was performed to follow its timedependent evolution in a perturbed random medium. The method was found to be analogous to the diffusive acoustic wave spectroscopy developed to study fluidized suspensions of particles.
Motsclés: Optical correlation; Perturbation techniques; Scattering; Sensitivity analysis; Spectroscopic analysis; Transducers; Diffusive wave spectroscopy; Multiple scattering; Ultrasonic waves


Generation of very high pressure pulses with 1bit time reversal in a solid waveguide Montaldo, G., P. Roux, A. Derode, C. Negreira, and M. Fink Journal of the Acoustical Society of America 110, no. 6, 28492857 (2001)
Résumé: The use of piezoelectric transducer arrays has opened up the possibility of electronic steering and focusing of acoustic beams to track kidney stones. However, owing to the limited pressure delivered by each transducer (typically 10 bar), the number of transducers needed to reach an amplitude at the focus on the order of 1000 bars is typically of some hundreds of elements. We present here a new solution based on 1bit time reversal in a solid waveguide to obtain, with a small number of transducers, a very high amplitude pulse in tissues located in front of the waveguide. The idea is to take advantage of the temporal dispersion in the waveguide to create, after time reversal, a temporally recompressed pulse with a stronger amplitude. The aim of this work is threefold: first, we experimentally demonstrate 1bit time reversal between a point source in water and several transducers fastened to one section of a finitelength cylindrical waveguide. Second, we numerically and experimentally study the temporal and spatial focusing at the source as a function of the characteristics of the "solid waveguidetime reversal mirror (TRM)" system: length and diameter of the guide, number of transducers of the TRM. Last, we show that the instantaneous power delivered in water at the focus of the solid waveguide is much higher than the power directly transmitted into water from a classically focused transducer. The combination of 1bit time reversal and a solid waveguide leads to shock wave lithotripsy with lowpower electronics. © 2001 Acoustical Society of America.
Motsclés: Arrays; Piezoelectric transducers; Shock waves; Steering; Waveguides; High pressure pulses; Lithotripsies; Acoustics; acoustic technique; pulse generator; acoustics; amplitude modulation; article; electronics; hyperbarism; lithotripsy; nephrolithiasis; pressure; priority journal; pulse generator; transducer; waveform


Proposed test of the time independence of the fundamental constants α and me/mp using monolithic resonators Braxmaier, C., O. Pradl, H. Müller, A. Peters, J. Mlynek, V. Loriette, and S. Schiller Physical Review D 64, no. 4 (2001)
Résumé: A novel method is proposed for a laboratory test of the time independence of the finestructure constant and of the electron to proton mass ratio, employing only electromagnetic resonators. For a resonator made out of a dielectric ionic or molecular crystal, the resonance frequencies depend on the product α2 √me/mp through the index of refraction. A test of the time independence of α and me/mp can be performed by a search for a longterm change in the difference of two resonance frequencies exhibiting different dispersion. The method can be advantageously implemented with a single resonator. The prospects of implementation using optical resonators, systematic effects, and experimental requirements for a test at the level of 4 × 1015/yr relative variation in α or me/mp are discussed. On the experimental side, birefringent monolithic sapphire optical resonators with highreflectivity mirrors are fabricated and characterized. The measurements of optical materials with ultralow absorption loss (ppm/cm level), required for the proposed method, are also reported. ©2001 The American Physical Society.
Motsclés: proton; absorption; article; calculation; crystal; dispersion; electromagnetic field; electron; measurement; methodology; physics


Observation of a coherent backscattering effect with a dipolar source for elastic waves: Highlight of the role played by the source De Rosny, J., A. Tourin, and M. Fink Physical Review E  Statistical, Nonlinear, and Soft Matter Physics 64, no. 6 II, 066604/1066604/4 (2001)
Résumé: Experimental evidence of the role played by the source on the coherent backscattering effect (CBE) for elastic waves was established. It was shown that using a dipolar source and a monopolar receiver, a "bicone" can be obtained instead of a simple cone that is well explained by describing dipolar as the superposition of two monopolar sources opposite in phases.
Motsclés: Aluminum; Computational methods; Eigenvalues and eigenfunctions; Elastic waves; Fourier transforms; Interferometers; Lenses; Modal analysis; Optical beam splitters; Silicon; Transducers; Bicone; Chaotic cavity; Coherent backscattering effect; Heterodyne interferometer; Multipolar emitterreceiver; Pointlike dipolar source; Silicon plate; Backscattering


Tomographic and topographic images using an interference microscope Vabre, L., A. Dubois, E. Beaurepaire, J. L. Stéhlé, and A. C. Boccara REE, Revue de L'Electricite et de L'Electronique, no. 1, 4547 (2001)
Résumé: An interference microscope using a low coherence source, based on a Michelson interferometer, is coupled with a CCD camera acquiring interference images at high frequency. Using a parallel lockin detection on all the pixels of the CCD array, we obtain intensity images (tomographic images) or phase images (proportional to the topography) of the observed object.
Motsclés: Coherent optics; Interference microscopy; Tomography


Infrared thermography revealed a role for mitochondria in presymptomatic cooling during harpininduced hypersensitive response Boccara, M., C. Boué, M. Garmier, R. De Paepe, and A.C. Boccara Plant Journal 28, no. 6, 663670 (2001)
Résumé: The establishment of Erwinia amylovora harpininduced hypersensitive response (HR) in Nicotiana sylvestris was followed by infrared thermography (IRT). Three to four hours after elicitation, the temperature decreased in the harpininfiltrated zone associated to stomatal opening. The marked drop in temperature which reached 2°C and preceded necrosis symptoms for several hours, is thus likely caused by higher transpiration. Neither of these effects was observed in a respiratory mutant, affected in complex I structure and function and overexpressing alternative oxidase, indicating that they are directly or indirectly mediated by mitochondrial function. However, as the HR establishment was similar in both wild type and mutant, cell death was either uncorrelated with the observed epidermal changes or occurred by a different signalling pathway in the two genotypes. IRT revealed a novel aspect of plantpathogen interactions and could be applied to screen for mutants affected in elicitor signalling and/or for respiratory mutants.
Motsclés: Alternative oxidase; Complex I respiratory mutant; Mitochondria; Nicotiana sylvestris; Plantpathogen interactions; Cells; Infrared radiation; Mutagens; Thermography (imaging); Cell death; Plants (botany); Embryophyta; Erwinia amylovora; Nicotiana sylvestris; harpin; HrpZ protein, Pseudomonas syringae; outer membrane protein; article; Erwinia; infrared spectrophotometry; microbiology; mitochondrion; physiology; polyacrylamide gel electrophoresis; temperature; tobacco; Western blotting; Bacterial


Diffraction effects in the parametric interaction of acoustic waves: Application to measurements of the nonlinearity parameter B/A in liquids Barrière, C., and D. Royer IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 48, no. 6, 17061715 (2001)
Résumé: This paper deals with diffraction effects related to the determination of nonlinearity parameters B/A in liquids using the parametric interaction. A diffraction model, based on plane wave expansions, is applied to weak nonlinear interactions between two ultrasonic waves propagating in an absorbing medium. A validation of the model has been carried out with an experiment performed using an optical interferometer. A method is proposed to measure nonlinearity parameters with the parametric interaction of two waves with a high frequency ratio. With respect to diffraction effects, the parametric interaction can then be identified with a phase modulation of the high frequency wave. A prototype, using the interaction of a 30MHz frequency continuous wave with an acoustic pulse of central frequency 2.5 MHz, has been designed. The proposed method is validated by nonlinearity parameters measurements in wellknown liquids (water and ethanol). The uncertainty on absolute measurements is discussed, and a relative method is proposed to increase the accuracy. This method enables measurements without any transducer calibration.
Motsclés: Approximation theory; Computational methods; Ethanol; Frequencies; Interferometers; Parameter estimation; Ultrasonic measurement; Ultrasonic propagation; Ultrasonic transducers; Water; Wave equations; Acoustic pulse; Nonlinear interaction; Optical interferometer; Plane wave expansion; Acoustic wave diffraction


Nearfield optical studies of semicontinuous metal films Ducourtieux, S., V. A. Podolskiy, S. Grésillon, S. Buil, B. Berini, P. Gadenne, A. C. Boccara, J. C. Rivoal, W. D. Bragg, K. Banerjee, V. P. Safonov, V. P. Drachev, Z. C. Ying, A. K. Sarychev, and V. M. Shalaev Physical Review B  Condensed Matter and Materials Physics 64, no. 16, 165403116540314 (2001)
Résumé: Local field distributions in random metaldielectric films near a percolation threshold are experimentally studied using scanning nearfield optical microscopy (SNOM). The surfaceplasmon oscillations in such percolation films are localized in small nanometerscale areas, "hot spots", where the local fields are much larger than the field of an incident electromagnetic wave. The spatial positions of the hot spots vary with the wavelength and polarization of the incident beam. Local nearfield spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance qualityfactor of hot spots increases from the visible to the infrared. Giant local optical activity associated with chiral plasmon modes has been obtained. The hot spot's large local fields may result in local, frequency and spatially selective photomodification of percolation films.
Motsclés: metal; article; dielectric constant; electromagnetic field; film; microscopy; molecular interaction; oscillation; polarization; spectroscopy; surface plasmon resonance


Correlation between the optical and the magnetic properties of defects in photorefractive CdTe:Ge Farid, B., F. Ramaz, O. Panchuk, P. Fochuk, T. Arnoux, J. C. Launay, and B. Briat Optical Materials 18, no. 1, 711 (2001)
Résumé: The optimisation of CdTe:Ge for photorefractive applications at useful wavelengths requires the characterisation of a number of deep defects and an understanding of their role in recharging processes. We have investigated crystals of several origins via optical absorption and photoabsorption (PA), magnetic circular dichroism (MCD) and the optical detection of magnetic resonance (ODMR). Photoabsorption provides quantitative information about optical crosssections of ionisation while MCD indicates the occurrence of several paramagnetic species and enables the monitoring of their relative concentrations. Besides a very clear identification of the ionised donor GeCd+, ODMR shows the presence of several uncompletely identified defects. © 2001 Elsevier Science B.V. All rights reserved.
Motsclés: Cadmium telluride; CdTe:Ge; Germanium; Photorefractive crystals; Cadmium compounds; Crystal defects; Germanium; Light absorption; Magnetic resonance; Paramagnetism; Photoionization; Magnetic circular dichroism (MCD); Photoabsorption; Photorefractive materials


Determination of Lamb mode eigenvalues Pagneux, V., and A. Maurel Journal of the Acoustical Society of America 110, no. 3 I, 13071314 (2001)
Résumé: An original method is presented to determine the complex Lamb wave spectrum by using a numerical spectral method applied to the elasticity equations. This method presents the advantage to directly determine complex wave numbers for a given frequency via a classical matricial eigenvalue problem, and allows the wave numbers to be determined at relatively high frequencies (i.e., corresponding to many propagating modes). It does not need initial guess values for the wave numbers, contrary to the usual method of root finding of the RayleighLamb frequency equations (dispersion relation) in the complex plane. Results are presented and the method is discussed. © 2001 Acoustical Society of America.
Motsclés: Acoustic wave propagation; Eigenvalues and eigenfunctions; Elasticity; Spectrum analysis; Wave numbers; Acoustics; eigenvalue; wave; analytic method; article; dispersion; elasticity; frequency modulation; mathematical analysis; mathematical computing; priority journal; sound; spectral sensitivity; waveform


Experimental observation of percolationenhanced nonlinear light scattering from semicontinuous metal films Breit, M., V. A. Podolskiy, S. Grésillon, G. Von Plessen, J. Feldmann, J. C. Rivoal, P. Gadenne, A. K. Sarychev, and V. M. Shalaev Physical Review B  Condensed Matter and Materials Physics 64, no. 12, 12510611251065 (2001)
Résumé: Strongly enhanced secondharmonic generation (SHG), which is characterized by a nearly isotropic intensity distribution, is observed for goldglass films near the percolation threshold. The diffuselike SHG scattering, which can be thought of as nonlinear critical opalescence, is in sharp contrast with highly collimated linear reflection and transmission from these nanostructured semicontinuous metal films. Our observations, which can be explained by giant fluctuations of local nonlinear sources for SHG due to plasmon localization, verify recent predictions of percolationenhanced nonlinear scattering.
Motsclés: glass; gold; article; chemical structure; film; light scattering; nanoparticle; polarimetry; prediction; reflectometry


In situ local tissue characterization and imaging by backscattering acoustooptic imaging LévêqueFort, S., J. Selb, L. Pottier, and A. C. Boccara Optics Communications 196, no. 16, 127131 (2001)
Résumé: We use an imaging system taking advantage of a focused ultrasonic field to mark the photon trajectories in a biological sample by modulating the optical paths in the focal zone. We show the applicability of this setup to various geometrical configurations, such as forward and backward scattering. The modulated speckle detected on a CCD camera reflects, after parallel processing, the local optical properties of the tissue in the focal zone. We propose to use the backscattering geometry to estimate the local effective attenuation coefficient. Finally we compare images which have been obtained through several centimeters of biological tissue, both in forward and in backward geometry. © 2001 Elsevier Science B.V. All rights reserved.
Motsclés: Acoustooptic imaging; Biological tissues; CCD camera; Speckle; Attenuation; Backscattering; Charge coupled devices; Light modulation; Photons; Tissue; Trajectories; Acoustooptic imaging; Imaging techniques


Numerical and experimental timereversal of acoustic waves in random media Derode, A., M. Tanter, A. Tourin, L. Sandrin, and M. Fink Journal of Computational Acoustics 9, no. 3, 9931003 (2001)
Résumé: In classical mechanics, a timereversal experiment with a large number of particles is impossible. Because of the high sensitivity to initial conditions, one would need to resolve the positions and velocities of each particle with infinite accuracy. Thus, it would require an infinite amount of information, which is of course out of reach. In wave physics however, the amount of information required to describe a wave field is limited and depends on the shortest wavelength of the field. Thus we can propose an acoustic equivalent of the experiment we mentioned above. We start with a coherent transient pulse, let it propagate through a disordered highly scattering medium, then record the scattered field and timereverse it: surprisingly, it travels back to its initial source, which is not predictable by usual theories for random media. Indeed, to study waves propagation in disordered media theoreticians, who find it difficult to deal with one realization of disorder, use concepts defined as an average over the realizations, which naturally leads to the diffusion approximation. But the corresponding equation is not timereversal invariant and thus fails in describing our experiment. Then, to understand our experimental results and try to predict new ones, we have developed a finite elements simulation based on the real microscopic timeinvariant equation of propagation. The experimental and numerical results are found to be in very good agreement.


Ultrasound propagation through a rotational flow: Numerical methods compared to experiments Manneville, S., C. Prada, M. Tanter, M. Fink, and J.F. Pinton Journal of Computational Acoustics 9, no. 3, 841852 (2001)
Résumé: Sound propagation through a vortex is studied numerically using two different techniques: raytracing and finitedifferences. Geometrical acoustics and raytracing are shown to yield a good picture of the interaction between a sound wave and a vortex when the ratio of the vortex radius to the acoustic wavelength is larger than one. In particular, this technique allows to take into account finitesize effects such as edge waves and the results are compared to experimental data. The interest of the finitedifference approach is demonstrated for cases where sound scattering occurs. We show the ability of such a simulation to account for both sound scattering and finitesize effects. Those two numerical techniques are compared and their validity is investigated.


Random multiple scattering of ultrasound. II. Is time reversal a selfaveraging process? Derode, A., A. Tourin, and M. Fink Physical Review E  Statistical, Nonlinear, and Soft Matter Physics 64, no. 3 II, 366061366113 (2001)
Résumé: The statistical moments of ultrasonic waves transmitted through a disordered medium with resonant multiple scattering were investigated. An ultrasonic pulsed wave was transmitted from a point source to a 128element receiving array through twodimensional samples with various thickness. The results show that a robust estimation on a single realization of disorder is obtained using wide frequency bandwidth.
Motsclés: Acoustic wave propagation; Backscattering; Bandwidth; Integral equations; Interfaces (computer); Mathematical models; Piezoelectric devices; Steel; Ultrasonic scattering; Ultrasonic pulsed waves; Ultrasonic waves


Random multiple scattering of ultrasound. I. Coherent and ballistic waves Derode, A., A. Tourin, and M. Fink Physical Review E  Statistical, Nonlinear, and Soft Matter Physics 64, no. 3 II, 366051366057 (2001)
Résumé: Random multiple scattering of ultrasound was analyzed using statistical approach techniques. An ultrasonic pulsed wave transmitted from a point source to a 128element receiving array was studied through two dimensional samples with various thickness. It is found that the transmitted wave forms exhibits a timedependent frequency spectrum. The results show that the secondary wave forms on the coherent wave form is produced due to the presence of elastic resonance in the given frequency bandwidth.
Motsclés: Backscattering; Ballistics; Coherent light; Integral equations; Mathematical models; Ultrasonic scattering; Ballistic waves; Resonant multiple scattering; Ultrasonic waves


Exact symmetries of electron Bloch states and optical selection rules in [001] GaAs/AlAs quantum wells and superlattices Tronc, P., and Y. E. Kitaev Physical Review B  Condensed Matter and Materials Physics 63, no. 20, 20532612053267 (2001)
Résumé: We determined the exact symmetries of conduction and valence Bloch states in typeI and typeII [001] (GaAs)m(AlAs)n superlattices at the Γ point and at some other symmetry points of the Brillouin zone of the superlattices and derived optical selection rules. Contrary to a result widely accepted in the envelopefunction approximation (EFA), pz atomic orbitals cannot mix with px and py orbitals to build Bloch states. The phononassisted transitions involving the Γ point as an initial or final state are allowed both without and with taking into account the spinorbit interaction whatever are the symmetries of the initial and final states. The electron band structure of the superlattices is discussed. Within the domain of validity of EFA (i.e., for not too small values of m and n), a detailed analysis of the Blochstate symmetry and selection rules is provided on imposing invariance of the superlattice structure under the change of z to z (the σz symmetry operation). It is shown that optical transitions between the conduction states arising from the Γ states of GaAs on one hand and the conduction states arising from the X states of AlAs on the other hand can be allowed from spinorbit coupling only. The correspondence is provided between the symmetry of a Bloch state and the parity with respect to σz of its associated envelope function. The effect of an electric field parallel to the growth axis is discussed. Quantum wells do not differ from superlattices with regard to Blochstate and envelopefunction symmetries or optical selection rules. All the above results are still valid for any pseudomorphic superlattice or quantum well made of two binary compounds with zincblend structure and identical cations or anions, such as, for example, in the GaN/AlN system.
Motsclés: aluminum; arsenic; gallium; article; conductance; electric field; electron; mathematical analysis; quantum theory; semiconductor


Optical measurement of large transient mechanical displacements Barrière, C., and D. Royer Applied Physics Letters 79, no. 6, 878880 (2001)
Résumé: A simple method suitable for extracting large mechanical displacements from the phase modulation of an optical beam reflected from the moving surface is presented. In the MHz range, transient displacements larger than 1 μm have been measured with a standard heterodyne interferometer. © 2001 American Institute of Physics.


Mixed matrix formulation for the analysis of lasergenerated acoustic waves by a thermoelastic line source Royer, D. Ultrasonics 39, no. 5, 345354 (2001)
Résumé: An analytical model has been developed for the generation of bulk and surface acoustic waves at the surface of an isotropic solid by an infinite thermoelastic laser line source. This model is based on the mixed matrix formulation. It leads to expressions in closed form for the longitudinal and transverse bulk displacements and also for the Rayleigh wave components. A comparison with other models shows that the mechanical effect of the free surface reduces the strength of the two dipoles of force equivalent to the point thermoelastic source. For Rayleigh waves, quantitative agreement has been found with a numerical model and with experiments previously carried out on a duraluminum plate. © 2001 Elsevier Science B.V.
Motsclés: Laser ultrasonics; Nondestructive evaluation; Surface acoustic waves; Thermoelastic line source; Laser applications; Mathematical models; Nondestructive examination; Surface waves; Thermoelasticity; Laser ultrasonics; Rayleigh waves; Ultrasonic waves


Optimal focusing by spatiotemporal inverse filter. I. Basic principles Tanter, M., J.F. Aubry, J. Gerber, J.L. Thomas, and M. Fink Journal of the Acoustical Society of America 110, no. 1, 3747 (2001)
Résumé: A focusing technique based on the inversion of the propagation operator relating an array of transducers to a set of control points inside a medium was proposed in previous work [Tanter et al., J. Acoust. Soc. Am. 108, 223234 (2000)] and is extended here to the time domain. As the inversion of the propagation operator is achieved both in space and time, this technique allows calculation of the set of temporal signals to be emitted by each element of the array in order to optimally focus on a chosen control point. This broadband inversion process takes advantage of the singularvalue decomposition of the propagation operator in the Fourier domain. The physical meaning of this decomposition is explained in a homogeneous medium. In particular, a definition of the number of degrees of freedom necessary to define the acoustic field generated by an array of limited aperture in a focal plane of limited extent is given. This number corresponds to the number of independent signals that can be created in the focal area both in space and time. In this paper, this broadband inversefocusing technique is compared in homogeneous media with the classical focusing achieved by simple geometrical considerations but also with timereversal focusing. It is shown that, even in a simple medium, slight differences appear between these three focusing strategies. In the companion paper [Aubry et al., J. Acoust. Soc. Am. 1107 4858 (2001)] the three focusing techniques are compared in heterogeneous, absorbing, or complex media where classical focusing is strongly degraded. The strong improvement achieved by the spatiotemporal inversefilter technique emphasizes the great potential of multiplechannel systems having the ability to apply completely different signal waveforms on each transducer of the array. The application of this focusing technique could be of great interest in various ultrasonic fields such as medical imaging, nondestructive testing, and underwater acoustics. © 2001 Acoustical Society of America.
Motsclés: Acoustic arrays; Acoustic fields; Decomposition; Degrees of freedom (mechanics); Transducers; Ultrasonics; Waveform analysis; Spatiotemporal inverse filters; Acoustic transducers; filter; article; comparative study; frequency modulation; priority journal; signal processing; simulation; sound transmission; spatial frequency discrimination; ultrasound transducer; Echoencephalography; Filtration; Fourier Analysis; Humans; Phantoms, Imaging; Transducers; Ultrasonography


Optimal focusing by spatiotemporal inverse filter. II. Experiments. Application to focusing through absorbing and reverberating media Aubry, J.F., M. Tanter, J. Gerber, J.L. Thomas, and M. Fink Journal of the Acoustical Society of America 110, no. 1, 4858 (2001)
Résumé: To focus ultrasonic waves in an unknown heterogeneous medium using a phased array, one has to calculate the optimal set of signals to be applied on the transducers of the array. (In most applications of ultrasound, medical imaging, medical therapy, nondestructive testing, the first step consists of focusing a broadband ultrasound beam deeply inside the medium to be investigated.) Focusing in a homogeneous medium simply requires to compensate for the varying focusarray elements geometrical distances. Nevertheless, heterogeneities in the medium, in terms of speed of sound, density, or absorption, may strongly degrade the focusing. Different techniques have been developed in order to correct such aberrations induced by heterogeneous media (time reversal, speckle brightness, for example). In the companion to this paper, a new broadband focusing technique was investigated: the spatiotemporal inverse filter. Experimental results obtained in various media, such as reverberating and absorbing media, are presented here. In particular, intraplate echoes suppression and highquality focusing through a human skull, as well as hyperresolution in a reverberating medium, will be shown. It is important to notice that all these experiments were performed with fully programmable multichannel electronics whose use is required to fully exploit the spatiotemporal technique. © 2001 Acoustical Society of America.
Motsclés: Absorption; Acoustic arrays; Echo suppression; Reverberation; Ultrasonic waves; Spatiotemporal inverse filters; Acoustic transducers; filter; amplifier; article; density; electronics; priority journal; signal processing; sound transmission; spatial frequency discrimination; ultrasound transducer; Echoencephalography; Filtration; Fourier Analysis; Humans; Phantoms, Imaging; Software; Transducers; Ultrasonic Therapy; Ultrasonography


Multiple scattering in a reflecting cavity: Application to fish counting in a tank De Rosny, J., and P. Roux Journal of the Acoustical Society of America 109, no. 6, 25872597 (2001)
Résumé: Classical fisheries acoustics techniques are useless in the presence of multiple scattering or reflecting boundaries. A general technique is developed that provides the number and the scattering strength of scatterers in motion placed inside a highly reflecting cavity. This approach is based on multiple scattering theory. The idea is to measure the average effect of the scatterers on the acoustic echoes of the cavity interfaces. This leads to the measure of the scattering mean free path, a typical length that characterizes the scattering strength of the cloud of scatterers. Numerical results are shown to agree with a simple theoretical analysis. Experiments are performed with fish in a tank at two different scales: ultrasonic frequency (400 kHz) in a 1.41 beaker with 1cmlong fish as well as fisheries acoustics frequency (12.8 kHz) in a 30m3 tank with 35cmlong fish. These results have interesting applications to fish target strength measurement and fish counting in aquaculture. © 2001 Acoustical Society of America.
Motsclés: Acoustics; Aquaculture; Fisheries; Reflection; Tanks (containers); Mean free path; Reflecting cavity; Scattering; water; acoustic method; fishery survey; scattering; acoustics; article; calculation; fish; mathematical analysis; motion; nonhuman; priority journal; sound transmission; strength; theory; ultrasound; Animals; Fisheries; Fishes; Mathematics; Models, Theoretical; Scattering, Radiation; Temperature; Time Factors; Ultrasonics


Impedance matching and acoustic absorption in granular layers of silica aerogels Forest, L., V. Gibiat, and A. Hooley Journal of NonCrystalline Solids 285, no. 13, 230235 (2001)
Résumé: Experimental results on the acoustic properties of silica aerogel granules of sizes 80 μm and 3.5 mm are presented. An acoustic absorber made of a twolayer granular aerogel structure, an impedance matching layer and an absorbing one, has been designed. The reflection coefficient, the attenuation, and the sound velocity of the granular aerogels have been measured as a function of frequency (between 20 and 2500 Hz) and compared with those of a glass wool sample. It has been found that the acoustic transmission losses in the aerogel absorber are at least 10 dB higher than in the glass wool sample of the same thickness (4 cm) for the frequency range 3001700 Hz. © 2001 Elsevier Science B.V.
Motsclés: Acoustic wave absorption; Acoustic wave reflection; Acoustic wave transmission; Attenuation; Grain size and shape; Impedance matching (acoustic); Silica; Granular layers; Silica aerogels; Aerogels


A study of the secular equation for Rayleigh waves using the root locus method Royer, D. Ultrasonics 39, no. 3, 223225 (2001)
Résumé: In a recent note Rahman and Barber [Trans. ASME 62 (1995) 250] proposed an exact expression for the roots of the secular equation for the Rayleigh waves in an isotropic halfspace. Using the root locus method, we describe in a very simpler manner the evolution of the roots versus the Poisson's ratio and we derive easily the critical value for which the nature of the roots changes. © 2001 Elsevier Science B.V.
Motsclés: Rayleigh wave equation; Root locus; Elastic moduli; Polynomials; Transfer functions; Rayleigh waves; Ultrasonic waves


Ultrasonic imaging using spatiotemporal matched field (STMF) processingapplications to liquid and solid waveguides Ing, R. K., and M. Fink IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 48, no. 2, 374386 (2001)
Résumé: This paper is devoted to imaging defects in liquid and solid ultrasonic waveguides. A new ultrasonic imaging technique, based on the spatiotemporal Green functions computation and crosscorrelation, is presented. This technique extends the concept of matched field processing (MFP) used in ocean acoustics. Results of experiments conducted in water and in a solid Duralumin bar show that a strong improvement of the spatial resolution is observed with this MFP.
Motsclés: Approximation theory; Computer simulation; Green's function; Oceanography; Ultrasonic transmission; Ultrasonic waves; Waveguides; Duralumin bar; Ocean acoustics; Spatiotemporal matched field processing; Ultrasonic imaging


Fluorine incorporation in plasmapolymerized octofluorocyclobutane, hexafluoropropylene and trifluoroethylene Sandrin, L., M. S. Silverstein, and E. Sacher Polymer 42, no. 8, 37613769 (2001)
Résumé: The need for increased signal transmission speed and device density in the next generation of multilevel integrated circuits (ICs) places stringent demands on materials performance. There is a requirement for interlayer dielectrics with permittivities under 3 (low κ dielectrics) that have compatibility with copper and copper processing. Plasma polymerization is a solventfree, room temperature process that can be used to rapidly deposit thin polymer films on a wide variety of substrates. This paper describes the deposition of plasma polymers from several fluorinated monomers (octofluorocyclobutane (OFCB), hexafluoropropylene (HFP) and trifluoroethylene (TrFE)), and evaluates their molecular structures. Films with relatively high F/C ratios were investigated in detail. The refractive index, n, of plasmapolymerized OFCB (PPOFCB), 1.37 at a wavelength of 900 nm, indicates that it has a high frequency permittivity (n2) of about 2.0. The plasma fluoropolymers were transparent, yellow films that adhered strongly to the substrates and were deposited at constant deposition rates that ranged from 0.03 μm/min for PPOFCB to 0.34 μm/min for PPHFP. The AFMdetermined roughness of PPOFCB on copper is 0.46 nm, half the 0.97 nm roughness of the substrate. The significantly rougher PPTrFE and PPHFP consist of spherical particles from predominantly gas phase polymerizations. The incorporation of fluorine in the polymer is greater and more efficient for PPOFCB and PPHFP than for PPTrFE. For PPOFCB, F/C increases with decreasing W/Fm (where W is the plasma power and Fm is the mass flow rate) and, in a less sensitive manner, with increasing pressure. A typical PPOFCB has an F/C of approximately 1.5 and approximately 1.5% oxygen resulting from the reaction of long lived radicals in the plasma polymer with atmospheric oxygen. PPOFCB and PPHFP have similar molecular structures, consisting of random assemblies of fluorinated carbon groups. CF2 groups are more prevalent in PPOFCB, reflecting the monomer structure and the low W/Fm. CF groups and unsaturation are more prevalent in PPHFP, reflecting the monomer structure and the high W/Fm. © 2001 Elsevier Science Ltd.
Motsclés: Fluoropolymer; Low κ dielectric; Plasma polymerization; copper; fluorine; hexafluoropropylene; octofluorocyclobutane; oxygen; polymer; solvent; trifluoroethylene; unclassified drug; article; crystal structure; dielectric constant; film; integrated circuit; polymerization; pressure; refraction index; surface property; temperature; Dielectric materials; Molecular structure; Monomers; Permittivity; Plasma polymerization; Refractive index; Sensitivity analysis; Substrates; Surface roughness; Hexafl


Nanometer scale apertureless near field microscopy Grésillon, S., S. Ducourtieux, A. Lahrech, L. Aigouy, J. C. Rivoal, and A. C. Boccara Applied Surface Science 164, no. 14, 118123 (2000)
Résumé: It is necessary to use the information contained in the near field to get subwavelength details in optical imaging which are not revealed through the farfield image. We have designed and built various setups able to perform nearfield measurements in the UV, visible and IR, both in transmission, reflection and dark field with a resolution of 10 nm, independent of the wavelength but related to the tip size. Images revealing local dielectric contrasts, small particle effects, as well as local field enhancements in random structures, are shown. © 2000 Published by Elsevier Science B.V.
Motsclés: Field enhancement; Local properties; Near field


Percolation and fractal composites: Optical studies Ducourtieux, S., S. Grésillon, A. C. Boccara, J. C. Rivoal, X. Quelin, P. Gadenne, V. P. Drachev, W. D. Bragg, V. P. Safonov, V. A. Podolskiy, Z. C. Ying, R. L. Armstrong, and V. M. Shalaev Journal of Nonlinear Optical Physics and Materials 9, no. 1, 105116 (2000)
Résumé: Local field distributions arc studied in random metaldielectric films near percolation (percolation films) and fractal aggregates of colloidal particles. For both systems, it is shown that optical excitations are localized in small nanometerscale areas, "hot spots," where the local fields are much larger than the field of an incident electromagnetic wave. The large local fields result in giant enhancement of various optical phenomena. The surfaceenhanced whitelight generation and secondharmonic generation have been obtained in percolation films. For fractal aggregates of silver particles, a giant effect of local optical activity has been observed. The effect is due to surfaceplasmon excitations localized on chiralactive particle configurations in fractals.


Multiple scattering of sound Tourin, A., M. Fink, and A. Derode Waves Random Media 10, no. 4, R31R60 (2000)
Résumé: We present a topical review which summarizes the main contributions to `multiple scattering of acoustic and elastic waves' including the most recent advances.The review is divided into five main parts.In the first part, the effects of multiple scattering on ultrasonic propagation are illustrated on the basis of three experimental examples.In the second and third parts, we present the two possible descriptions for the propagation of an acoustic wave in a random medium. The first one is based on the study of the coherent wave, i.e. the wave amplitude averaged over disorder, whereas the second one deals with the propagation of the incoherent intensity, i.e. the intensity averaged over disorder. We especially insist on the microscopic basis for the phenomenological radiative transfer equation and show how it can be solved in the diffusion approximation. The theory is illustrated with experimental results obtained on a twodimensional multiplescattering prototype made of thousands of steel rods randomly distributed and immersed in water. In the fourth part, we present experimental evidence that the diffusion equation fails in describing all the aspects of the propagation of an acoustic wave in a random medium: e.g. the coherent backscattering effect recently observed for ultrasonic waves. We show that this effect arises as a consequence of reciprocity. Finally, in the fifth part, we discuss another property which is not taken into account in the radiative transfer theory: the reversibility of an acoustic wave propagating in a disordered medium.
Motsclés: Acoustic wave backscattering; Approximation theory; Diffusion; Mathematical models; Ultrasonic propagation; Coherent backscattering effects; Radiative transfer equation; Acoustic wave scattering


Experimental and theoretical waveforms of Rayleigh waves generated by a thermoelastic laser line source Royer, D., and C. Chenu Ultrasonics 38, no. 9, 891895 (2000)
Résumé: An analytical model has been developed for the generation of surface acoustic (Rayleigh) waves in an isotropic solid by a thermoelastic laser line source. For a Gaussian light intensity profile, this model leads to an expression in closed form for the normal surface displacement of the Rayleigh wave either in the near field or in the far field domain. Quantitative agreement has been found for experiments carried out with an interferometric optical probe on a duraluminum plate.
Motsclés: Laser beam effects; Mathematical models; Nondestructive examination; Photoacoustic effect; Surface waves; Waveform analysis; Gaussian light; Laser ultrasonics; Rayleigh waves; Thermoelastic line sources; Ultrasonic propagation; acoustics; article; laser; theoretical model; ultrasound; Acoustics; Lasers; Models, Theoretical; Ultrasonics


Transport parameters for an ultrasonic pulsed wave propagating in a multiple scattering medium Tourin, A., A. Derode, A. Peyre, and M. Fink Journal of the Acoustical Society of America 108, no. 2, 503512 (2000)
Résumé: A set of ultrasonic experimental methods was developed to characterize a multiple scattering medium in terms of l(s), l*, l(a), respectively, the elastic, transport, and absorption mean free paths and D the diffusion constant. Actually, these quantities are the key parameters for a wave propagating in a disordered medium. Although they are widely used in optics, they are less common in acoustics. The underlying model is based on the expansion of the average solution for the heterogeneous Green's function equation. To validate this theoretical approach, a sample made of randomly located steel rods was used as a prototype. Through timeresolved measurements of the transmitted amplitude, the difference between the ballistic and the coherent wave is highlighted. In varying the sample thickness, l(s) is determined, the coherent and diffusive regime are distinguished, and the transition from one to the other is followed. Furthermore, as a limit to a description of the average intensity based on the diffusion approximation, the existence of a coherent backscattering effect is shown. This latter gives a method to estimate D and l*. These quantities being determined, it becomes possible to infer l(a) using average timeresolved intensity measurements. Finally, some applications to coarsegrain stainless steels are discussed. (C) 2000 Acoustical Society of America.
Motsclés: acoustics; article; calculation; mathematical analysis; priority journal; sound; theory; ultrasound


Time reversal and the inverse filter Tanter, M., J.L. Thomas, and M. Fink Journal of the Acoustical Society of America 108, no. 1, 223234 (2000)
Résumé: To focus ultrasonic waves in an unknown inhomogeneous medium using a phased array, one has to calculate the optimal set of signals to be applied on the transducers of the array. In the case of timereversal mirrors, one assumes that a source is available at the focus, providing the Green's function of this point. In this paper, the robustness of this timereversal method is investigated when loss of information breaks the timereversal invariance. It arises in dissipative media or when the field radiated by the source is not entirely measured by the limited aperture of a timereversal mirror. However, in both cases, linearity and reciprocity relations ensure time reversal to achieve a spatiotemporal matched filtering. Nevertheless, though it provides robustness to this method, no constraints are imposed on the field out of the focus and sidelobes may appear. Another approach consists of measuring the Green's functions associated to the focus but also to neighboring points. Thus, the whole information characterizing the medium is known and the inverse source problem can be solved. A matrix formalism of the propagation operator is introduced to compare the timereversal and inverse filter techniques. Moreover, experiments investigated in various media are presented to illustrate this comparison. (C) 2000 Acoustical Society of America.
Motsclés: Fourier transformation; priority journal; review; time; transducer; ultrasound; Humans; Models, Theoretical; Skull; Time


Period doubling occurences in wind instruments musical performance Gibiat, V., and M. Castellengo Acustica 86, no. 4, 746754 (2000)
Résumé: As a special class of Non Linear Dynamical Systems, self sustained musical instruments can exhibit peculiar behaviours related to bifurcation and chaos theories. While many quasi periodic occurrences in woodwinds and bowed strings have been reported, only a few period doubling scenarios have been observed in musical instruments, and most of the time they have been obtained through numerical simulations or specially designed hybrid experimental setups far from musical situations. We present here a set of experimental results, all related to this period doubling scenario and always in direct connection with musical performance on wind instruments and voice. We show that at least three perioddoublings can be obtained on a trombone, that the crumhorn and the bassoon can oscillate in the same way and that traditional singers, and in some cases classical singers, exhibit phenomena clearly linked with this scenario. Finally, from the whole set of experimental results available for this particular set of multiphonic sounds we give some hints for a general schema governing their musical production.
Motsclés: Bifurcation (mathematics); Chaos theory; Computer simulation; Dynamics; Musical performance; Wind instruments; Musical instruments


Limits of timereversal focusing through multiple scattering: Longrange correlation Derode, A., A. Tourin, and M. Fink Journal of the Acoustical Society of America 107, no. 6, 29872998 (2000)
Résumé: Experimental results of timereversal focusing in a highorder multiple scattering medium are presented and compared to theoretical predictions based on a statistical model. The medium consists of a random collection of parallel steel rods. An ultrasonic source (3.2 MHz) transmits a pulse that undergoes multiple scattering and is recorded on an array. The timereversed waves are sent by the array back to the source through the scattering medium. The quality of temporal focusing is very well predicted by a simple statistical model. However, for thicker samples, persistent temporal side lobes appear. We interpret these sidelobes as a consequence of the growing number of crossing paths in the sample due to highorder multiple scattering. As to spatial focusing, the resolution is practically independent from the array's aperture. With a 16element array, the resolution was found to be 30 times finer than in a homogeneous medium. Resolutions of the order of the wavelength (0.5 mm) were attained. These results are discussed in relation with the statistical properties of timereversal mirrors in a random medium. (C) 2000 Acoustical Society of America.
Motsclés: acoustics; article; priority journal; signal noise ratio; signal transduction; sound transmission; statistical analysis; thickness


Time reversal in a waveguide: Study of the temporal and spatial focusing Roux, P., and M. Fink Journal of the Acoustical Society of America 107, no. 5 I, 24182429 (2000)
Résumé: Temporal and spatial focusing properties of timereversal mirrors (TRMs) are studied in a waveguide. The experiments are done using an ultrasonic TRM in an idealized waveguide. The width of the focal spot, and the spatial and temporal sidelobe levels are experimentally and numerically analyzed with respect to the characteristics of the waveguideTRM system. An algorithm is developed to compute directly in the time domain the timereversed field. This algorithm is based on the application of the mirror theorem to both the source and the TRM placed in the waveguide. Because time reversal is a stable and robust process, some of the ultrasonic results can be extended to ocean acoustics. Applications to underwater acoustic transmissions as well as ultrasonic medical imaging are discussed. (C) 2000 Acoustical Society of America.
Motsclés: acoustics; algorithm; amplitude modulation; article; imaging; mathematical computing; priority journal; sound transmission; ultrasound


Thermal characterization of filmonsubstrate systems with modulated thermoreflectance microscopy Li, B., L. Pottier, J. P. Roger, D. Fournier, and E. Welsch Review of Scientific Instruments 71, no. 5, 21542160 (2000)
Résumé: Computer simulations are performed in this article to show the feasibility of simultaneous determination of the film diffusivity, the substrate diffusivity, and the thermal boundary resistance of filmonsubstrate systems by modulated thermoreflectance microscopy and multiparameter fitting. The dependences of the phase on the probetopump beam separation, measured at four different modulation frequencies, are simultaneously fitted to an appropriate thermal diffusion model to extract the three thermal parameters. The selection of the optimal frequency combination is analyzed. Three samples: an 80 nm gold film on LaAlO3 and diamond substrates, and a 300 nm YBaCuO film on LaAlO3 substrate, are simulated. Experimental results are also presented to discuss the influence of the shape and irregularity of the beam on the fitting. The simulation results show that the statistical mean values of the three thermal parameters are very close to the input values, and the statistical errors of the film diffusivity and substrate diffusivity are comparable to the overall experimental error. However, the error of the thermal boundary resistance depends largely on the absolute thermal resistance value and the diffusivity difference between the film and the substrate. Under typical experiment conditions, the measurement errors of the two diffusivities are ∼5%, and error of the thermal boundary resistance is 10%20%. © 2000 American Institute of Physics.


Direct observation of locally enhanced electromagnetic field Gadenne, P., X. Quelin, S. Ducourtieux, Samuel Gresillon, L. Aigouy, J.C. Rivoal, V. Shalaev, and A. Sarychev Physica B: Condensed Matter 279, no. 13, 5255 (2000)
Résumé: Surface enhanced Raman scattering and other nonlinear enhanced optical effects are well known to be induced by the surface of discontinuous or rough metal thin films. In the percolating range of concentration, theoretical calculations lead to locally enhanced field distributions at the surface of the films, due to huge fluctuations close to the phase transition threshold. Using a scanning nearfield optical microscope (SNOM) of extremely high lateral resolution (10 nm), we have been able to record the field distribution close to the surface of discontinuous gold films in both transmission and reflection modes. We report here the direct observations, at a scale much shorter than the wavelength, of the giant field peaks, the socalled `hot spots'. Their intensities and spatial distribution are found in good agreement with the theoretical predictions.
Motsclés: Electromagnetic fields; Gold; Light reflection; Light transmission; Optical microscopy; Percolation (solid state); Raman scattering; Thin films; Anderson localization; Scanning nearfield optical microscopes (SNOM); Surface enhanced Raman scattering (SERS); Surface plasmon modes; Metallic films


Point defect 3D mapping in thick transparent substrates Loriette, V. Journal of Optics A: Pure and Applied Optics 2, no. 2, 7076 (2000)
Résumé: In order to control the performance of optical components for interferometric gravitational wave detectors, an instrument to detect point defects inside thick transparent substrates is presented. The main feature of this metrology tool is to localize, in three dimensions, the presence of defects. A rough estimate of their sizes is also obtained by analysis of the collected signals.
Motsclés: Bubbles (in fluids); Gravity waves; Interferometers; Light scattering; Particle size analysis; Point defects; Substrates; Transparency; Gravitational wave detectors; Optical materials


Coherent Backscattering of an Elastic Wave in a Chaotic Cavity De Rosny, J., A. Tourin, and M. Fink Physical Review Letters 84, no. 8, 16931695 (2000)
Résumé: We report the first experimental evidence of coherent backscattering enhancement for transient elastic waves propagating in a twodimensional chaotic cavity. The timeintegrated squared amplitude at the point source is twice as large as at the other points around the source. Contrary to analogous optical experiments, this effect is already clearly observable on a single realization. Especially, the spatial shape of the coherent backscattering enhancement is well predicted by a generalization of the existing theory.


Application of the parabolic equation method to medical ultrasonics Roux, P., H. C. Song, M. B. Porter, and W. A. Kuperman Wave Motion 31, no. 2, 181196 (2000)
Résumé: The application of the parabolic equation (PE) method is extended to hyperthermia and imaging problems in medical ultrasonics. It is shown that the PE provides an accurate solution of the acoustic field generated by a prefocused array through an inhomogeneous medium. To investigate hyperthermia therapy, the PE is coupled with the heat equation. Simulations provide evidence that this interaction defocuses the acoustic field and degrades the efficiency of the hyperthermia treatment. We propose a procedure to maintain the focus in the zone of interest using two arrays and phaseconjugation invariance properties. For medical imaging, an algorithm is developed to move the focal spot of a linear array in any direction using a frequency shift in a timereversal mirror (TRM). Simulations demonstrate that the proposed algorithm can be applied to an inhomogeneous medium in the presence of an aberrating layer without the knowledge of the medium properties. © 2000 Elsevier Science B.V.


Revealing a lognormal cascading process in turbulent velocity statistics with wavelet analysis Arneodo, A., S. Manneville, J. F. Muzy, and S. G. Roux Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 357, no. 1760, 24152438 (1999)
Résumé: We use the continuous wavelet transform to extract a cascading process from experimental turbulent velocity signals. We mainly investigate various statistical quantities such as the singularity spectrum, the selfsimilarity kernel and spacescale correlation functions, which together provide information about the possible existence and nature of the underlying multiplicative structure. We show that, at the highest accessible Reynolds numbers, the experimental data do not allow us to distinguish various phenomenological cascade models recently proposed to account for intermittency from their lognormal approximation. In addition, we report evidence that velocity fluctuations are not scaleinvariant but possess more complex selfsimilarity properties, which are likely to depend on the Reynolds number. We comment on the possible asymptotic validity of the multifractal description.
Motsclés: Cascade models; Intermittency; Multifractals; Selfsimilarity; Turbulence; Wavelet analysis


Electronic and vibrational levels of the photochromic Mn in sillenites Beneventi, P., B. Briat, R. Capelletti, M. Gospodinov, L. Kovács, E. Mazzocchi, and A. Ruffini Radiation Effects and Defects in Solids 150, no. 14, 243247 (1999)
Résumé: Visible and high resolution FTIR spectroscopy (at 9K) and MCD (at 1.4 K) measurements are applied to study the valency and the coordination of Mn in photorefractive and photochromic sillenites (BSO, BOO, BTO and mixed BSOBMnO) single crystals. Vibrational transitions of Mn5+ in Td symmetry, due to excitation of an F mode, its overtone and combination with an A mode are detected. The crystal field spectra and MCD results strongly support the d2 configuration for Mn in Td symmetry. Vibronic replicas of the 3A2→1E electronic transition, in which ones or two vibrational modes are also excited, are reported. © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint.
Motsclés: Crystal field transitions; FTIR spectroscopy; Impurities; Magnetic circular dichroism; Sillenites; Vibrational modes; Boron compounds; Electron energy levels; Fourier transform infrared spectroscopy; Molecular vibrations; Phase transitions; Photorefractive materials; Positive ions; Single crystals; Coordination; Crystal field spectra; Electronic transition; Magnetic circular dichroism; Sillenites; Valency; Vibrational levels; Vibrational modes; Magnesium


Nanoscale observation of enhanced electromagnetic field Grésillon, S., J.C. Rivoal, P. Gadenne, X. Quélin, V. Shalaev, and A. Sarychev Physica Status Solidi (A) Applied Research 175, no. 1, 337343 (1999)
Résumé: The surface of nanosized discontinuous or rough metal thin films is able to induce Raman scattering enhanced by several orders of magnitude. This effect has been theoretically attributed to the local field distribution at the surface of the films. As for the relevant parameter in phase transitions, the fields experience here huge fluctuations, leading to localized giant peaks so called `hot spots'. Using a Scanning NearField Optical Microscope (SNOM) of extremely high lateral resolution (10 nm), we have been able to record the field distribution close to the surface of gold films. We report here the first direct observation of the hot spots with such lateral resolution. Their intensities and spatial distribution are found in good agreement with the theoretical predictions. We also have performed local spectroscopy, which shows up sharp variations at nanometric scale (much smaller than the wavelength).
Motsclés: Electromagnetic field effects; Gold; Image analysis; Image quality; Nanostructured materials; Optical microscopy; Phase transitions; Raman scattering; Surface roughness; Thin films; Nanoscale observations; Scanning nearfield optical microscopy (SNOM); Metallic films


Timereversal mirrors and rough surfaces: Experiment Roux, P., J. De Rosny, M. Fink, and J. H. Rose Journal of the Acoustical Society of America 106, no. 2, 724732 (1999)
Résumé: A novel acoustic timereversal technique has been tested for determining the surfaceheight autocorrelation function and rms height of rough surfaces. A timereversal mirror (TRM) was used to insonify periodically and 'randomly' rough surfaces of otherwise flat samples immersed in water. The standard use of the TRM is as follows: transmit an initial planar pulse, record the signals at each array element, digitally time reverse each signal, and then retransmit the timereversed signal. As expected from timereversal symmetry, one approximately recovers the incident planar pulse after the reflection of the retransmitted wave. The new technique is a simple modification of this procedure. Namely, as before, we record and time reverse the initial reflection. However, we next translate the transducer a fixed distance parallel to the surface before retransmitting. For very small displacements, little change is observed in the TRM's signal. For larger and larger translations, the TRM's signal decorrelates, i.e., it less and less resembles the incident pulse. The signal's decorrelation as a function of displacement is directly related to the autocorrelation function of the rough surface within the limits set by the system's point response function. The TRM was used both in reflection mode and in transmission mode. Samples consisted of 'randomly' rough surfaces of metal and plastic plates, as well as metal plates machined to have periodically rough surfaces. Evidence is provided that the timereversal mirror is sensitive to the surfaceheight autocorrelation and, in favorable cases, determines the autocorrelation function and rms height.
Motsclés: acoustics; article; geometry; measurement; physical parameters; priority journal; sound transmission; ultrasound


Timereversal mirrors and rough surfaces: Theory Rose, J. H., M. Bilgen, P. Roux, and M. Fink Journal of the Acoustical Society of America 106, no. 2, 716723 (1999)
Résumé: A systematic study is presented of the sensitivity of acoustic time reversal mirrors (TRMs) to errors. One result is a novel acoustic method for determining the rootmeansquare, rms, height and the surfaceheight autocorrelation function of rough surfaces. In particular, the effects of misregistering a TRM are studied with respect to a rough surface the reflected wavefield is recorded in one place, time reversed, and then rebroadcast in a second place. This displacement causes the signal to depend importantly on the surface's rms height and autocorrelation function. Simple closedform formulas are presented for determining the rms height and the normalized surfaceheight autocorrelation function for both deterministic and randomly rough surfaces. Also presented is a numerical study that illustrates the statistical uncertainty in and the spatial resolution of the proposed method. An accompanying experimental article tests the proposed method with measurements made on a number of different rough surfaces.
Motsclés: acoustics; article; measurement; physical parameters; priority journal; sound; statistical model; theory


Dynamic time reversal of randomly backscattered acoustic waves Tourin, A., A. Derode, and M. Fink Europhysics Letters 47, no. 2, 175181 (1999)
Résumé: We report the first experiments using the reversibility of a transient acoustic wave in a multiplescattering medium to simulate either a stationary or a dynamic acoustic lens. The method is based on time reversal experiments performed in a backscattering configuration. In the stationary case, we show that we take advantage of multiple scattering to focus better than with a perfect reflecting interface. In the dynamic case, we explain the refocused spot time evolution by a simple model based on the timedependent ability to recover the angular spectrum thanks to both single and multiplescattering paths.


Characterization of a large vortex using acoustic timereversal mirrors Manneville, S., A. Maurel, P. Roux, and M. Fink European Physical Journal B 9, no. 3, 545549 (1999)
Résumé: We report new results on the ultrasonic characterization of a fluid flow using an acoustic timereversal mirror (TRM). The structure of a large vortex generated by a rotating disk in a hollow cylinder is investigated both inside and below the cylinder. For meanflow characterization, the TRM is shown to be a powerful vorticity detector. Experimental timeofflight data are successfully compared to a numerical simulation of the flow and the orthoradial velocity is reconstructed using simple geometrical acoustics. Realtime measurements allow us to extract the precession motion of the vortex, providing direct, nonintrusive, and dynamical information on the flow.
Motsclés: 43.30+m underwater sound; 43.35+d ultrasonics, quantum acoustics, and physical effects of sound; 47.32y rotational flow and vorticity


Experimental observation of localized optical excitations in random metaldielectric films Grésillon, S., L. Aigouy, A. C. Boccara, J. C. Rivoal, X. Quelin, C. Desmarest, P. Gadenne, V. A. Shubin, A. K. Sarychev, and V. M. Shalaev Physical Review Letters 82, no. 22, 45204523 (1999)
Résumé: The localized optical excitations in random metaldielectric films were examined by scanning nearfield optical microscopy. The patterns of the observed surface plasmon modes localized in hot spots are attributed to Anderson electron localization and are found to agree with theoretical predictions.
Motsclés: Dielectric films; Electric fields; Electron resonance; Electron transitions; Mathematical models; Metallic films; Optical microscopy; Scanning; Semiconducting films; Anderson electron localization; Optical excitation; Plasmons; Scanning nearfield optical microscopy (SNOM); Optical films


Electric field diffraction by a semiinfinite perfectly conducting plane of small thickness: application to nearfield microscopy Cory, H., A. C. Boccara, J. C. Rivoal, and S. Grésillon Microwave and Optical Technology Letters 21, no. 3, 177183 (1999)
Résumé: A perfectly conducting half plane of finite thickness much smaller than the wavelength of the incident electromagnetic wave is investigated. The geometry of the structure is defined according to a transformation which is a particular case of the SchwarzChristoffel transformation, representing a step with a slightly concave edge instead of the usual straight one. The solutions of the partial differential equations describing the structure are given for specific ranges of the variables, and the results obtained with this method are compared to those obtained with an independent numerical method for a step with a straight edge. Good agreement is obtained between the two sets of results, except right around the edges of the steps in the two structures, due to the different shapes involved. It is shown that the width of the structure has but a slight influence on the value of the electric field, while the angle of incidence of the incoming wave has a strong influence on this value.
Motsclés: Diffraction; Electromagnetic fields; Electromagnetic wave propagation; Light polarization; Mathematical transformations; Numerical methods; Optical microscopy; Partial differential equations; Thickness measurement; Angle of incidence; Electric field diffraction; Electric field intensity; Near field microscopy; Electric fields


Transmissionmode apertureless nearfield microscope: optical and magnetooptical studies Grésillon, S., H. Cory, J. C. Rivoal, and A. C. Boccara Journal of Optics A: Pure and Applied Optics 1, no. 2, 178184 (1999)
Résumé: A new nearfield optical microscope working in transmission is presented. Lateral optical resolution less than 10 nm is obtained with a pure metallic probe. Polarization images of a metallic step confirmed the good resolution by comparison with an analytical model. We also demonstrate the capability of the microscope to obtain images with polarization effects. The good resolution is used for the observation of small gold aggregates which confirm that this microscope is able to make spectroscopic measurements of the optical effect induced by a nanometric scale particle. The polarization sensibility allows us to measure nearfield magnetooptical contrast on a multilayer sample with magnetic domains. These results are promising for magnetooptical characterization with nanometre resolution.
Motsclés: Computer simulation; Gold; Light polarization; Light transmission; Magnetic domains; Magnetooptical effects; Mathematical models; Optical resolving power; Spectroscopy; Near field optics; Near field simulation; Optical microscope; Polarization effects; Optical microscopy


Spectroscopic characterization of photorefractive CdTe: Ge Briat, B., F. Ramaz, B. Farid, K. Shcherbin, and H. J. Von Bardeleben Journal of Crystal Growth 197, no. 3, 724728 (1999)
Résumé: We report on the photoabsorption, magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) of Gedoped CdTe. Strong photoinduced absorption changes have been detected for two samples at ambient and low temperatures (down to 4.3 K). They reveal the existence of four absorption bands around 0.94, 1.10, 1.22 and 1.36 eV at 4.3 K. MCD experiments at 1.4 K allowed the tagging of different paramagnetic defects to the two bands at lower energies. EPR demonstrated that one of them is Ge+Cd. We propose that the second one is an isolated or Gerelated Acentre. A diagram showing the various transport processes is given. © 1999 Elsevier Science B.V. All rights reserved.
Motsclés: Cadmium telluride; CdTe: Ge; Germanium; Photorefractive crystals; Absorption; Binding energy; Cadmium compounds; Carrier mobility; Crystal defects; Doping (additives); Germanium; Paramagnetic resonance; Photorefractive materials; Spectroscopic analysis; Magnetic circular dichroism (MCD); Photoinduced absorption changes; Photorefractive crystals; Crystal growth


Optical transmission of ZnSe crystals grown by solid phase recrystallization Rzepka, E., J. P. Roger, P. Lemasson, and R. Triboulet Journal of Crystal Growth 197, no. 3, 480484 (1999)
Résumé: Solid phase recrystallization of CVD grown ZnSe polycrystals is achieved using thermal treatments involving different stoichiometric conditions. Relevant color changes of the crystals appear after thermal treatment and/or doping. The optical transmission is measured in the visible and near IR range. Contributions to the color changes are interpreted as due to structural modifications by using light scattering theory. On the other hand, a new absorption band appearing in the near IR at 0.85 eV seems to be correlated with native point defects associated with deviation from stoichiometry induced by thermal treatment. © 1999 Elsevier Science B.V. All rights reserved.
Motsclés: Grain boundary; Light scattering; Optical transmission; ZnSe; Crystal growth; Crystallization; Grain boundaries; Heat treatment; Infrared radiation; Light absorption; Light scattering; Light transmission; Point defects; Semiconductor doping; Semiconductor growth; Stoichiometry; Zinc selenide; Semiconducting zinc compounds


Ultrasound puts materials to the test Fink, M. Physics World 11, no. 2, 4145 (1998)


Nearfield optical imaging of light propagation in semiconductor waveguide structures Bourzeix, S., J. M. Moison, F. Mignard, F. Barthe, A. C. Boccara, C. Licoppe, B. Mersali, M. Allovon, and A. Bruno Applied Physics Letters 73, no. 8, 10351037 (1998)
Résumé: We have investigated light propagation in optical devices by nearfield scanning optical microscopy (NSOM) at the telecommunication wavelength of 1.55 μm. NSOM images obtained on the top of channel waveguides measure the mode profile perpendicular to the propagation direction and show a modulation of intensity along this direction. This modulation demonstrates the periodic variation of the mode size predicted for the propagation in small guides and marks the direction of propagation. We show that NSOM analysis can completely assess complex optical devices with subwavelength resolution: determination of the optical path, variation of the light intensity along this path, and measurement of local radiative losses. © 1998 American Institute of Physics.


Acoustic time reversal with mode conversion at a solidfluid interface Draeger, C., D. Cassereau, and M. Fink Applied Physics Letters 72, no. 13, 15671569 (1998)
Résumé: Acoustic timereversal experiments are mostly carried out in fluid media. This letter presents experiments proving the capability of a timereversal mirror to obtain simultaneous focusing of both propagation modes inside a solid. The mirror is located in a surrounding fluid and records the longitudinal and transverse wavefronts (created by a laser impact on the solid) after conversion into pressure waves at the solidfluid interface. We show that the timereversed pressure wavefronts are reconverted mostly to their original propagation mode and focus simultaneously at the location of the laser impact. © 1998 American Institute of Physics.


Optimisation of parameters for photothermal/mirage sensor design in view of in situ environmental and industrial agricultural applications Zimering, B. L., and A. C. Boccara Instrumentation Science and Technology 26, no. 23, 261279 (1998)
Résumé: We define recommendations for compact setup design for real time in situ trace gas detection based on both theoretical analysis of certain optical parameters (parallelism and collimation) and practical considerations in environmental and industrial agriculture applications. Criteria are proposed for the selection of parallel photothermal beam deflection (PTBD or mirage effect) spectroscopy, based on advantages discussed in this analysis. Furthermore, this analysis allows sensitivity optimisation and highlights local (nonremote) continuous detection applications of systems based on this principle. We illustrate these applications with recent gas phase measurements for coffee process control.
Motsclés: Agriculture; Food processing; Food products; Industrial applications; Optimization; Process control; Real time systems; Spectroscopic analysis; Trace analysis; Mirage effects; Photothermal beam deflection (PTBD) spectroscopy; Chemical sensors


Influence of boundary conditions on timereversal focusing through heterogeneous media Tanter, M., J.L. Thomas, and M. Fink Applied Physics Letters 72, no. 20, 25112513 (1998)
Résumé: This letter presents a way to overcome ultrasonic focusing degradations through strongly diffracting nondissipative layers. A first set of experiments shows that, using finite aperture transducer arrays, neither the timereversal technique, nor other focusing techniques are able to achieve proper focusing through this kind of aberrator. These experimental results show the limits of a finite aperture timereversal mirror compared to the theoretical behavior of a timereversal cavity. To simulate a timereversal cavity, totally reflecting walls are set between the timereversal mirror and the aberrator. The experiments and numerical simulations presented in this letter show that as soon as we introduce these reflecting boundaries, the timereversal focusing becomes optimal and the spatial resolution is strongly improved. © 1998 American Institute of Physics.


Optical and magnetooptical study of photorefractive germaniumdoped cadmium telluride Briat, B., K. Shcherbin, B. Farid, and F. Ramaz Optics Communications 156, no. 46, 337340 (1998)
Résumé: A photorefractive CdTe:Ge single crystal was investigated via photoabsorption at room temperature, 98 K and 4.3 K, and magnetic circular dichroism at 1.4 K. At all wavelengths, except near 1064 nm, absorption spectra suffer relatively large changes under various illuminations. Magnetic circular dichroism demonstrates unambiguously the presence of two paramagnetic defects, one of them being the ionised donor Ge+ at the Cd site. Our results further reveal the existence of four absorption components around 0.94 eV, 1.10 eV, 1.22 eV and 1.36 eV. The two extreme ones are assigned to the photoneutralisation of Ge+Cd and to the photoionisation of Ge0Cd respectively. © 1998 Elsevier Science B.V. All rights reserved.
Motsclés: Cadmium telluride; CdTe:Ge; Germanium; Photorefractive crystals; Absorption spectroscopy; Germanium; Photoionization; Semiconducting cadmium telluride; Semiconductor doping; Photoneutralisation; Photorefractive materials


Electric field intensity variation in the vicinity of a perfectly conducting conical probe: Application to nearfield microscopy Cory, H., A. C. Boccara, J. C. Rivoal, and A. Lahrech Microwave and Optical Technology Letters 18, no. 2, 120124 (1998)
Résumé: A vibrating metallic conical probe has been used in nearfield microscopic measurements to obtain optical resolutions much smaller than the wavelength. Numerical simulations of these complex problems have drawn most of the attention in the past. We propose here an analytical expression for the electric field intensity in the near region of a probe situated above the surface of a semiinfinite dielectric medium. It is given as a function of the observation point location, with the cone aperture, the dielectric medium permittivity, and the characteristics of the incident plane wave as parameters. © 1998 John Wiley & Sons, Inc.
Motsclés: Conical probe; Electromagnetics; Nearfield microscopy; Electric fields; Electromagnetism; Field emission microscopes; Numerical analysis; Simulation; Conical metallic probe; Near field microscopy; Optical sensors


Focusing and steering through absorbing and aberrating layers: Application to ultrasonic propagation through the skull Tanter, M., J.L. Thomas, and M. Fink Journal of the Acoustical Society of America 103, no. 5 I, 24032410 (1998)
Résumé: The timereversal process is applied to focus pulsed ultrasonic waves through the human skull bone. The aim here is to treat brain tumors, which are difficult to reach with classical surgery means. Such a surgical application requires precise control of the size and location of the therapeutic focal beam. The severe ultrasonic attenuation in the skull reduces the efficiency of the time reversal process. Nevertheless, an improvement of the time reversal process in absorbing media has been investigated and applied to the focusing through the skull [J.L. Thomas and M. Fink, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 43, 11221129 (1996)]. Here an extension of this technique is presented in order to focus on a set of points surrounding an initial artificial source implanted in the tissue volume to treat. From the knowledge of the Green's function matched to this initial source location a new Green's function matched to various points of interest is deduced in order to treat the whole volume. In a homogeneous medium, conventional steering consists of tilting the wave front focused on the acoustical source. In a heterogeneous medium, this process is only valid for small angles or when aberrations are located in a layer close to the array. It is shown here how to extend this method to aberrating and absorbing layers, like the skull bone, located at any distance from the array of transducers.
Motsclés: article; brain surgery; human; mathematical analysis; priority journal; signal transduction; skull; skull surgery; ultrasound; Brain Neoplasms; Humans; Models, Biological; Skull; Ultrasonic Therapy; Ultrasonics


Nondestructive evaluation of polymer coatings by mirageFTIR spectroscopy: A tool for surface characterization Pandey, C. C., A. Kumar, A. C. Boccara, and D. Fournier Journal of Applied Polymer Science 67, no. 7, 12491252 (1998)
Résumé: We demonstrate the unusual applications of the recently developed mirageFourier transform infrared (FTIR) spectroscopy for nondestructive characterization of polymer coatings, especially on absorbing substrates or very thin coatings, where conventional FTIR techniques tend to fail. The mirageFTIR has been briefly described. A few specific examples have been given to demonstrate the potential of this technique as a tool for surface characterization. © 1998 John Wiley & Sons, Inc.
Motsclés: MirageFourier transform infrared spectroscopy; Nondestructive; Surface coating; Characterization; Fourier transform infrared spectroscopy; Nondestructive examination; Protective coatings; Substrates; Surfaces; MirageFourier transform infrared spectroscopy; Polymer coatings; Polymers


Time reversal in multiply scattering media Derode, A., A. Tourin, and M. Fink Ultrasonics 36, no. 15, 443447 (1998)
Résumé: The application of timereversal mirrors (TRM) to media with very highorder multiple scattering is presented. Random sets of up to 2500 steel rods are considered. When a pulsed wave traverses such a medium, it undergoes many scatterings before reaching the TRM. The resulting pressure field spreads in time, up to 300 times the initial pulse duration; it is recorded, timereversed and retransmitted through the same disordered medium. Surprisingly, the timereversed waves are found to converge to their source and recover their original waveform and duration, unlike one could have expected given the high order of multiple scattering involved and the usual sensitivity to initial conditions of timereversal processes. In addition to this, the observed resolution of the timereversed waves was greatly increased, and found to be smaller than the theoretical limit for the array's aperture. Theoretical limits of timereversed experiments are discussed. © 1998 Elsevier Science B.V.
Motsclés: Multiple scattering; Time reversal; Steel; Ultrasonic transmission; Ultrasonics; Time reversal mirrors (TRM); Ultrasonic scattering


Evolution of the acoustical properties of silica alcogels during their formation Forest, L., V. Gibiat, and T. Woignier Ultrasonics 36, no. 15, 477481 (1998)
Résumé: Silica aerogels are materials of very high porosity obtained through a solgel process. Since their structural properties greatly influence their acoustical and mechanical properties, it is important to follow how the different chemical reactions occur in this process. We present a study of this evolution for two different types of alcogels through acoustical measurements in an ultrasonic range. These samples have been chosen because of the significantly different behaviour of aerogels which have the same structure. The data obtained during the gelation show fluctuations of velocity that can be interpreted as chemical changes. These chemical changes give information about the evolution and the kind of structures one can expect from different catalyses. © 1998 Elsevier Science B.V.
Motsclés: Chemical changes; Gelation mechanism; Porous media; Structure characterization; Acoustic properties; Aerogels; Chemical reactions; Silica; Solgels; Silica alcogels; Ultrasonic testing


Correlation length of ultrasonic speckle in anisotropic random media: Application to coherent echo detection Derode, A., and M. Fink Journal of the Acoustical Society of America 103, no. 1, 7382 (1998)
Résumé: In a recent paper [Derode and Fink, J. Acoust. Soc. Am. 101, 690704 (1997)], a technique for studying spatial coherence properties of backscattered speckle noise by means of a transducers array was described. In this paper another approach is presented. Here, a single transducer, focusing inside a scattering medium, is moved in a plane; correlation is studied between signals acquired in two different positions. A study investigates how the correlation function of the medium determines that of the speckle noise This is applied to fully or partially incoherent media, and media with anisotropic scatterer distribution such as unidirectional and crossply composites; we show that it is possible to extract information about the medium from a measurement of the backscattered field correlation length. The influence of correlation length on defect detection is discussed. Experimental as well as theoretical results are presented, they are found in good agreement.
Motsclés: anisotropy; article; correlation function; echography; fourier transformation; noise; priority journal; sound detection; sound transmission; ultrasound; ultrasound transducer


Aberration correction in ultrasonic medical imaging with timereversal techniques Fink, M., and C. Dorme International Journal of Imaging Systems and Technology 8, no. 1, 110125 (1997)
Résumé: Degradation of image quality is currently observed with ultrasonic scanners owing to distortion of the ultrasonic beams through inhomogeneous tissue layers. Adaptive timedelay focusing techniques allow an efficient correction of these effects when the inhomogeneous layer is close to the transducer array. If the aberrating layers are far from the array, these techniques are no longer appropriate to correct the propagation effects between the layer and the transducer array. In this article we show that timereversal mirrors can solve this problem. In the matchedfilter approach that extends the concept of time reversal mirrors, the Green's function of a dominant scatterer available in the medium is recorded in digital memory and used to focus on the scatterer in both transmit and receive modes. An extension of this technique is also presented to focus, in the presence of an aberrating layer, not only on the dominant scatterer, but also around it to image the surrounding zone. From the knowledge of the Green's function needed to focus on the initial scatterer, new Green's functions matched to the new point of interest are calculated. The algorithm uses the concept of timereversal propagation, and theoretical and experimental results obtained with this technique are presented. The calculation of each Green's function matched to each new desired focal point allows us to realize a Bscan image of the zone surrounding the reflector. © 1997 John Wiley & Sons, Inc.
Motsclés: Aberration correction; Beam forming; Matched filter; Medical imaging; Time reversal; Ultrasound; Aberrations; Algorithms; Green's function; Image quality; Medical imaging; Mirrors; Aberration correction; Beam forming; Matched filter; Time reversal mirrors; Ultrasonic imaging


Theoretical and experimental investigations of lateral modes in 13 piezocomposites Certon, D., O. Casula, F. Patat, and D. Royer IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 44, no. 3, 643651 (1997)
Résumé: Materials with a periodic microstructure show resonances caused by the elastic wave Bragg diffraction. This paper presents a simple approach to describe these resonances (called lateral resonances) in 13 piezoelectric composite materials which have a 2D periodicity. Our model is based on the analysis of the propagation of transverse waves in a 2D periodic medium of infinite thickness and takes into account the periodic and interfacial boundary conditions. This model predicts the displacement field vectors and frequencies of lateral resonances from which the phase velocity of lateral waves is determined. The theoretical and experimental variations of this velocity versus the ceramic rod width to pitch ratio are compared. It is shown that the first lateral mode frequency is maximum when the ceramic volume fraction is around 0.65. Theoretical predictions of the mechanical displacement at the composite surface are compared with measurements obtained by an interferometric laser technique. A good agreement is observed, showing that lateral waves are mainly vertically polarized. © 1997 IEEE.


Measurement of thermal diffusivity of C60 fullerenes by microscopic infrared radiometry Pandey, G. C., A. C. Boccara, and D. Fournier Fullerene Science and Technology 5, no. 5, 10671074 (1997)
Résumé: Microscopic I.R. radiometry has been used to determine thermal diffusivity of C60 fullerenes both for crystalline and powder (as compact disc) samples. The data have been found to be comparable with those obtained by conventional laser flash method and analysed in the light of microstructural variations.
Motsclés: Crystalline materials; Infrared radiation; Microscopic examination; Powders; Radiometry; Thermal diffusion in solids; Microscopic infrared radiometry; Fullerenes


Multiple receiver wind profiling techniques for the boundary layer in the presence of hydrometeors Van Baelen, J., L. Hirsch, and C. Prada Geophysical Research Letters 24, no. 21, 26672670 (1997)
Résumé: A UHF boundary layer wind profiler has been operated in a multiple receiver mode. The data sets collected are used to estimate the horizontal wind using different multiple receiver analysis techniques performed in the time and frequency domains. Those results are also compared with simultaneous Doppler beam swinging measurements. The particularity of the data sets used for this intercomparison is that they were recorded while hydrometeors were present in the boundary layer. The large mean fall speed and the broad fall velocity distribution of rain drops causes a much more rapid decay of diffraction patterns than in the case of scattering by snow flakes or micro turbulence. Therefore, analysis methods which do not account for the effect of the decay of the diffraction patterns show an overestimation of the wind velocity, and do so in a different way whether snow or rain is present. To the contrary, algorithms that account for the effect of diffraction pattern decay appear to reasonably estimate the actual horizontal wind and compare well with the Doppler measurements. Copyright 1997 by the American Geophysical Union.
Motsclés: Atmospheric turbulence; Boundary layers; Electromagnetic wave diffraction; Frequency domain analysis; Light scattering; Rain; Snow; Time domain analysis; Hydrometeors; Multiple receiver wind profiling; Wind


Study of woodwindlike systems through nonlinear differential equations. Part I. Simple geometry Barjau, A., V. Gibiat, and N. Grand Journal of the Acoustical Society of America 102, no. 5 I, 30233031 (1997)
Résumé: Idealized woodwind models assume a welllocalized nonlinearity coupled to a linear bore whose mathematical description is usually expressed through a convolution product. For the case of the simplest bore geometries, cylindrical and conical, the convolution can be transformed into a delayed differential equation. When including in it the nonlinearity, the usual concepts of nonlinear dynamical systems allow a better understanding of the system's evolution. In this paper they are applied to the cylindrical and conical geometries and some characteristics of their behaviors are analyzed at the threshold of oscillation through an analytical study. It is shown that their dynamics cannot be reduced to a finite number of degrees of freedom (effective dynamics). Finally numerical simulations reveal peculiar characteristics of the direct and inverse bifurcations involved in such simple systems for quadratic and cubic nonlinearities respectively.
Motsclés: acoustics; article; geometry; mathematical analysis; nonlinear system; priority journal


Study of woodwindlike systems through nonlinear differential equations. Part II. Real geometry Barjau, A., and V. Gibiat Journal of the Acoustical Society of America 102, no. 5 I, 30323037 (1997)
Résumé: In a previous paper [Barjau et al., J. Acoust. Soc. Am. 102, 30233031 (1997)], a differential formulation has been used to study the stability and bifurcations in geometrically simple woodwindlike systems. For general bore geometries, such differential formulation can be obtained by representing the system through a lumped equivalent mechanical model whose parameters are extracted from a real impedance curve on which a modal analysis has been performed. The present procedure is then applied to a particular case and several timedomain simulations are carried out in order to both assess the validity of the systematically developed procedure and explore the possible system behaviors far from the threshold of oscillation.
Motsclés: acoustics; article; geometry; mathematical analysis; nonlinear system; priority journal


Experimental study of a submerged fountain Maurel, A., S. Cremer, and P. Jenffer Europhysics Letters 39, no. 5, 503508 (1997)
Résumé: We present an experimental study of a vertical planar water jet impinging from below on a water/air interface. Varying the jet velocity and the cavity depth, three regimes are observed: at small velocities or high cavity depths, the jet is stable and the surface bump vertically above the jet is stationary. Increasing the velocity, the bump starts oscillating along the free surface at a welldefined frequency. This motion corresponds to the destabilization of the jet confined in the bulk of the cavity to a selfsustained oscillation regime, characteristic of the behavior of confined jets. Increasing further the velocity induces a transition to a behavior with more complex frequency spectrum. This paper presents a detailed study of the selfsustained oscillation regime. The frequency at the onset is studied as a function of the cavity depth and we show an unusual behavior of the frequency with the distance from the threshold.


Study of the birefringence by optical reflectivity in YBa2Cu3O7 and its variation with a superconducting current Durrell, J., G. Hauchecorne, J. Bok, J. Bouvier, A. C. Boccara, J. P. Contour, and J. P. Roger Physica C: Superconductivity and its Applications 282287, no. PART 2, 10251026 (1997)
Résumé: The birefringence of caxis thin films of YBa2Cu3O7 has been evaluated by the change in polarization of reflected light. It has been observed that the flow of an electric current through superconducting YBCO thin film induces an increase in the optical birefringence. That is tentatively explained by a the shift of the oxygens in the chains. The polarization and the complex refractive index have been computed as a function of the shift of the O atoms: the result is in good agreement with the experimental relative phase difference.
Motsclés: Birefringence; Electric currents; High temperature superconductors; Light polarization; Light reflection; Oxide superconductors; Refractive index; Yttrium compounds; Yttrium barium copper oxides; Superconducting films


Partial coherence of transient ultrasonic fields in anisotropic random media: Application to coherent echo detection Derode, A., and M. Fink Journal of the Acoustical Society of America 101, no. 2, 690704 (1997)
Résumé: In this paper the spatial coherence properties of the speckle noise backscattered from random media insonified by a broadband pulse are studied. The approach presented is an adaptation of the optical notion of coherence introduced by Zernike. How the statistical parameters (particularly the secondorder moment) of the medium determine those of the speckle noise is studied. This is applied to fully or partially incoherent media, and media with anisotropic scatterer distribution such as unidirectional and crossply composites; also investigated is the possibility of detecting a coherent reflector among speckle noise through measurements of coherence length. Experimental as well as theoretical results are presented. Measurements were carried out with a linear array of 128 programmable transmitters/receivers.
Motsclés: acoustics; anisotropy; article; fourier transformation; noise; priority journal; statistical parameters; transducer; ultrasound


Ultrasonic beam steering through inhomogeneous layers with a time reversal mirror Dorme, C., and M. A. Fink IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 43, no. 1, 167175 (1996)
Résumé: Adaptive time delay focusing techniques allow an efficient correction of the effects due to an inhomogeneous layer close to the transducer array. If the layer is far from the array, these techniques are no longer appropriate to correct the diffraction effects between the layer and the transducer array. This problem was overcome by the use of acoustic time reversal mirrors [1J. In this technique, the Green's function of a dominant scatterer available in the medium is recorded in digital memories and used to focus on the scatterer in both transmit and receive modes [2]. We present in this paper an extension of this technique to focus, in the presence of an aberrating layer, not only on the dominant scatterer, but also around it in order to image the surrounding zone. From the knowledge of the Green's function needed to focus on the initial scatterer, we calculate the new Green's function matched to the new point of interest. The algorithm uses the concept of time reversal propagation, and we shall present here theoretical and experimental results obtained with this technique. Finally, the knowledge of each Green's function matched to each new desired focal point allows the realization of a Bscan image of the zone surrounding the reflector. © 1996 IEEE.
Motsclés: Aberrations; Algorithms; Calculations; Data storage equipment; Focusing; Green's function; Mirrors; Ultrasonic diffraction; Ultrasonic propagation; Ultrasonic scattering; Ultrasonic transducers; Aberrating layer; Adaptive time delay focusing technique; B scan image; Inhomogeneous layers; Time reversal mirror; Transducer array; Ultrasonic beam steering; Ultrasonic reflection


Selffocusing Rayleigh wave using a time reversal mirror Ing, R. K., M. Fink, and O. Casula Applied Physics Letters 68, no. 2, 161163 (1996)
Résumé: Detection of surface and subsurface flaws is achieved using the time reversal process leading to selffocusing Rayleigh waves. The generation of surface waves is accomplished with an array of transducers coupled to the specimen with a Plexiglas wedge. The advantages of this technique are demonstrated with the detection of surface and subsurface holes of subwavelength dimensions. An optical interferometer is used to control the surface acoustic field obtained with such a time reversal process. © 1996 American Institute of Physics.
Motsclés: Acoustic arrays; Acoustic transducers; Acoustic wave propagation; Acoustic waves; Boundary conditions; Focusing; Interferometers; Mirrors; Signal detection; Surfaces; Optical interferometer; Plexiglass wedge; Rayleigh waves; Self focusing; Subsurface flaw; Surface acoustic field; Surface flaw; Time reversal; Surface waves


Beam size and collimation effects in spectroscopic ellipsometry of transparent films with optical thickness inhomogeneity El Rhaleb, H., N. Cella, J. P. Roger, D. Fournier, A. C. Boccara, and A. Zuber Thin Solid Films 288, no. 12, 125131 (1996)
Résumé: In spectroscopic ellipsometry (SE) measurements of thin films with inhomogeneous optical thickness, the spot size of the probe beam on the sample surface is a highly critical parameter in the treatment of the experimental data. SE measurements were performed on transparent samples prepared by various techniques: plasma enhanced chemical vapor deposition of diamond film, spincoated PMMA films and thermally grown SiO2 film. The first two deposition techniques can produce films with inhomogeneous optical thickness. By comparing SE measurements using different setup configurations, we demonstrate that a combination of lenses and small diaphragms is most suitable for minimizing the effects related to the optical thickness distribution and light beam divergence. In this way mistakes are avoided in extracting physical parameters from SE optical modeling of these films.
Motsclés: Diamond; Ellipsometry; Optical coatings; Polymers; Chemical vapor deposition; Computer simulation; Diamond films; Ellipsometry; Optical collimators; Optical properties; Plasma applications; Polymethyl methacrylates; Silica; Spectroscopic analysis; Thin films; Inhomogeneous optical thickness; Plasma enhanced chemical vapor deposition; Spectroscopic ellipsometry (SE); Transparent films; Optical coatings


Photothermal characterization of vertical and slanted thermal barriers: A quantitative comparison of mirage, thermoreflectance, and infrared radiometry Ocariz, A., A. SanchezLavega, A. Salazar, D. Fournier, and A. C. Boccara Journal of Applied Physics 80, no. 5, 29682982 (1996)
Résumé: We present a quantitative and comparative study of three different photothermal techniques based on the signal produced on vertical/slanted thermal barriers under a variety of barrier/sample conditions. Models based on integral methods are developed to calculate the amplitude and phase of the sample surface temperature and mirage deflection in surrounding air. Thegeometries studied include: vertical, tilted, buried, and finite size barriers separating identical or different media. The models incorporate the probe and pump beam sizes, the thermal resistance of the barrier and the optothermal characteristics of the sample. Experimental measurements are performed on a variety of fabricated barriers with three modulated photothermal techniques: the thermoreflectance, infrared radiometry, and mirage detection. We discuss in a comparative way the limits, drawbacks and he applicability of each Technique. Model fits to the experimental results allow characterization of the thermal barrier (spatial localization, geometry, orientation, and size) and provide an accurate determination of its thermal resistance. © 1996 American Institute of Physics.


A compensated optical profilometer for wavefront control of Virgo gravitational wave antenna optics Gounelle, M., V. Loriette, A. C. Boccara, and R. Nahoum Measurement Science and Technology 7, no. 7, 10321037 (1996)
Résumé: Wavefront control is crucial to the Virgo experiment. We have developed a profilometer sensitive to the sample local slope, which measures mirror profiles with nanometre sensitivity over a large frequency range, without any need for a reference surface. In order to reduce measurement errors due to atmospheric turbulence and mechanical defects, a compensation system is included as part of the bench.
Motsclés: Atmospheric turbulence; Gravity waves; Interferometers; Measurement errors; Mirrors; Surface measurement; Wavefronts; Nanometre sensitivity; Optical profilometer; Virgo antenna; Wavefront control; Optical instruments


Compact design for real time in situ atmospheric trace gas detection based on mirage effect (photothermal deflection) spectroscopy Zimering, B., and A. C. Boccara Review of Scientific Instruments 67, no. 5, 18911895 (1996)
Résumé: We present a compact and rugged setup for real time in situ trace gas detection based on photothermal deflection spectroscopy. The use of high quality Ge windows permits a true parallel geometry, resulting in a design resistant to ambient air motion while preserving the advantages of the technique such as good sensitivity, local probing, and open air measurement capability. Detection of ethylene within an enclosed sample cell has been performed in a N2 atmosphere, and we estimate the practical noise limited detection minimum to be 250 ppt, an order of magnitude improvement from a previous setup. We have measured absorption spectra of open laboratory air and real time ethylene exhaust from polymer and biological samples to investigate industrial, agricultural, and environmental applications. © 1996 American Institute of Physics.


Microscopic measurements of the local heat conduction in polycrystalline diamond films Plamann, K., D. Fournier, B. C. Forget, and A. C. Boccara Diamond and Related Materials 5, no. 68, 699705 (1996)
Résumé: It is generally known that the microstructure of polycrystalline CVD diamond samples has a strong impact on their thermal properties. Despite the fact that nowadays layers can be deposited with macroscopic thermal conductivities or diffusivities rivalling those of type II natural diamonds, the samples are highly thermally inhomogeneous and sometimes show local values differing by up to two orders of magnitude. To examine these phenomena more closely, we present a microscopic photo thermal measuring method for the local thermal diffusivity. We demonstrate the feasibility of diffusivity measurements at a sample surface of ca. 20 × 20 μm. We show results obtained on a reference sample of known diffusivity and present measurements on a small single crystal diamond, a local measurement at the substrate side of a CVD diamond layer, and a measurement of the diffusivity inside a microcrystal at the growth side of a CVD diamond layer.
Motsclés: CVD diamond; Photothermal microscopy; Thermal conductivity; Thermal diffusivity


Thermal diffusivity of anisotropic materials by photothermal methods Salazar, A., A. SánchezLavega, A. Ocáriz, J. Guitonny, G. C. Pandey, D. Fournier, and A. C. Boccara Journal of Applied Physics 79, no. 8, 39843993 (1996)
Résumé: In this paper we analyze the possibility of extending the photothermal methods developed for thermal diffusivity measurements of isotropic materials [A. Salazar and A. SánchezLavega, Rev. Sci. Instrum. 65, 2896 (1994)] to the case of anisotropic specimens. A full theoretical treatment of the photothermal signal generation for the case of an anisotropic sample with its principal axes aligned with the sample surface is presented. Three photothermal detection schemes have been studied: infrared radiometry, photothermal reflectance, and optical beam deflection (mirage effect). The fundamental result we have obtained is that when using infrared radiometry, photothermal reflectance and collinear mirage setups, the thermal magnitude retrieved is the resistivity to heat diffusion (a tensor defined as the inverse of the thermal diffusivity tensor). Only the perpendicular mirage experiment allows one to directly retrieve the thermal diffusivity along any direction of the material. Experimental measurements performed on two highly anisotropic materials confirm the abovementioned results. © 1996 American Institute of Physics.


Time reversal focusing applied to lithotripsy Thomas, J.L., F. Wu, and M. Fink Ultrasonic Imaging 18, no. 2, 106121 (1996)
Résumé: Time reversal of ultrasonic field allows a very efficient approach to focusing pulsed ultrasonic waves through lossless inhomogeneous media. Time reversal mirrors (TRM) are made of large transducer arrays, allowing the incident acoustic field to be sampled, time reversed and reemitted. Time reversal processing permits a choice of any temporal window to be time reversed, allowing operation in an iterative mode. In multitarget media, this process converges on the most reflective target, i.e., the dominant scatterer. In this paper, the time reversal process is applied to track, in real time, a moving gall bladder or kidney stone embedded in its surrounding medium. We investigate the feasibility of a piezoelectric shock wave generator in which the focal zone is moved electronically to track the stone during a lithotripsy treatment. We show that TRM allows us to obtain sharp focusing on one bright point of the stone. The time of flight profile is then determined and used in a leastmeansquare method to calculate the spatial coordinates of the stone.
Motsclés: Electronic focusing, extrapolation; lithotripsy; piezoelectric generator; stone tracking; time delay estimation; time reversal mirrors; ultrasound; article; cholelithiasis; lithotripsy; piezoelectricity; shock wave; time; ultrasound; Cholelithiasis; Humans; Kidney Calculi; Lithotripsy; Ultrasonography, Interventional


An electron paramagnetic resonance and magnetooptical study of vanadium in ZnxCd1xTe:V Von Bardeleben, H. J., C. Miesner, J. Monge, B. Briat, J. C. Launay, and X. Launay Semiconductor Science and Technology 11, no. 1, 5862 (1996)
Résumé: The vanadiumrelated defects in highresistivity Bridgmangrown ZnxCd1xTe (x = 0.04, 0.10) crystals have been studied by electron paramagnetic resonance (EPR) and magnetic circular dichroism absorption spectroscopy (MCDA). Only one vanadiumrelated spectrum is observed by EPR, which is attributed to a V2+ defect in orthorhombic point symmetry. Its spin Hamiltonian parameters are electron spin S = 3/2, Landé gtensor gxx = 1.976, gyy = 1.959, 9zz = 1.974, crystal field splitting parameters D = 0.933 cm1 and E = 0.073 cm1, nuclear spin I = 7/2, hyperfine tensor Axx = 65 × 104 cm1, Ayy = 57 × 104 cm1, Azz = 65 × 104 cm1 with the principal axes x, y, z oriented along the [110] , [001], [110] directions respectively. The MCDA measurements performed on the same crystals show in addition to the absorption bands of the V2+ ion those of the V3+ ion, a defect not observed by EPR; the nonobservation of the V3+ EPR spectrum is ascribed to a strong zerofield splitting of the 3A2 groundstate. The low point symmetry of the V2+ defect is attributed to the formation of VCdXCd second nearest neighbour complexes in the Znalloyed CdTe samples.
Motsclés: Absorption spectroscopy; Crystal defects; Crystal growth; Electric conductivity; Electrons; Ions; Magnetooptical effects; Paramagnetic resonance; Semiconducting zinc compounds; Tensors; Crystal field splitting parameters; Magnetic circular dichroism absorption spectroscopy; Orthorhombic point symmetry; Spin Hamiltonian parameters; Zinc cadmium telluride; Vanadium


Selffocusing Rayleigh wave using a time reversal mirror Ing, R. K., M. Fink, and O. Casula Applied Physics Letters, 161 (1995)
Résumé: Detection of surface and subsurface flaws is achieved using the time reversal process leading to selffocusing Rayleigh waves. The generation of surface waves is accomplished with an array of transducers coupled to the specimen with a Plexiglas wedge. The advantages of this technique are demonstrated with the detection of surface and subsurface holes of subwavelength dimensions. An optical interferometer is used to control the surface acoustic field obtained with such a time reversal process. © 1996 American Institute of Physics.


A sensitive ultrasonic method for measuring transient motions of a surface Royer, D., and O. Casula Applied Physics Letters 67, 3248 (1995)
Résumé: A method for measuring surface displacements is described. The principle is based on the detection of the phase shift of a high frequency continuous ultrasonic wave reflected from the moving surface. The analysis shows that the long time delay undergone by the ultrasonic wave produces a significant enhancement of the Doppler phase shift signal. Experiments were carried out in water with a 30MHz focused transducer probe and transient mechanical displacements smaller than 1 Å (surface velocities smaller than 1 mm/s) were detected in a 10 MHz bandwidth.© 1995 American Institute of Physics.


Test of mirror pair reflectivity coefficient equality and homogeneity for interferometric gravitational wave detectors Loriette, V. Journal of Optics 26, no. 1, 3538 (1995)
Résumé: A bench for evaluating the homogeneity of coated optical surfaces specular reflection coefficient is presented. Comparison of the local reflectivity of a sample with a reference surface leads to the determination of the relative homogeneity delta R/R of the sample. Various arrangements that allow the study of high reflectivity mirrors or beamsplitter plates are described. When testing high reflectivity mirrors a sensitivity of 6*105 on delta R/R was achieved and a similar sensitivity was obtained when comparing two mirrors.


Near field optical microscopy using a metallic vibrating tip Gleyzes, P., A. C. Boccara, and R. Bachelot Ultramicroscopy 57, no. 23, 318322 (1995)
Résumé: We present here an alternative experimental approach to achieve a local selection of a field distribution smaller than the wavelength. The sample surface is irradiated by a strongly converging beam issued from a microscope objective. A tungsten tip is used to perturb locally and periodically the electromagnetic field in the diffraction limited spot. The vibration amplitude is kept at a constant value when scanning the sample. We expect this to discard the topographical response and to retain mainly the optical response of the sample. We can use our microscope as an ordinary scanning metallographic one by monitoring the DC level of the reflected light and compare this signal with the AC signal induced by the tip oscillation on the same detector. Preliminary results obtained show a clear improvement of the resolution compared to the diffraction limited one.
We present here an alternative experimental approach to achieve a local selection of a field distribution smaller than the wavelength. The sample surface is irradiated by a strongly converging beam issued from a microscope objective. A tungsten tip is used to perturb locally and periodically the electromagnetic field in the diffraction limited spot. The vibration amplitude is kept at a constant value when scanning the sample. We expect this to discard the topographical response and to retain mainly the optical response of the sample. We can use our microscope as an ordinary scanning metallographic one by monitoring the DC level of the reflected light and compare this signal with the AC signal induced by the tip oscillation on the same detector. Preliminary results obtained show a clear improvement of the resolution compared to the diffraction limited one.
Motsclés: Detectors; Diffraction; Electromagnetic fields; Light reflection; Metallography; Microscopes; Monitoring; Optical resolving power; Scanning; Surfaces; Tungsten; Vibrations (mechanical); Metallic vibrating tip; Microscope objective; Near field optical microscopy; Optical response; Topographical response; Optical microscopy; conference paper; microscopy; scanning near field optical microscopy


The iterative time reversal process: Analysis of the convergence Prada, C., J.L. Thomas, and M. Fink Journal of the Acoustical Society of America 97, no. 1, 6271 (1995)
Résumé: The efficiency of a time reversal acoustic mirror to focus on a reflective target through an inhomogeneous media has been demonstrated. In a multitarget media, the ability of such a mirror to work in an iterative mode in order to focus selectively on the strongest target was shown [C. Prada, F. Wu, and M. Fink, J. Acoust. Soc. Am. 90, 1119 (1991)]. The theory of how the iterative time reversal process is built is based on a matrix formalism and treats the array of L transducers in a given medium as a linear system of L inputs/L outputs. The system is characterized at each frequency by its transfer matrix K and the time reversal iterative process is then described by a time reversal operator K*K. Because of the reciprocity principle, this operator is Hermitian. The following result is shown: If the scattering medium is a set of well resolved targets of different reflectivities then each eigenvector of the operator K*K with nonzero eigenvalue corresponds to one of the targets in the set and provides the optimum phase law to focus on it. Furthermore, the eigenvalue is proportional to the reflectivity of the target. In particular, the 'brightest' target is associated to the eigenvector of greatest eigenvalue so that the iterative time reversal process leads to a wave focusing on this target. This analysis is illustrated by numerical and experimental results.
Motsclés: acoustics; article; brightness; mathematical analysis; priority journal; signal transduction; sound; surface property; waveform; acoustic transducers; convergence; experimental study; iterative time reversal


Influence of initial conditions on the optical and electrical characterisation of sillenitetype crystals Hamri, A., M. Secu, V. Topa, and B. Briat Optical Materials 4, no. 23, 197201 (1995)
Résumé: We compare a set of results concerning our measurement of the optical absorption (OA), photoconductivity (PC), thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) on the same crystal. We also investigate the effects of various preillumination treatments at room or liquid nitrogen temperature. We conclude that illumination with infrared light (rather than solely red) plays an important role which has been underestimated in the past. © 1995.
Motsclés: Infrared radiation; Light absorption; Luminescence; Optical materials; Optical variables measurement; Photochromism; Photoconductivity; Sillenite; Thermally stimulated currents; Thermally stimulated luminescence; Crystals


Optical and transport measurements of YBa2Cu3O6+x thin films (0.38≤x≤1) El Azrak, A., L. A. De Vaulchier, N. Bontemps, C. Thivet, M. GuillouxViry, A. Perrin, B. Wuyts, and E. Osquiguil Physica C: Superconductivity and its applications 235240, no. PART 2, 14311432 (1994)
Résumé: We report optical reflectivity and transmission measurements at room temperature on a set of five YBa2Cu3O6+x films for 0.38≤x≤, and ρ{variant}(T) resistivity data down to 10K, prepared from a single YBa2Cu3O7 thin film. In contrast with x=1, as x decreases, the scattering rate deduced from the optical conductivity departs from its linear variation with ω, in correlation with the absence of linearity of ρ{variant}(T) versus temperature. © 1994.


Infrared properties of YBa2Cu3O7 and Bi2Sr2Can1CunO2n+4 thin films El Azrak, A., R. Nahoum, N. Bontemps, M. GuillouxViry, C. Thivet, A. Perrin, S. Labdi, Z. Z. Li, and H. Raffy Physical Review B 49, no. 14, 98469856 (1994)
Résumé: We report roomtemperature optical reflectivity and transmission measurements on a variety of YBa2Cu3O7 and Bi2Sr2Can1CunO2n+4 thin films in the 50025 000 cm1 frequency range. Reflectivity data when not biased by the substrate are used to compute the optical conductivity. A comparison of our results with published reflectivity data on singledomain YBa2Cu3O7 crystals suggests an overwhelming contribution of the CuO2 planes (with respect to the chains) to the optical conductivity. The data can be analyzed in terms of a Drude lowfrequency contribution and a socalled midinfrared band. However, the anomalous infrared response appears to be uniquely described by a generalized Drude response: for all the samples investigated, the relaxation rate (which increases linearly with frequency) and the effective mass are found to be the same as a function of frequency. © 1994 The American Physical Society.


Magnetic circular and linear dichroism saturation studies: Determination of the "crystal field" splitting in krypton and xenon matrixisolated iron atoms Rivoal, J. C., J. Zoueu, X. Blanchard, R. Nahoum, M. Eyring, M. Pyka, and M. Vala The Journal of Chemical Physics 101, no. 4, 26842692 (1994)
Résumé: The site symmetry and magnitude of the crystal field splitting of metal atoms trapped in rare gas matrices may be determined by a combination of the temperature and magnetic field dependence of magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD). General theoretical expressions are derived for the MCD and MLD considering the Zeeman effect and the matrix (i.e., crystal field) as perturbations on the free atom JM J〉 states. These expressions are applied to the observed MCD and MLD T and Bsaturation curves for Fe atoms isolated in krypton and xenon matrices in the range 01.5 T and 2.322 K. An octahedral crystal field model accounts well for all four independent sets of data (MCD vs B, MCD vs 1/T, MLD vs B 2, and MLD vs 1/Ã2) for two different, isolated electronic transitions. A crystal field parameter of +0.06±0.05 cm 1 has been determined, corresponding to an overall electronic groundstate splitting of 3.2±2.5 cm 1 . This splitting is less than the optical bandwidth and is shown to be consistent with previous studies of rare gas matrixisolated Fe atoms by Mössbauer and laserexcited emission spectroscopies. © 1994 American Institute of Physics.


Relationship between frequency and temperature dependence of the carrier scattering rate in the normal state of YBa2Cu3O7 thin films Bontemps, N., A. El Azrak, C. Thivet, M. GuillouxViry, and A. Perrin Physica C: Superconductivity and its applications 235240, no. PART 2, 14291430 (1994)
Résumé: We have measured the transmission spectra in the infrared range (10006000 cm1) of several thin films YBaCuO/MgO as a function of temperature: we show that the temperature dependence of the transmission between 90 K and 300 K is consistent with a linear variation of the carrier scattering rate with temperature. Our results appear in quantitative agreement with the marginal Fermi liquid (MFL) theory. © 1994.


Inverse scattering analysis with an acoustic timereversal mirror Thomas, J.L., P. Roux, and M. Fink Physical Review Letters 72, no. 5, 637640 (1994)
Résumé: Timereversal mirror (TRM) utilizes timereversal invariance of wave equation. Acoustic TRM are made of large transducer arrays allowing the incident acoustic field to be sampled, time reversed, and reemitted. The ability of TRM to time reverse separately each scattered wave permits us to isolate spatially and temporally the equivalent sources of extended scatterer illuminated by broadband pulse. We investigate more particulary the problem of a cylindrical scatterer: We show that TRM allows us to determine the location of the mode conversions of incident waves into circumferential waves. © 1994 The American Physical Society.


Current induced optical birefringence in superconducting YBaCuO (123) and BiSrCaCuO (2212) thin films Hauchecorne, G., A. C. Boccara, J. Bok, J. P. Contour, J. Demoment, J. P. Roger, H. Raffy, and A. Sacuto Physica C: Superconductivity and its applications 235240, no. PART 3, 20992100 (1994)
Résumé: We have observed that the flow of an electric current through superconducting YBCO and BiSrCaCuO thin films induces an increase in the optical birefringence, characteristic of a chance in the crystal symmetry. This symmetry breaking is confirmed by Raman spectroscopy and is tentatively explained by ionic displacement within the CuO2 planes and lattice distorsion. © 1994.


Giant surface acoustic waves generated by a multiple beam laser: Application to the detection of surface breaking slots Chenu, C., M.H. Noroy, and D. Royer Applied Physics Letters 65, no. 9, 10911093 (1994)
Résumé: Giant surface acoustic wave (SAW) pulses have been generated by implementing on the surface of a solid an array of strip thermoelastic sources. It is shown experimentally, with a 16beam pulsed YAG laser, that the delays introduced between the laser emission times allow the constructive summation of the SAW pulses. Mechanical displacements larger than 20 nm on a steel sample and 50 nm on aluminum were detected by a heterodyne optical interferometer. Examples of noncontact and nondestructive detection of thin surface breaking slots are presented.


Enhancement of the 1 Stark ladder transition in a superlattice: a proposal for a new electrooptical modulator with a high extinction ratio Depeyrot, J., P. Tronc, G. Wang, and J.F. Palmier Superlattices and Microstructures 15, no. 3, 297 (1994)
Résumé: We present computations of the oscillator strengths of superlattice optical transitions for an applied electric field such that the electron envelope function is localized over three wells. To calculate the electron envelope function we use a pertubative model whereas for the heavyhole one, we use variationnal calculations of the eigenstates in an isolated quantum well structure since the holes are strongly localized. Our computations show that a strong asymmetry does exist between the oscillator strengths of the 1 and the +1 optical transitions of the WannierStark ladder. Moreover, the calculated ratio of the oscillator strengths of the 1 and the 0 transitions agrees reasonably well with previous reported data. When a pertubation even with respect to z (growth direction) is added to the external electric potential, the intensity of the 1 transition increases, leading us to propose a structure for a new electrooptical modulator with a high extinction ratio. © 1994 Academic Press. All rights reserved.
Motsclés: Computational methods; Eigenvalues and eigenfunctions; Electric field effects; Electron transport properties; Electrooptical devices; Mathematical models; Modulators; Oscillators (electronic); Perturbation techniques; Semiconductor device structures; Semiconductor quantum wells; Variational techniques; Electron envelope function; Electrooptical modulator; Extinction ratio; Oscillator strengths; Stark ladder transition; Superlattices


Luminescence, absorption and magnetic circular dichroism of Pb centers in KCl crystals Topa, V., B. Briat, J. C. Rivoal, C. Pedrini, and B. Moine Journal of Luminescence 6061, no. C, 603606 (1994)
Résumé: We demonstrate that several kinds of colored T centers can be prepared by an electrical coloring of Pbdoped KCl crystals. Ta is diamagnetic and it contains predominantly the [Pb]2 species. Te is paramagnetic and we tentatively assign it to [Pb2] units perturbed by Ca2+. Oxygen centers are thought to occur in Tb. © 1994.
Motsclés: Calcium; Coloring; Cyclotron resonance; Lead; Light absorption; Light polarization; Luminescence of inorganic solids; Paramagnetic resonance; Potassium compounds; Spectroscopy; Colored T centers; Magnetic circular dichroism (MCD); Oxygen centers; Color centers


Probing dynamic processes in multilayered structures by stimulated spectroscopic ellipsometry Jin, G., J. Roger, A. C. Boccara, and J. L. Stehle Journal of Physics D: Applied Physics 26, no. 11, 20962099 (1993)
Résumé: Spectroscopic ellipsometry is a widely used technique for the determination of the optical and geometrical characteristics of complex multilayered structures. We show in this communication that photostimulated ellipsometry is able to account for the time ( or frequency) dependence of heat and carrier diffusion processes in such structures. The results of various experiments on SIMOX structures are reported.
Motsclés: Photoinduced plasma; Photoreflectance; Spectroscopic ellipsometry; Nondestructive examination; Optical variables measurement; Spectroscopic analysis; Ellipsometry


Transient elastic wave generation by an array of thermoelastic sources Noroy, M.H., D. Royer, and M. Fink Applied Physics Letters 63, no. 24, 32763278 (1993)
Résumé: A set of 16 thermoelastic strip sources, equivalent to a phased array of ultrasonic transducers has been implemented on a solid by irradiating its surface with a multiple beam pulsed YAG laser. Longitudinal elastic waves were focused in the sample by time delaying each laser pulse. Results of experiments performed on duraluminum in the thermoelastic regime are presented and compared with simulations based on the surface center of the expansion model. It is shown that with this technique, a high energy focusing, together with an ultrasonic beam scanning can be achieved in a wide angular range leading to a significant improvement in the signaltonoise ratio of a laser based ultrasonic system.


Optical spectra of a conducting polymer (polymethyl3thiophene) at several stages of the electrodeposition process Tian, C., G. Jin, F. Chao, M. Costa, and J. P. Roger Thin Solid Films 233, no. 12, 9195 (1993)
Résumé: The electrodeposition of polymethyl3thiophene on a platinum electrode was carried out potentiostatically. The variation in optical parameters was followed in situ in real time, by modulated ellipsometry at a fixed wavelength. On the contrary, exsitu spectra were determined by spectroscopic ellipsometry, for doped and undoped films, at several stages of the film growth. The structure of the film for each stage was observed by atomic force manoscopy. Nucleation, first monolayer, thin dense fibrous layers and thicker granular layers were successively studied. © 1993.


Optical, magnetooptical and EPR study of chromium impurities in Bi4Ge3O12 single crystal Moya, E., C. Zaldo, B. Briat, V. Topa, and F. J. López Journal of Physics and Chemistry of Solids 54, no. 7, 809816 (1993)
Résumé: The spectroscopic properties of Cr impurities in four Bi4Ge3O12 single crystals of different origins have been investigated in detail with three complementary techniques. Optical absorption, electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) demonstrate that Cr4+ at the tetrahedral germanium site is by far the dominant species in these crystals. Low temperature MCD is shown to be appropriate for a nondestructive evaluation of the amount of dopant. Lattice defects appear to be present in asgrown crystals. Diamagnetic centers, contributing to the nearu.v. absorption, are also formed by u.v. irradiation at 310 nm. Finally Xray irradiation at 15 K produces similar but more drastic effects on the absorption spectra. © 1993.
Motsclés: BGO/Cr; Bi4Ge3O12; bismuth germanate; electron paramagnetic resonance; lattice defects; magnetooptical effects; photoinduced changes; photorefractive materials; Chromium compounds; Doping (additives); Electron paramagnetic resonance; Electron resonance; Impurities; Magnetooptical effects; Optical properties; Bismuth germanates; Single crystals; Crystals


Optical conductivity and carrier relaxation rate in the normal state of high Tc thin films El Azrak, A., R. Nahoum, A. C. Boccara, N. Bontemps, M. GuillouxViry, C. Thivet, A. Perrin, Z. Z. Li, and H. Raffy Journal of Alloys and Compounds 195, no. C, 663666 (1993)
Résumé: We report the optical conductivity of YBa2Cu3O7 and Bi2Sr2CaCu2O8+δ thin films at room temperature. The optical conductivity in the 5007000 cm1 frequency range is analysed in terms of a frequency dependent relaxation time τ and effective mass of the normal carriers. The relaxation rate 1/τ exhibits the same linear dependence on frequency whatever the sample. The assumption that 1/τ also depends linearly on temperature accounts satisfactorily for the temperature variation (90300K) of the transmission in the 10007000 cm1 range. These results are consistent with the marginal Fermi liquid model. © 1993.
Motsclés: Barium compounds; Bismuth compounds; Calcium compounds; Ceramic materials; Copper compounds; Copper oxides; Optical properties; Oxides; Strontium compounds; Superconducting films; Thin films; Yttrium compounds; Carrier relaxation rate; High temperature thin film superconductors; Optical conductivity; High temperature superconductors


Self and mutual timedependent interaction forces between ultrasonic transducers  Application to the computation of radiation impedances Cassereau, D., and D. Guyomar Journal of the Acoustical Society of America 93, no. 1, 552558 (1993)
Résumé: In this paper, diffraction theory is used to describe the self and mutual timedependent interaction forces between ultrasonic transducers. This concept is interesting since it is equivalent, up to a temporal Fourier transform, to the radiation impedance. The formalism allows a description of self and mutual radiation impedances in terms of the aperture functions of the emitter and the receiver, and this description does not suppose that the transducers work in the piston mode. Then, the formalism is used to retrieve the wellknown results about the radiation impedance of a circular piston. Finally, the mutual radiation impedance between two transducers is analyzed, and the particular case of small transducers compared to the wavelength is discussed. The results are similar to those obtained by Stepanishen and Pritchard [R. L. Pritchard, J. Acoust. Soc. Am. 32, 730737 (1960); P. R. Stepanishen, ibid. 49, 283292, 841849 (1971)], except that the results here exhibit a correction term that increases the validity domain of the approximations.
Motsclés: article; diffraction; fourier transformation; impedance; mathematical analysis; radiation; spectral sensitivity; ultrasound transducer


Optical properties and fluctuations of composition in Ga0.77In0.23As0.19Sb0.81 alloys Tronc, P., B. Reid, R. Maciejko, J. L. Leclercq, and J. L. Lazzari Solid State Communications 85, no. 2, 177181 (1993)
Résumé: Low temperature (T < 10 K) photoluminescence experiments can be used to measure the bandgap fluctuations in a quaternary GaxIn1xAsySb1y alloy latticematched to GaSb. At x = 0.77 and y = 0.19, the bandgap variations of layers prepared by liquid phase epitaxy are fitted by a Gaussian distribution with a root mean square value of 4.2 meV. It is shown that the local fluctuations of composition and the correlative stress within the layer partially compensate to reduce the fluctuations of bandgap. © 1993.
Motsclés: Band structure; Composition; Crystal lattices; Electron energy levels; Energy gap; Epitaxial growth; Excitons; Interpolation; Low temperature effects; Optical properties; Photoluminescence; Stresses; Bandgap fluctuations; Composition fluctuations; Correlative stress; Gallium indium arsenic antimony alloys; Gaussian distributions; Liquid phase epitaxy; Semiconducting gallium compounds


Photothermal beam deflection using the reverse mirage geometry: Theory and experiment Manning, C. J., R. A. Palmer, J. L. Chao, and F. Charbonnier Journal of Applied Physics 71, no. 5, 24332440 (1992)
Résumé: This paper presents a mathematical model and supporting experimental data for the reverse mirage effect with absorption in the deflecting medium. The reverse mirage effect is defined as transverse photothermal beam deflection with the probe laser on the nonilluminated side of the interface. The model takes into account the Gaussian distribution of intensity in the probe beam, and is used to consider the phase of the photothermal beam deflection (PBD) response. The results of the mathematical treatment are shown to be in good accord with experiment. The theory shows, in agreement with Bićanić, that a plot of the natural log of the response magnitude versus probe distance is linear, and has a slope equal to the absorptivity. As an example of its application, this relationship is used to obtain absolute absorption coefficients in the midinfrared for acetonitrile from stepscan Fourier transform ir PBD data. The theory is used to show that the largest absorptivity that can be measured by this method is approximately 650 cm1. The phase is shown to exhibit monotonic behavior to a larger absorptivity, estimated to be greater than 3000 cm1. The designs of the PBD accessory and sample cells are described in some detail, and a number of experimental considerations are discussed.


Optical probing of pulsed, focused ultrasonic fields using a heterodyne interferometer Royer, D., N. Dubois, and M. Fink Applied Physics Letters 61, no. 2, 153155 (1992)
Résumé: The pulsed acoustic field of a piezoelectric focused transducer transmitted in water has been investigated by using an optical heterodyne interferometer. The probe beam is reflected by a thin membrane that follows the motion of the fluid particle. Absolute measurements of the mechanical displacement have been performed in the frequency range 515 MHz with a spatial resolution better than 0.1 mm and a sensitivity of 0.1 nm with a 20 MHz detection bandwidth that corresponds to a minimum detectable acoustic energy density of 2 μJ/m 2. The experimental results all agree quantitatively with diffraction theory predictions of impulse response at the focus of the transducer.


Excitonic transitions in GaAsGaAlAs superlattices under a weak electric field Depeyrot, J., P. Tronc, E. Umdenstock, B. Etienne, J. F. Palmier, and A. Sibille Superlattices and Microstructures 12, no. 4, 565570 (1992)
Résumé: We report a study of the photoluminescence spectra of GaAsGa0.65Al0.35As superlattice under an electric field of about 10 kVcm1, from 9 K up to 80 K; the ratio ρ{variant} = I1 I0 of the intensities of the 1 and 0 peaks of the WannierStark ladder varies with the temperature T. To fit these variations, we assume for both transitions, a trapped exciton density of states with a gaussian distribution of eigenenergies, and a two dimensional density of states for free excitons. We show that above about 50 K there is a thermal equilibrium between direct and crossed free excitons whereas at low temperatures, the equilibrium statistics are not respected: the excitons trap into the direct state rather than to the crossed one. © 1992.
Motsclés: Band structure; Eigenvalues and eigenfunctions; Electric fields; Excitons; Photoluminescence; Population statistics; Eigenenergies; Excitonic transitions; Photoluminescence spectra; Thermal equilibrium; Trapped exciton density; Weak electric field; Semiconductor superlattices


Sites and valencies of chromium in bismuth germanates: a magnetic circular dichroism and absorption study Briat, B., V. Topa, C. L. Boudy, and J. C. Launay Journal of Luminescence 53, no. 16, 524528 (1992)
Résumé: Bismuth germanate exists as Bi4Ge3O12 (BGO 2:3) which is used as a scintillator material and Bi12GeO20 (BGO 6:1) which is photorefractive. We present a magnetic circular dichroism (MCD) and absorption study of the two varieties doped with Cr. The site and valency of Cr in the 2:3 crystal is found to depend upon whether the doping is done with a piece of highly Crdoped BGO 6:1 or with chromium oxide. In the first case, we provide very clear evidence that Cr3+ lies at the Bi site in a high field environment. In the second, Cr enters the lattice as Cr4+, at both the Ge and the Bi sites. The MCD of Crdoped BGO 6:1 also depends upon the history of the sample investigated. We suggest that Cr enters this lattice as Cr5+ (Ge site) and Cr4+. © 1992.
Motsclés: Bismuth Compounds  Crystal Lattices; Bismuth Compounds  Doping; Chromium and Alloys; Color; Light  Absorption; Bismuth Germanium Oxide; Magnetic Circular Dichroism (MCD); Sites; Valency; Bismuth Compounds


The van CittertZernike theorem in pulse echo measurements Mallart, R., and M. Fink Journal of the Acoustical Society of America 90, no. 5, 27182727 (1991)
Résumé: A classical theorem of statistical optics, the van CittertZernike theorem, is generalized to pulse echo ultrasound. This theorem fully describes the secondorder statistics of the spatial fluctuations (the spatial covariance) of the field produced by an incoherent source. As a random scattering medium is insonified, it behaves as an incoherent source. The van CittertZernike theorem can thus predict the spatial covariance of the pressure field backscattered by a random medium. It is shown that this spatial covariance and the incident energy diagram are Fourier pairs. In the case of a focused illumination, the spatial covariance of the backscattered pressure field is proportional to the autocorrelation of the transmitting aperture function. This is independent of frequency and of F/ number. Experimental results obtained with a linear array are in good agreement with theoretical expectations. The implications of this theorem in speckle reduction and in focusing in nonhomogenous media are discussed.
Motsclés: acoustics; article


Atomic structure and local electrical behaviour of grain boundaries in YBa2Cu3O7x Laval, J. Y., M. Drouet, C. Delamarre, C. Cabanel, W. Swiatnicki, and M. Leliboux Physica C: Superconductivity and its applications 185189, no. PART 4, 24492450 (1991)
Résumé: The influence of the grain boundaries types and of their distribution on the electrical behaviour of YBCO superconductors has been analyzed. Three kinds of materials have been considered: polycrystals, textured materials and bicrystals. The relationships between the structure and the local electrical behaviour have been investigated by combining a local electrical probe method with transmission electron microscopy. © 1991.
Motsclés: Atoms; Ceramic Materials; Yttrium CompoundsGrain Boundaries; Microscopic ExaminationTransmission Electron Microscopy; Oxide Superconductors; Polycrystals; Yttrium Barium Copper Oxides; High Temperature Superconductors


Double excitations of Ti2+Mn2+ spin clusters in MgCl2, MnCl2, and MnBr2 Herren, M., M. A. Aebersold, B. Briat, and H. U. Güdel Inorganic Chemistry 30, no. 17, 32803282 (1991)
Résumé: Blueshifted sidebands on the wellknown Mn2+ absorptions are observed in the visible absorption and MCD spectra of manganese chloride and bromide doped with Ti2+ and of magnesium chloride doped with both Ti2+ and Mn2+. These sidebands are assigned to Ti2+Mn2+ double excitations of exchangecoupled Ti2+Mn2+ pairs (MgCl2) and Ti2+(Mn2+)6 clusters (MnCl2, MnBr2), respectively. The energy differences ΔE of roughly 680 (chlorides) and 600 cm1 (bromide) to the Mn2+ absorptions correspond to the trigonal splitting of the Ti2+ 3T1g(Oh) ground state. In the 4A1g 4Eg (Mn2+) region, the absorption lines are sharp and ΔE in the visible absorption spectrum of MgCl2:Ti2+,Mn2+ matches the known groundstate splitting of a Ti2+Mn2+ pair in MgCl2. The trigonal groundstate splitting of Ti2+ in MnBr2 has not been measured before. The principle of coupling an infrared electronic excitation on one ion with a visible excitation on a neighboring ion should be applicable to many other pairs of ions. © 1991 American Chemical Society.


Infrared conductivity versus doping from reflectivity in YBa2Cu3O6+x and Nd1+yBa2yCu3O6+x ceramics Bouvier, J., N. Bontemps, M. Nanot, and F. Queyroux Physica C: Superconductivity and its applications 185189, no. PART 2, 10151016 (1991)
Résumé: We have measured the room temperature reflectivity spectra in the range 50025000 cm1 on YBa2Cu3O6+x (0<x<1) and Nd1+yBa2yCu3O6+x (0<y<0.5) polished ceramics as a function of doping (x and y). In the metallic range, in both systems, the Drude contribution and the midinfrared band exhibit a correlated increase with hole doping level (x or y). The conductivity is compared to recent simulations of the tJ model as a function of hole doping. © 1991.
Motsclés: Ceramic Materials; Infrared Radiation; Neodymium Compounds; Yttrium Compounds; Neodymium Barium Copper Oxides; Oxide Superconductors; Yttrium Barium Copper Oxides; High Temperature Superconductors


Bistable behavior of a vibrating tip near a solid surface Gleyzes, P., P. K. Kuo, and A. C. Boccara Applied Physics Letters 58, no. 25, 29892991 (1991)
Résumé: Experiments using a vibrating tip close to a solid surface have shown a bistable behavior of the motion. These measurements have been interpreted in terms of perturbed harmonic oscillators both numerically and analytically.


The iterative time reversal mirror: A solution to selffocusing in the pulse echo mode Prada, C., F. Wu, and M. Fink Journal of the Acoustical Society of America 90, no. 2 I, 11191129 (1991)
Résumé: Accurate focusing in an inhomogeneous medium is difficult to implement. In order to focus on a reflective target we have extended the concept of optical phaseconjugate mirrors, valid for monochromatic signals, to broadband pulses such as those used in ultrasound echography. The transducer's linear response to the acoustic pressure allows one to replace the phase conjugation by a time reversal operation on the pulse echo signals. The time reversal mirror is an array of transmitreceive transducers. A first incident wave is reflected by the target. The received signals are stored in shift registers, reversed in time, and then reemitted. The main advantage of this process is that waves distorted by the propagation through an aberrating medium are corrected by the mirror operation and the back propagation through the medium. When the medium contains several reflectors, this time reversal process can be iterated in order to focus on the most reflective one. Theoretical results on this principle, numerical simulations, and experimental results with a 1D array working at a central frequency of 3 MHz are presented.
Motsclés: acoustics; article


Photothermal imaging of copperdecorated grain boundary in silicon Inglehart, L. J., A. Broniatowski, D. Fournier, A. C. Boccara, and F. Lepoutre Applied Physics Letters 56, no. 18, 17491751 (1990)
Résumé: Using a scanning photothermal reflectance microscope we have observed a large enhancement of the modulated reflectance signal accompanied by a phase change of π in the region of a copperdecorated grain boundary in silicon. A preliminary analysis of the data is given in terms of the thermal and plasma waves generated in the specimen. Orders of magnitude of recombination velocities of the surface and the boundary are determined in reasonable agreement with electrical measurements.


Geometry of the ND3 group in a metallic Ca(ND3)6 compound and in solid and liquid deuteroammonia as measured by neutron scattering Damay, P., F. Leclercq, and P. Chieux Physical Review B 41, no. 14, 96769682 (1990)
Résumé: The Ca(ND3)6 compound is a metallic crystal which is subject to considerable diffusional motion above 40 K; at lower temperature the protons are in a disordered state (molecular glass). It is then very difficult to refine the molecular structure of the ammonia group from classical Bragg scattering. The geometry of the ND3 group has been measured with high precision in Ca(ND3)6 as well as in liquid and solid deuteroammonia from diffuse scattering using neutron scattering at very large momentumtransfer vector (up to 23 A1). It is found that the tetrahedral geometry of ND3 in the compound is the same as in liquid ammonia; the bond length RND in the compound is nevertheless significantly longer than in the gas phase (RND1.025 as compared with 1.01) while the bond angle is decreased (DND103°instead of 106°). The present results are in contradiction with the model of distorted ammonia as proposed by Von Dreele et al. for this compound. © 1990 The American Physical Society.


Variablephotonenergy photoelectron spectroscopic studies of highspin d6 tetrahedral FeCl4 2: Electronic relaxation effects on ionization Butcher, K. D., S. V. Didziulis, B. Briat, and E. I. Solomon Inorganic Chemistry 29, no. 9, 16261637 (1990)
Résumé: Variablephotonenergy photoelectron spectra (PES) are reported for the valence band region of ferrous chloride over the energy range 25150 eV. Changes in peak intensity as a function of photon energy are compared to atomic photoionization cross sections, allowing experimental assignment and quantitation of the PES features. These results indicate that the groundstatebonding description in ferrous chloride corresponds to the normal description for transitionmetal complexes, with the highest occupied levels containing mostly metal character, but with significant spinpolarization effects. The relatively large exchange splitting in d6 splits the d levels into spinup and spindown sets, with the majority spinup levels showing greater covalent mixing in both the experimental data and in spinunrestricted SCFXαSW calculations. The PE spectra further show satellite peaks with significant offresonance intensity (∼6% of the mainband intensity), indicating that a large electronic relaxation occurs upon ionization. PE spectra taken at the Fe 3p absorption edge show dominant resonance enhancement in the deeper bindingenergy region of the main band as well as in the satellites. The XαSW calculations reproduce both the large relaxation effects and the observed resonance behavior and assign the satellites as arising from ligand ionization plus ligandtometal chargetransfer shakeup. The resonance profiles of ferrous chloride are very similar to those of ferric chloride, which shows no relaxation on ionization, but has an inverted energy level description for the majority spinup levels in the ground state (with the highest occupied orbital containing mostly ligand character). The resonance PES profiles thus indicate that ionized ferrous chloride has relaxed sufficiently to become inverted due to the decrease in electron repulsion and increase in exchange stabilization present in the highspin d5 final state. The role of exchange in determining both the groundstate bonding and its change on ionization are considered, and the implications of electronic relaxation for ferrous redox chemistry are discussed. © 1990 American Chemical Society.


Reflectivity and transmission spectra of Bi2Sr2Ca1Cu2O8 sputtered thin films Bouvier, J., N. Bontemps, A. C. Boccara, S. Labdi, and H. Raffy Journal of The LessCommon Metals 164165, no. PART 2, 10921098 (1990)
Résumé: We report on the roomtemperature reflectivity and transmission spectra of Bi2Sr2Ca1Cu2O8 thin films. By fitting simultaneously both spectra, we determine the plasma frequency and characterize several oscillators in the infrared and visible range. The role of the mid infrared band is emphasized. © 1990.


Stark localization in GaAsGaAlAs superlattices under a low electric field Tronc, P., C. Cabanel, J. F. Palmier, and B. Etienne Solid State Communications 75, no. 10, 825829 (1990)
Résumé: We show, from the photoluminescence of a GaAsGa0.65Al0.35As superlattice of period 70 Å, that the localization of the electrons takes place at low field (≈ 1 × 104 V cm1). Before the localization, the energy of the photoluminescence line does not vary appreciably with the electric field. When the localization begins, the binding energy of the heavyhole exciton is approximately the same whether the electron wavefunction is centered on the same well as the hole or an adjacent well. © 1990.
Motsclés: Photoluminescence; Semiconducting Gallium Arsenide; Semiconducting Gallium Compounds; Electron WaveFunction; Heavy Hole Exciton; Stark Localization; Superlattices


Thermal diffusivity of amorphous semiconductor superlattice films Zhang, Z. C., J. P. Roger, D. Fournier, A. C. Boccara, and J. C. Wang Thin Solid Films 186, no. 2, 361366 (1990)
Résumé: The thermal diffusivity of a series of amorphous semiconductor superlattice films ( aSi:H aSiNx:H, where aSi:H is hydrogenated amorphous silicon and aSiNx:H is hydrogenated amorphous SiNx) has been investigated both experimentally and theoretically. The mirage effect detection technique has been used to determine the difference between two phase measurements of photothermal signals, where the heating beam illuminates the front and the rear surface of the sample successively. Using a onedimensional model and an approximate theoretical calculation, we have been able to determine the thermal diffusivity of the superlattices by the experimental measurement of the phase lag. These results are in good quantitative agreement with a model based on the effective medium approximation. © 1990.
Motsclés: Semiconductor Devices; Semiconductor MaterialsAmorphous; Spectroscopy, Absorption; Thermal DiffusionSolids; Photothermal Signals; Superlattices; Semiconducting Films


Variable photon energy photoelectron spectroscopy on FeCl4 : An unusual electronic structure for highspin d5 complexes Butcher, K. D., S. V. Didziulis, B. Briat, and E. I. Solomon Journal of the American Chemical Society 112, no. 6, 22312242 (1990)
Résumé: Variable energy photoelectron spectroscopy (PES) is used to elucidate the valence band electronic structure and bonding in tetrahedral d5 FeCl4 . PES spectra obtained over the photon energy range 25150 eV show intensity changes in the valence band features which indicate that more metal character is present in the deepest bonding levels. This is inverted from the normal electronic structure description of transitionmetal complexes. The lack of offresonance intensity in the deep binding energy satellite, which corresponds to a twoelectron transition involving metal ionization plus ligandtometal charge transfer, indicates that little relaxation occurs on ionization. This result is confirmed by analysis of the satellite structure in the core level XPS Fe 2p spectra. PES spectra taken at the Fe 3p absorption edge, which provide insight into the bonding description of the ionized final state, show dramatic resonance intensity enhancement of the main band peaks as well as the satellite. The resonance enhancement of the main band indicates that it contains significant metal character after ionization and thus provides further evidence that the relaxation is small. A configuration interaction analysis shows that the resonance profiles of the photoelectron peak intensities at the absorption edge are also consistent with an inverted groundstate bonding scheme with little relaxation occurring upon ionization. Quantitative analysis of the resonance intensity data gives an experimental estimate of the covalent mixing in the HOMO as 38% Fe, 62% Cl. Both the inverted bonding scheme and the very small relaxation are reproduced by spinunrestricted but not by the spinrestricted SCFXαSW calculations. The origin of this unusual electronic structure in highspin d5 complexes and its implications with respect to redox chemistry are discussed.


Dynamics of the spinglass freezing in semimagnetic semiconductors Zhou, Y., C. Rigaux, A. Mycielski, M. Menant, and N. Bontemps Physical Review B 40, no. 11, 81118114 (1989)
Résumé: Accurate measurements of the imaginary part of the complex susceptibility are used in order to compare the validity of different dynamic scaling models on the two related spinglass compounds Hg1xMnxTe and Cd1xMnxTe (x=0.3). The conventional powerlaw scaling yields in both compounds a dynamic exponent z=91, Tc=8.4 K for Hg0.7Mn0.3Te and 6.45 K for Cd0.7Mn0.3Te. The generalized infield scaling leads to an independent and consistent determination of Tc. Good scalings may also be achieved according to activated dynamics, but the P values appear to differ in various systems, a result at odds with the expected universality of the critical exponents. © 1989 The American Physical Society.


Selective excitation of Ti2+Mn2+ and Mn 2+Ti2+Mn2+ spin clusters in MgCl2 Herren, M., S. M. Jacobsen, H. U. Güdel, and B. Briat The Journal of Chemical Physics 90, no. 2, 663671 (1989)
Résumé: Ti2+Mn2+ dimers and three types of Mn 2+Ti2+Mn2+ trimers obtained by codoping MgCl2 with Ti2+ and Mn2+ were studied by siteselective dyelaser spectroscopy. Very high selectivity was achieved by using the 6A1g →4A1g, 4Eg excitation on Mn2+, which does not coincide with any Ti2+ singleion excitation. Sharpline 1E g (T2g) →3T1g a luminescence from Ti2+ was observed in the near infrared (NIR). Electronic groundstate splittings resulting from Ti2+Mn 2+ exchange interactions were observed for the various spin clusters and, as a result, accurate exchange parameters could be obtained. The luminescence behavior of Ti2+Mn2+ pairs was investigated as a function of temperature and compared with the behavior of Ti2+ single ions in MgCl2. As in Ti2+:MgCl2 luminescence from Ti2+Mn2+ clusters was not only observed in the NIR but also in the visible, it corresponds to 3T 1g b →3T1g a transitions. © 1989 American Institute of Physics.


Fluxcreeplimited critical currents in YBa2Cu3O 7δ ceramics Dai, U., N. Hess, L. R. Tessler, G. Deutscher, G. Vetter, F. Queyroux, N. Bontemps, R. Mahoum, M. Lagues, and P. Mocaer Applied Physics Letters 55, no. 11, 11351137 (1989)
Résumé: Critical currents of YBa2Cu3O7δ bulksintered samples were measured at and above 77 K in the presence of magnetic fields. In moderate magnetic fields a Hn dependence with n<1 was observed. Transport measurements give n≅0.5, indicating that critical currents are limited by flux creep rather than by field quenching of Josephson junctions.


Heterogeneous media and rough surfaces: A fractal approach for heat diffusion studies Fournier, D., and A. C. Boccara Physica A: Statistical Mechanics and its Applications 157, no. 1, 587592 (1989)
Résumé: After a brief survey of Euclidean heat sources distribution behaviour in Euclidean spaces we will show how a distribution of fractal sources behaves in Euclidean spaces. The main applications deal with rough surfaces which can be mapped to fractal structures. In order to describe heat diffusion in heterogeneous media, the dimensionality of the diffusion process has to be introduced. We will illustrate this theroetical approach with experimental data obtained by using a short laser light pulse for heating and an infrared detection to monitor the surface temperature. © 1989.


Evidence for differing short and longtime decay behavior in the dynamic response of the insulating spinglass Eu0.4Sr0.6S Bontemps, N., and R. Orbach Physical Review B 37, no. 9, 47084713 (1988)
Résumé: The time decay of the thermoremanent magnetization of the insulating spinglass Eu0.4Sr0.6S has been measured with use of Faraday rotation to probe short and longtime regimes (3 ms 3000 s) between 0.86Tg and 1.04Tg. Aging effects are negligible in this compound to within our experimental accuracy, so that equilibrium response is accessible. We find evidence for two time regimes. The decay is of a powerlaw form at short times. Beyond a welldefined crossover time, the decay is of a stretched exponential form. The crossover time decreases as the temperature increases from below Tg, and drops abruptly at Tg. No field dependence of the crossover time is observed for the field range investigated (540 Oe). Connection is made with computer simulations of Ogielski, and with the existence of a finite length scale in the equilibrium spinglass state. © 1988 The American Physical Society.


The memory effect for dynamic behaviour: The case of a perfect ferromagnet Rajchenbach, J. Journal of Physics C: Solid State Physics 21, no. 13, L447L451 (1988)
Résumé: In ferromagnets, irreversible changes in the domain topography induce memory effect and hysteresis properties for the macroscopic behaviour. The author investigates the connection between the dynamic behaviour and the domain structure. In the simple case of a perfect ferromagnet with a regular array of up and down domains, he establishes here that the relaxation rate is inversely proportional to the mean size of domains. The author then considers the memory effect that this dependence implies for the dynamic behaviour. © 1988 IOP Publishing Ltd.


Moment analysis for absorption and magnetic circular dichroism bands of atomic P←S transitions: Application to matrixisolated chromium Vala, M., R. Pyzalski, J. Shakhsemampour, M. Eyring, J. Pyka, T. Tipton, and J. C. Rivoal The Journal of Chemical Physics 86, no. 11, 59515957 (1987)
Résumé: Equations useful for the moment analysis of magnetic circular dichroism (MCD) and absorption bands of P←S electronic transitions of atoms or ions for any spin multiplicity have been derived. In particular, it is shown that the first through third MCD moments, together with the zeroth and second absorption moments may be employed to obtain such electronic and vibonic parameters as (1) the excited state spinorbit coupling constant, (2) the excited state orbital g factor, and (3) the contributions of the cubic and noncubic metal/lattice cage modes to the spectral absorption bandwidths. The MCD and absorption spectra of Cr atoms isolated in krypton and xenon matrices are presented. Forbidden transitions between the two allowed 7P←7S bands are observed. Their appearance is ascribed to secondorder spinorbit coupling to the nearlying 7P states. The moment equations are applied in an appropriate manner to the observed allowed bands. Excited state spinorbit splitting (14 cm1:Cr/Kr and 56 cm1:Cr/Xe) are found to be significantly reduced from the gas phase value (+194 cm1) for the z7P←7S transition. For the y7P←S band, an enhancement (+275 cm1:Cr/Kr) and a reduction (+171 cm 1:Cr/Xe) from the gas phase value of 206 cm1 are observed. It is further shown that the JahnTeller effect is strongly operative in the z7P←7S Cr band in both matrices. © 1987 American Institute of Physics.


Moment analysis of matrixenhanced forbidden transitions: Application to Cr atoms isolated in Kr and Xe matrices Rivoal, J.C., and M. Vala The Journal of Chemical Physics 86, no. 11, 59585962 (1987)
Résumé: The band moment analysis of allowed transitions in magnetic circular dichroism/absorption may be complicated by the presence of nearlying forbidden transitions. The rare gas matrix enhances the spinorbit coupling between the forbidden and allowed states. The theory necessary to extract the spinorbit constant of the allowed excited state is presented for the case of a matrixisolated metal atom possessing nP←nS transitions. Application is made to matrixembedded Cr atoms which possess two fully allowed states each of which is coupled to a different forbidden state: the orbitally forbidden z 7D state is mixed with the orbitally allowed y 7P state and the spinforbidden z 5P state is coupled with the spinallowed z 7P state. The mixing coefficients for both these pairs (in Kr and Xe matrices) have been determined quantitatively. It is shown that both spin and orbital designations of these atomic states in rare gas matrices is meaningless. © 1987 American Institute of Physics.


Status of the Orsay FEL experiment Billardon, M., P. Elleaume, J. M. Ortega, C. Bazin, M. Bergher, M. E. Couprie, Y. Lapierre, Y. Petroff, R. Prazeres, and M. Velghe Nuclear Inst. and Methods in Physics Research, A 259, no. 12, 7276 (1987)
Résumé: The FEL installed on the storage ring ACO at Orsay has been operated down to a wavelength of 463 nm, at a ring energy of 200 MeV. This is probably the lowest wavelength attainable on ACO with TiO2 SiO2 dielectric mirrors because of the nearUV TiO2 broad absorption band. A careful analysis of the optical gain, taking into account the measured beam characteristics, shows a spectral dependence scaling as λ1,1, with a maximum at λ = 650 nm close to 0.4%. Therefore the ACO FEL may be tuned over almost the whole visible spectrum at the expense of time consuming mirror changes. Due to the poor transmission of these very high reflectivity mirrors the average extracted power is only a few mW throughout the visible region. However the maximum intracavity peak power (at low frequency Qswitching) reaches about 0.5 MW. The next step will be to test new Al2O3 SiO2 coated mirrors in the nearUV spectral range. © 1987.
Motsclés: ACCELERATORS  Storage Rings; MIRRORS; OPTICAL GAIN; OPTICAL KLYSTRON; ORSAY STORAGE RING; LASERS, FREE ELECTRON


Magnon and phonon Raman scattering of the planar ferromagnet Rb 2CrCl4 Abdalian, A. T., B. Briat, C. Dugautier, and P. Moch Journal of Physics C: Solid State Physics 20, no. 16, 24652477 (1987)
Résumé: Magnon and phonon Raman scattering is studied in the canted ferromagnet Rb2CrCl4. The phonon spectra obtained at about 8 and 200 K are discussed on the basis of the exact crystal structure (D2h 18) and of the appropriate quadratic one (D4h 5) previously proposed for the isostructural crystal K2CuF4. Among the twelve modes predicted in D4h5 symmetry, only one (belonging to Gamma 4 +) is missing. The most important feature of this work is the first direct observation of the optic gap predicted by the theory at about 170 cm1 and the study of its magnetic field dependence for B along the easy direction (110) and the hard direction (001). The behaviour of the line is discussed in relationship with spin wave calculations by considering a possible slight misorientation of the field off the principal axes. The acoustic magnon line, previously observed directly via ferromagnetic resonance, is also seen for B>2T along (110). Polarisation measurements on the two lines indicate that the acoustic and optic modes must be assigned to γ2 + and γ1 + (C2h) respectively. A quantitative analysis of the authors results shows that the average Lande factor is close to the spinonly value and it suggests that g⊥ to is slightly larger than g∥. It also leads to an estimate of P+2D where P and D stand for the two anisotropy parameters. © 1987 IOP Publishing Ltd.


Synthesis, structural characterization, and magnetic properties of V3+ dimer compounds. Neutron scattering and magnetic circular dichroism study of Cs3V2Cl9 and Rb3V2Br9 Leuenberger, B., B. Briat, J. C. Canit, A. Furrer, P. Fischer, and H. U. Güdel Inorganic Chemistry 25, no. 17, 29302935 (1986)
Résumé: The two title compounds were synthesized from a melt and characterized by neutron powder diffraction. The structure is confirmed to be hexagonal, P63/mmc, with two dimers V2X9 3 per unit cell. The following lattice parameters were obtained at room temperature: Cs3V2Cl9, a = 7.240 (2) Å, c = 17.95 (1) Å; Rb3V2Br9, a = 7.361 (2) Å, c = 18.53 (1) Å. The groundstate properties of both V3+ compounds were investigated by means of inelastic neutron scattering, magnetic circular dichroism, and magnetic susceptibility measurements. The ground state of V3+ is a 3A2 state, and the dimer is described by a spin Hamiltonian containing exchange interaction (J) and zerofield splitting (D) parameters. Both parameters could be determined from the experimental results: Cs3V2Cl9, J = 11 cm1, D = 7.2 cm1; Rb3V2Br9, J = 5.2 cm1, D = 12 cm1. Though the exchange interaction is ferromagnetic, the ground level of the dimer is a nonmagnetic singlet. The presence of antiferromagnetic interdimer interactions emerges from our experimental results. Neither compound shows a magnetic ordering above 1.5 K. © 1986 American Chemical Society.

