Articles avant 2005

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 (20-1000 Hz). Transient elastography experiments based on plane shear waves that propagate in an Agar-gelatin 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.

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 specular-diffuse 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.

Résumé: One-channel time reversal (TR) experiments through multiple scattering media were discussed. The hyperresolution and the self-averaging property was described. It was found that the developed formalism leads to a deeper understanding of the role of the ladder and most-crossed diagrams in a TR experiment. Results show that when the initial source and the time reversal point are at the same location, the time-reversed amplitude is twice higher.

Résumé: A method for tracking the 3-D motion of tissues in real-time is combined with a 2-D high-intensity focused ultrasound (US), or HIFU, multichannel system to correct for respiratory motion during HIFU therapy. Motion estimation is based on an accurate ultrasonic speckle-tracking method. A pulse-echo 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 1-D cross-correlation of the backscattered signals acquired at two different times. The 3-D 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 real-time 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 motion-estimation 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.

Résumé: We present a method to transmit digital information through a highly scattering medium in a MIMO-MU (multiple input multiple output multiple users) context. It is based on iterations of a time-reversal 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 inter-symbol interference and lower error rate) than classical time-reversal 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 non-null singular values of the time-reversal 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.

Résumé: A one-channel electromagnetic time-reversal 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.

Résumé: The feasibility of temperature estimation during high-intensity focused ultrasound therapy using pulseecho diagnostic ultrasound data has been demonstrated. This method is based upon the measurement of thermally-induced 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 thermo-acoustic 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 two-dimensional 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 two-dimensional maps from the multiple steered plane wave insonincations. Experiments were conducted in a tissue-mimicking gelatin-based phantom and in fresh bovine liver.

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.

Résumé: The measurements of the acoustic intensity scattered and absorbed by humans were presented. The study used long-duration acoustic reverberation to obtain the total scattering and absorption cross-sections 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.

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 so-called 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.

Résumé: The transient radiation of low-frequency 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 semi-infinite 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.

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), 1533-1543 (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 spatio-temporal 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.

Résumé: The vortex burst in a laminar-flow 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 k-5/3 law. In addition, this structure seems to have some common feature with the theoretical one of Lundgren.

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.

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.

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 one-dimensional analytical calculation taking into account the dispersion effect was developed.

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.

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 two-dimensional cross-correlation 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 UMa-system. 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.

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 time-reversal 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 so-called super-resolution 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 time-reversal focusing towards small modifications in the medium, such as people in motion or temperature variations. Sound focusing through walls using the time-reversal approach is also experimentally demonstrated. © 2003 Acoustical Society of America.

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.

Résumé: Developing minimally invasive brain surgery by high-intensity 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 3-D 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.

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.

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 block-copolymer micelles and demonstrated both in vitro and in vivo imaging. When conjugated to DNA, the nanocrystal-micelles acted as in vitro fluorescent probes to hybridize to specific complementary sequences. Moreover, when injected into Xenopus embryos, the nanocrystal-micelles were stable, nontoxic (<5 × 109 nanocrystals per cell), cell autonomous, and slow to photobleach. Nanocrystal fluorescence could be followed to the tadpole stage, allowing lineage-tracing experiments in embryogenesis.

Résumé: Self-sustained 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 (hole-tone, 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.

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 time-reversed 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 time-reversal 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 time-reversal focusing results. © 2002 American Institute of Physics.

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 contra-propagating 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 cross-correlation 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 cross-correlation coefficient versus the number of turns is proportional to the depth of the slot. © 2002 Elsevier Science B.V. All rights reserved.

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.

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.

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 1-bit 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 1-bit time reversal between a point source in water and several transducers fastened to one section of a finite-length 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 waveguide-time 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 1-bit time reversal and a solid waveguide leads to shock wave lithotripsy with low-power electronics. © 2001 Acoustical Society of America.

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.

Résumé: The establishment of Erwinia amylovora harpin-induced hypersensitive response (HR) in Nicotiana sylvestris was followed by infra-red thermography (IRT). Three to four hours after elicitation, the temperature decreased in the harpin-infiltrated 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 over-expressing 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 plant-pathogen interactions and could be applied to screen for mutants affected in elicitor signalling and/or for respiratory mutants.

Résumé: Local field distributions in random metal-dielectric films near a percolation threshold are experimentally studied using scanning near-field optical microscopy (SNOM). The surface-plasmon oscillations in such percolation films are localized in small nanometer-scale 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 near-field spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance quality-factor 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.

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 Rayleigh-Lamb frequency equations (dispersion relation) in the complex plane. Results are presented and the method is discussed. © 2001 Acoustical Society of America.

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.

Résumé: Sound propagation through a vortex is studied numerically using two different techniques: ray-tracing and finite-differences. Geometrical acoustics and ray-tracing 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 finite-size effects such as edge waves and the results are compared to experimental data. The interest of the finite-difference approach is demonstrated for cases where sound scattering occurs. We show the ability of such a simulation to account for both sound scattering and finite-size effects. Those two numerical techniques are compared and their validity is investigated.

Résumé: Random multiple scattering of ultrasound was analyzed using statistical approach techniques. An ultrasonic pulsed wave transmitted from a point source to a 128-element receiving array was studied through two dimensional samples with various thickness. It is found that the transmitted wave forms exhibits a time-dependent 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.

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.

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, 223-234 (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 singular-value 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 inverse-focusing technique is compared in homogeneous media with the classical focusing achieved by simple geometrical considerations but also with time-reversal 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 48-58 (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 spatio-temporal inverse-filter technique emphasizes the great potential of multiple-channel 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.

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 1.4-1 beaker with 1-cm-long fish as well as fisheries acoustics frequency (12.8 kHz) in a 30-m3 tank with 35-cm-long fish. These results have interesting applications to fish target strength measurement and fish counting in aquaculture. © 2001 Acoustical Society of America.

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 half-space. 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.

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 solvent-free, 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 plasma-polymerized 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 AFM-determined 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.

Résumé: Local field distributions arc studied in random metal-dielectric films near percolation (percolation films) and fractal aggregates of colloidal particles. For both systems, it is shown that optical excitations are localized in small nanometer-scale 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 surface-enhanced white-light generation and second-harmonic 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 surface-plasmon excitations localized on chiral-active particle configurations in fractals.

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.

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 time-reversal 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 time-reversal method is investigated when loss of information breaks the time-reversal invariance. It arises in dissipative media or when the field radiated by the source is not entirely measured by the limited aperture of a time-reversal 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 time-reversal and inverse filter techniques. Moreover, experiments investigated in various media are presented to illustrate this comparison. (C) 2000 Acoustical Society of America.

Résumé: Experimental results of time-reversal focusing in a high-order 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 time-reversed 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 side-lobes as a consequence of the growing number of crossing paths in the sample due to high-order multiple scattering. As to spatial focusing, the resolution is practically independent from the array's aperture. With a 16-element 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 time-reversal mirrors in a random medium. (C) 2000 Acoustical Society of America.

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 film-on-substrate systems by modulated thermoreflectance microscopy and multiparameter fitting. The dependences of the phase on the probe-to-pump 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.

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.

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 pre-focused 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 phase-conjugation 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 time-reversal 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.

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 BSO-BMnO) 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.

Résumé: A novel acoustic time-reversal technique has been tested for determining the surface-height autocorrelation function and rms height of rough surfaces. A time-reversal 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 time-reversed signal. As expected from time-reversal 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 time-reversal mirror is sensitive to the surface-height autocorrelation and, in favorable cases, determines the autocorrelation function and rms height.

Résumé: We report the first experiments using the reversibility of a transient acoustic wave in a multiple-scattering 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 time-dependent ability to recover the angular spectrum thanks to both single- and multiple-scattering paths.

Résumé: The localized optical excitations in random metal-dielectric films were examined by scanning near-field 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.

Résumé: A new near-field 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 near-field magneto-optical contrast on a multi-layer sample with magnetic domains. These results are promising for magneto-optical characterization with nanometre resolution.

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.

Résumé: We have investigated light propagation in optical devices by near-field 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.

Résumé: We define recommendations for compact set-up 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 (non-remote) continuous detection applications of systems based on this principle. We illustrate these applications with recent gas phase measurements for coffee process control.

Résumé: A photorefractive CdTe:Ge single crystal was investigated via photo-absorption 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.

Résumé: The time-reversal 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, 1122-1129 (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.

Résumé: The application of time-reversal mirrors (TRM) to media with very high-order 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, time-reversed and retransmitted through the same disordered medium. Surprisingly, the time-reversed 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 time-reversal processes. In addition to this, the observed resolution of the time-reversed waves was greatly increased, and found to be smaller than the theoretical limit for the array's aperture. Theoretical limits of time-reversed experiments are discussed. © 1998 Elsevier Science B.V.

Résumé: In a recent paper [Derode and Fink, J. Acoust. Soc. Am. 101, 690-704 (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 cross-ply 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.

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 1-3 piezoelectric composite materials which have a 2-D periodicity. Our model is based on the analysis of the propagation of transverse waves in a 2-D 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.

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 inter-comparison 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 over-estimation 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.

Résumé: In a previous paper [Barjau et al., J. Acoust. Soc. Am. 102, 3023-3031 (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 time-domain 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.

Résumé: The birefringence of c-axis 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.

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 B-scan image of the zone surrounding the reflector. © 1996 IEEE.

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, spin-coated 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 set-up 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.

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.

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.

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 re-emitted. 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 least-mean-square method to calculate the spatial coordinates of the stone.

Résumé: Detection of surface and subsurface flaws is achieved using the time reversal process leading to self-focusing 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.

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*10-5 on delta R/R was achieved and a similar sensitivity was obtained when comparing two mirrors.

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.

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.

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 B-saturation curves for Fe atoms isolated in krypton and xenon matrices in the range 0-1.5 T and 2.3-22 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 ground-state 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 matrix-isolated Fe atoms by Mössbauer and laser-excited emission spectroscopies. © 1994 American Institute of Physics.

Résumé: Time-reversal mirror (TRM) utilizes time-reversal 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.

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 16-beam 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.

Résumé: We demonstrate that several kinds of colored T centers can be prepared by an electrical coloring of Pb-doped 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.

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 signal-to-noise ratio of a laser based ultrasonic system.

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 as-grown crystals. Diamagnetic centers, contributing to the near-u.v. absorption, are also formed by u.v. irradiation at 310 nm. Finally X-ray irradiation at 15 K produces similar but more drastic effects on the absorption spectra. © 1993.

Résumé: In this paper, diffraction theory is used to describe the self and mutual time-dependent 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 well-known 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, 730-737 (1960); P. R. Stepanishen, ibid. 49, 283-292, 841-849 (1971)], except that the results here exhibit a correction term that increases the validity domain of the approximations.

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 mid-infrared for acetonitrile from step-scan 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 cm-1. The phase is shown to exhibit monotonic behavior to a larger absorptivity, estimated to be greater than 3000 cm-1. The designs of the PBD accessory and sample cells are described in some detail, and a number of experimental considerations are discussed.

Résumé: We report a study of the photoluminescence spectra of GaAsGa0.65Al0.35As superlattice under an electric field of about 10 kVcm-1, from 9 K up to 80 K; the ratio ρ{variant} = I-1 I0 of the intensities of the -1 and 0 peaks of the Wannier-Stark 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.

Résumé: A classical theorem of statistical optics, the van Cittert-Zernike theorem, is generalized to pulse echo ultrasound. This theorem fully describes the second-order 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 Cittert-Zernike 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.

Résumé: Blue-shifted sidebands on the well-known 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 exchange-coupled Ti2+-Mn2+ pairs (MgCl2) and Ti2+(Mn2+)6 clusters (MnCl2, MnBr2), respectively. The energy differences ΔE of roughly 680 (chlorides) and 600 cm-1 (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 ground-state splitting of a Ti2+-Mn2+ pair in MgCl2. The trigonal ground-state 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.

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.

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 copper-decorated 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.

Résumé: Variable-photon-energy photoelectron spectra (PES) are reported for the valence band region of ferrous chloride over the energy range 25-150 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 ground-state-bonding description in ferrous chloride corresponds to the normal description for transition-metal complexes, with the highest occupied levels containing mostly metal character, but with significant spin-polarization effects. The relatively large exchange splitting in d6 splits the d levels into spin-up and spin-down sets, with the majority spin-up levels showing greater covalent mixing in both the experimental data and in spin-unrestricted SCF-Xα-SW calculations. The PE spectra further show satellite peaks with significant off-resonance intensity (∼6% of the main-band 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 binding-energy region of the main band as well as in the satellites. The Xα-SW calculations reproduce both the large relaxation effects and the observed resonance behavior and assign the satellites as arising from ligand ionization plus ligand-to-metal charge-transfer 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 spin-up 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 high-spin d5 final state. The role of exchange in determining both the ground-state bonding and its change on ionization are considered, and the implications of electronic relaxation for ferrous redox chemistry are discussed. © 1990 American Chemical Society.

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 cm-1). 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 heavy-hole exciton is approximately the same whether the electron wave-function is centered on the same well as the hole or an adjacent well. © 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 25-150 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 transition-metal complexes. The lack of off-resonance intensity in the deep binding energy satellite, which corresponds to a two-electron transition involving metal ionization plus ligand-to-metal 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 ground-state 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 spin-unrestricted but not by the spin-restricted SCF-Xα-SW calculations. The origin of this unusual electronic structure in high-spin d5 complexes and its implications with respect to redox chemistry are discussed.

Résumé: Ti2+Mn2+ dimers and three types of Mn 2+Ti2+Mn2+ trimers obtained by codoping MgCl2 with Ti2+ and Mn2+ were studied by site-selective dye-laser spectroscopy. Very high selectivity was achieved by using the 6A1g →4A1g, 4Eg excitation on Mn2+, which does not coincide with any Ti2+ single-ion excitation. Sharp-line 1E g (T2g) →3T1g a luminescence from Ti2+ was observed in the near infrared (NIR). Electronic ground-state 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.

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.

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.

Résumé: The band moment analysis of allowed transitions in magnetic circular dichroism/absorption may be complicated by the presence of near-lying forbidden transitions. The rare gas matrix enhances the spin-orbit coupling between the forbidden and allowed states. The theory necessary to extract the spin-orbit constant of the allowed excited state is presented for the case of a matrix-isolated metal atom possessing nP←nS transitions. Application is made to matrix-embedded 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 spin-forbidden z 5P state is coupled with the spin-allowed 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.

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 cm-1 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 spin-only 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.