Articles 2013

Résumé: Weintroduce an open microwave cavity that has a wall replaced by a sub-wavelength grating. Usually, sub-wavelength gratings show very low transmission. In our experiment, this phenomenon is compensated by the microwave cavity that finally allows all the energy to be transmitted. We study the far field emission of this system and show that coupling the cavity with a sub-wavelength grating gives rise to a zero order emission only at discrete angles and frequencies. We study the relations between angles of emissions and frequencies, the influence of geometric parameters such as the grating fill factor and the behavior of a chaotic cavity. We show that it allows us to make a configurable system that may have many applications in the fields of communications, detection and imaging, and may allow the study of open microwave cavities on a fundamental point of view. © 2013 IEEE.

Résumé: Optical forces can affect the motion of a Brownian particle. For example, optical tweezers use optical forces to trap a particle at a desirable position. Using more complex force fields it is possible to generate more complex configurations. For example, by using two optical traps placed next to each other, it is possible to obtain a bistable potential where a particle can jump between the two potentials with a characteristic time scale. In this proceeding, we discuss a simple finite difference algorithm that can be used to simulate the motion of a Brownian particle in a one-dimensional field of optical forces. © 2013 SPIE.

Résumé: Scanned at very high ultrasound frame rates, injectable microbubbles can be activated sequentially as isolated punctual sources of acoustic echoes. These signals can thus be localized far beyond the diffraction limit. The resolution improvement granted by Sono-Activated Ultrasound Localization Microscopy (SAULM) was demonstrated within microfluidic channels 20 times smaller than the imaging wavelength (λ = 870 μm). The width of the channels mapped with SAULM was 13 times smaller than as they appeared under conventional ultrasound imaging. Two channels separated by λ/4.5 could be distinguished. Implementing SAULM in-vivo could lead to a complete reconstruction of the vascular tree down to the smallest capillaries at several centimeter depth. © 2013 AIP Publishing LLC.

Résumé: An as yet unexploited Magneto Optical Kerr Effect (MOKE) at evanescent distance from a surface is introduced. In the case of a magnetic particle-metallic surface system, an extraordinary intensity is discovered and fully explained by the excitation of Surface Plasmon Polariton. © OSA 2013.

Résumé: In this article, we investigate composite media which present both a local resonance and a periodic structure. We numerically and experimentally consider the case of a very academic and simplified system that is a quasi-one dimensional split ring resonator medium. We modify its periodicity to shift the position of the Bragg bandgap relative to the local resonance one. We observe that for a well-chosen lattice constant, the local resonance frequency matches the Bragg frequency thus opening a single bandgap which is at the same time very wide and strongly attenuating. We explain this interesting phenomenon by the dispersive nature of the unit cell of the medium, using an analogy with the concept of white light cavities. Our results provide new ways to design wide and efficient bandgap materials.

Résumé: Using scanning probe microscopy with modulated illumination, we demonstrate simultaneous measurement of topography and optical forces exerted on a probe. Broadband optical field detection is possible using a single probe. © OSA 2013.

Résumé: We show that through-the-objective evanescent microscopy (epi-EM) is a powerful technique to image membranes in living cells. Readily implementable on a standard inverted microscope, this technique enables full-field and real-time tracking of membrane processes without labeling and thus signal fading. In addition, we demonstrate that the membrane/interface distance can be retrieved with 10 nm precision using a multilayer Fresnel model. We apply this nano-axial tomography of living cell membranes to retrieve quantitative information on membrane invagination dynamics. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Résumé: Audio source separation consists in recovering different unknown signals called sources by filtering their observed mixtures. In music processing, most mixtures are stereophonic songs and the sources are the individual signals played by the instruments, e.g. bass, vocals, guitar, etc. Source separation is often achieved through a classical generalized Wiener filtering, which is controlled by parameters such as the power spectrograms and the spatial locations of the sources. For an efficient filtering, those parameters need to be available and their estimation is the main challenge faced by separation algorithms. In the blind scenario, only the mixtures are available and performance strongly depends on the mixtures considered. In recent years, much research has focused on informed separation, which consists in using additional available information about the sources to improve the separation quality. In this paper, we review some recent trends in this direction. © 2013 IEEE.

Résumé: High Intensity Focused Ultrasound (HIFU) therapy is an innovative approach for tissue ablation, based on high intensity focused ultrasound beams. At the focus, HIFU induces a temperature elevation and the tissue can be thermally destroyed. In fact, this approach has been tested in a number of clinical studies for the treatment of several tumors, primarily the prostate, uterine, breast, bone, liver, kidney and pancreas. For transcranial brain therapy, the skull bone is a major limitation, however, new adaptive techniques of phase correction for focusing ultrasound through the skull have recently been implemented by research systems, paving the way for HIFU therapy to become an interesting alternative to brain surgery and radiotherapy. © 2013 Elsevier Masson SAS.

Résumé: We characterize the local properties of an optomechanical system comprising the movable mirror of a resonator and its intracavity field, mutually coupled via radiation pressure. Our approach shows that both the state of the mirror and the field can be interpreted as squeezed thermal states whose dynamical properties can be tuned by properly choosing the working parameters. This allows us to design conditional procedures for the amplification of the correlation properties of the optomechanical state. Our study is a step forward in the understanding of the physics that rules a system of current enormous experimental interest. © 2013 American Physical Society.

Résumé: We report measurements of the transmitted coherent (ensemble-averaged) wave resulting from the interaction of an ultrasonic shock wave with a two-dimensional random medium. Despite multiple scattering, the coherent waveform clearly shows the steepening that is typical of nonlinear harmonic generation. This is taken advantage of to measure the elastic mean free path and group velocity over a broad frequency range (2-15 MHz) in only one experiment. Experimental results are found to be in good agreement with a linear theoretical model taking into account spatial correlations between scatterers. These results show that nonlinearity and multiple scattering are both present, yet uncoupled. © 2013 American Physical Society.

Résumé: Acoustic communication is widespread in animals. According to the sensory drive hypothesis [Endler JA (1993) Philos Trans R Soc Lond B Biol Sci 340(1292):215-225], communication signals and perceptual systems have coevolved. A clear illustration of this is the evolution of the tetrapod middle ear, adapted to life on land. Here we report the discovery of a bone conduction-mediated stimulation of the ear by wave propagation in Sechellophryne gardineri, one of the world's smallest terrestrial tetrapods, which lacks a middle ear yet produces acoustic signals. Based on X-ray synchrotron holotomography, we measured the biomechanical properties of the otic tissues and modeled the acoustic propagation. Our models show how bone conduction enhanced by the resonating role of the mouth allows these seemingly deaf frogs to communicate effectively without a middle ear.

Résumé: Full-Field OCT (FF-OCT) is able to image biological tissues in 3D with micrometer resolution. In this study we add elastographic contrast to the FF-OCT modality. By combining FF-OCT with elastography, we create a virtual palpation map at the micrometer scale. We present here a proof of concept on multi-layer phantoms and preliminary results on ex vivo biological samples such as porcine cornea, human breast tissues and rat heart. The 3D digital volume correlation that is used in connection with the 3D stack of images allows to access to the full 3D strain tensor and to reveal stiffness anisotropy. © 2013 Optical Society of America.

Résumé: MR ARFI measures the displacement induced by the ultrasonic radiation force and provides the location of the focal spot without significant heating effects. Displacements maps obtained with MR ARFI provide an indirect estimation of the acoustic beam intensity at the target. This measure is essential for dose estimation prior to focused ultrasound treatments (FUS) and adaptive focusing procedures of MR-guided transcranial and transribs FUS. In the latter case, the beam correction is achieved by maximizing the displacement at focus. A significant number of serial MR ARFI images are required and thus, a partial k-space updating method, such as keyhole appears as a method of choice. The purpose of this work is to demonstrate via simulations and experiments the efficiency of the keyhole technique combined with a two-dimensional spin-echo MR ARFI pulse sequence. The method was implemented in an ex vivo calf brain taking advantage of the a priori knowledge of the focal spot profile. The coincidence of the phase-encoding axis with the longest axis of the focal spot makes the best use of the technique. Our approach rapidly provides the focal spot localization with accuracy, and with a substantial increase to the signal-to-noise ratio, while reducing ultrasound energy needed during MR-guided adaptive focusing procedures. © 2013 Elsevier Inc. All rights reserved.

Résumé: We show how disorder can be used to guide a broadband ultrasonic wave. The idea is to exploit the transverse localization regime that has been reported for light. Our waveguide consists of a set of parallel cylindrical scatterers randomly distributed in the transverse plane. An ultrasonic beam propagating along the direction of scatterers is found to remain confined in the two other directions on a size smaller than the waveguide diameter and driven by the localization length. Interestingly, the guided wave is also found to propagate with a very limited temporal dispersion. © 2013 American Physical Society.

Résumé: Homogenization theory is used to derive the effective properties of gratings with complex subwavelength structures. Going beyond the effect of the filling fraction, geometrical effects are analyzed using a two-step homogenization process. An explicit expression for the transmission spectrum is derived, able to predict the Fabry-Perot resonances and the Brewster angle realizing broadband extraordinary transmission. With the same filling fraction, one expects from this analytical expression that gratings with different geometries may display very different transmission properties. This sensitivity to the microstructure geometry is exemplified in the case of gratings made of hard material and made of dielectric material. The analytical results are shown to be within a few percentage points as compared to full-wave numerical simulations, paving the way for transmission properties tuned by structural and geometrical manipulations. © 2013 American Physical Society.

Résumé: Due to the simple properties of plane waves, non lossy straight pipes and their concatenation have been extensively used to derive digital waveguide synthesis and to compute acoustic immittances (input impedance, transmittance, etc) of wind instruments. This paper focuses on some possible refinements of such 1D wave propagation models, especially in the case of smooth horns. Four key points are examined: a refined curvilinear 1D horn equation, the smooth connection of constant-flared acoustic pipes, a radiation model consistent with spherical wavefronts, the effect of visco-thermal losses at the wall. They allow the definition of a complete model, from which a standard matrix formalism is recovered, as for plane or spherical waves. The comparison with measurements shows that each of these refinements is relevant, making one-dimensional models useful and efficient even for the quite sensitive case of brass instruments. Moreover, compared to the standard descriptions based on straight or conical pipes, the model proposed here gives accurate descriptions with only a few segments. © S. Hirzel Verlag · EAA.

Résumé: In this paper, we present a CMOS image sensor architecture coupling a spatial light modulator to a photodiode, for medical imaging based on acousto-optical coherence tomography with a digital holographic detection scheme. Our architecture is able to measure an interference pattern between a scattered beam transmitted through a scattering media and a reference beam, on an array with 16 μm pixel pitch, at 4000 Hz, which is compliant with correlation time of breast tissues. In-pixel processing allows generating from the incident light, a signal to polarize an embedded light modulator used to control the phase of the reflected beam. This reflected beam can then be focused on a region of interest of a scattering media, for therapy. The stacking of a photosensitive element with a spatial light modulator on the same device brings a significant robustness over the state of the art techniques such as perfect optical matching and reduced time delay in controlling light. © 2013 IEEE.

Résumé: The ultrasonic inspection of multiple scattering media gives rise to structural noise which makes it difficult to detect potential damage or crack inside the component. In order to predict the performances of ultrasonic inspection over such complex media, scattering models can be used. Such models rely on specific key parameters describing the multiple scattering process, which can be determined by specific measurements and post-processing techniques. Such experiments were carried out on stainless steel plates using linear phased-arrays. They consist in recording the response matrix constituted by impulse responses between all the elements of the array. By conducting post-processing on this matrix, we measure the elastic mean free path le and the correlation distance dc of the recorded noise. Additionally, the dynamic behaviour of the coherent backscattering effect was studied in order to measure the diffusion constant D. Plane-wave beamforming has been applied to the response matrix to improve the angular resolution and the signal-to-noise ratio of the backscattered intensity. Details of postprocessing techniques will be shown. © Published under licence by IOP Publishing Ltd.

Résumé: The assessment of coagulating blood elasticity has gained importance as a result of several studies that have correlated it to cardiovascular pathologic conditions. In this study we use supersonic shear wave imaging (SSI) to measure viscoelastic properties of blood clots. At the same time, classical rheometry experiments were carried out on the same blood samples taken within the first few seconds of coagulation. Using SSI, phase velocities of the shear wave indicated increasing dispersion with time. In all cases, the frequency bandwidth of propagating shear waves changed from 20-50 Hz at the first few min of coagulation to around 300 Hz toward the end of experiments. Using the values of G′ and G″ from the rheometry studies, the theoretical shear wave velocities were calculated and correlated with SSI measurements. Results of the two techniques were in very good agreement, confirming that SSI provides accurate measurements of viscoelastic properties as corroborated by conventional rheometric measurements. © 2013 World Federation for Ultrasound in Medicine & Biology.

Résumé: We present a novel technique to remotely measure and control the local temperature within a medium. This technique is based on the observation of the rotational Brownian motion of gold nanocrescent particles, which possess a strong anisotropic light interaction due to their plasmonic properties. Rotational scattering correlation spectroscopy performed on a single nanoparticle is able to determine the local temperature with high accuracy. These nano-thermometers can simultaneously play the role of nano-heaters when absorbing the light of a focused laser beam. © 2013 IOP Publishing Ltd.

Résumé: The scattering of an acoustic wave propagating in a non-uniform waveguide is inspected by revisiting improved multimodal methods in which the introduction of additional modes, so-called boundary modes, allows to better satisfy the Neumann boundary conditions at the varying walls. In this paper, we show that the additional modes can be identified as evanescent modes. Although non-physical, these modes are able to tackle the evanescent part of the field omitted by the truncation and are able to restore the right boundary condition at the walls. In the low-frequency regime, the system can be solved analytically, and the solution for an incident plane wave including one or two boundary modes is shown to be an improvement of the usual Webster equation. © 2013 The Author(s) Published by the Royal Society. All rights reserved.

Résumé: We investigate the nonlinear response of pulsed sound transmission in weakly compressed granular materials, composed of glass beads and sand. Amplitude and velocity of longitudinal waves are simultaneously measured. We observe that weakly compressed packings can both exhibit strengthening and weakening of the sound velocity and transmission amplitude when excited by high amplitude sound. These effects are due to the changes of the contacts between the particles and although the effective medium theory qualitatively describes these effects it fails to quantitatively account for them. © 2013 AIP Publishing LLC.

Résumé: Purpose: Low-intensity focused ultrasound has been shown to stimulate the brain noninvasively and without noticeable tissue damage. Such a noninvasive and localized neurostimulation is expected to have a major impact in neuroscience in the coming years. This emerging field will require many animal experiments to fully understand the link between ultrasound and stimulation. The primary goal of this paper is to investigate transcranial ultrasonic neurostimulation at low frequency (320 kHz) on anesthetized rats for different acoustic pressures and estimate the in situ pressure field distribution and the corresponding motor threshold, if any. The corresponding acoustic pressure distribution inside the brain, which cannot be measured in vivo, is investigated based on numerical simulations of the ultrasound propagation inside the head cavity, reproducing at best the experiments conducted in the first part, both in terms of transducer and head geometry and in terms of acoustic parameters. Methods: In this study, 37 ultrasonic neurostimulation sessions were achieved in rats (N = 8) using a 320 kHz transducer. The corresponding beam profile in the entire head was simulated in order to investigate the in situ pressure and intensity level as well as the spatial pressure distribution, thanks to a rat microcomputed tomography scan (CT)-based 3D finite differences time domain solver. Results: Ultrasound pulse evoked a motor response in more than 60% of the experimental sessions. In those sessions, the stimulation was always present, repeatable with a pressure threshold under which no motor response occurred. This average acoustic pressure threshold was found to be 0.68 ± 0.1 MPa (corresponding mechanical index, MI = 1.2 and spatial peak, pulse averaged intensity, Isppa = 7.5 W cm-2), as calibrated in free water. A slight variation was observed between deep anesthesia stage (0.77 ± 0.04 MPa) and light anesthesia stage (0.61 ± 0.03 MPa), assessed from the pedal reflex. Several kinds of motor responses were observed: movements of the tail, the hind legs, the forelimbs, the eye, and even a single whisker were induced separately. Numerical simulations of an equivalent experiment with identical acoustic parameters showed that the acoustic field was spread over the whole rat brain with the presence of several secondary pressure peaks. Due to reverberations, a 1.8-fold increase of the spatial peak, temporal peak acoustic pressure (Psptp) (±0.4 standard deviation), a 3.6-fold increase (±1.8) for the spatial peak, temporal peak acoustic intensity (Isptp), and 2.3 for the spatial peak, pulse averaged acoustic intensity (Isppa), were found compared to simulations of the beam in free water. Applying such corrections due to reverberations on the experimental results would yield a higher estimation for the average acoustic pressure threshold for motor neurostimulation at 320 KHz at 1.2 ± 0.3 MPa (MI = 2.2 ± 0.5 and Isppa = 17.5 ± 7.5 W cm-2). Conclusions: Transcranial ultrasonic stimulation is pressure- and anesthesia-dependent in the rat model. Numerical simulations have shown that the acoustic pattern can be complex inside the rat head and that special care must be taken for small animal studies relating acoustic parameters to neurostimulation effects, especially at a low frequency. © 2013 American Association of Physicists in Medicine.

Résumé: We demonstrate Optical-Resolution Photoacoustic Microscopy (OR-PAM), where the optical field is focused and scanned using Digital Phase Conjugation through a multimode fiber. The focus is scanned across the field of view using digital means, and the acoustic signal induced is collected by a transducer. Optical-resolution photoacoustic images of a knot made by two absorptive wires are obtained and we report on resolution smaller than 1.5 μm across a 201 μm × 201 μm field of view. The use of a multimode optical fiber for the optical excitation part can pave the way for miniature endoscopes that can provide optical-resolution photoacoustic images at large optical depth. © 2013 AIP Publishing LLC.

Résumé: Backward waves propagating on shell are guided modes with opposite phase and group velocities. For a shell in vacuum, backward modes are linked to zero group velocity modes and resonances which have been the object of recent studies. For a shell embedded in water, the group velocity does not vanish because of the leakage into the fluid. However, the group velocity of the backward mode has a minimum associated to a quasi-resonance. These phenomena are studied on air filled steel and zircaloy hollow cylinders, using a 3MHz linear array in pulse echo mode. The circumferential guided modes are generated and their radiation into water detected by the array. The modes are separated using the decomposition of the time reversal operator (TRO), each pair of counter-propagating modes being associated to 2 invariants of the TRO [Prada & al. J. Acoust. Soc. Am. 1998]. Two resonances are revealed by the eigenvalues of the TRO, one is associated with the first longitudinal thickness resonance and the other, very high, occurring at a slightly lower frequency, corresponds to the minimum of the group velocity of the backward mode. The back-propagations of the eigenvectors of the TRO provide the phase velocities of these modes. © 2013 Acoustical Society of America.

Résumé: In shallow water, active detection of a small moving target can be difficult because of strong echoes from large fixed obstacles. To cancel strong unwanted echoes, differences between successive acquisitions can be achieved, however they are very sensitive to fluctuations. A projection method combined with a fast acquisition technique is proposed as a robust alternative. An ultrasonic experiment is presented: a 64 transducers linear vertical array is used to detect a small target moving above a large obstacle in a waveguide. To reduce acquisition time, 8 groups of adjacent elements transmit linear frequency modulations with increasing delays in a single emission. The 8×64 array response matrix is then obtained by correlations and time windowing. The projection is achieved between two acquisitions obtained while the target is moving, in order to remove the obstacle's contribution. Namely, the second acquired matrix is projected on the space orthogonal to the 8 singular vectors of the first acquired matrix. Then, it is shown that the first singular vector of the projected matrix focuses on the second target's position. Comparisons are made with the decomposition of the time reversal operator in differential mode and conventional beamforming. © 2013 Acoustical Society of America.

Résumé: A realization of a reflectionless power splitter is proposed by use of a metamaterial junction. To design the junction, the electromagnetic wave transmission in multiple connected leads is investigated theoretically and numerically. A closed analytical form is derived for the scattering matrix of any geometry of the interconnected leads. We show that the use of a junction made of ∈-near-zero (ENZ) material allows production of perfect transmission. This can be achieved by reducing the area of the ENZ junction (squeezing effect) and by tuning the widths of the output leads with respect to the input lead. It is also shown that the same effect is obtained without squeezed junction by using a match impedance zero index material (MIZIM junction). © 2013 Optical Society of America.

Résumé: The effect of periodic and disordered photonic structures on the absorption efficiency of amorphous and crystalline Silicon thin-film solar cells is investigated numerically. We show that disordered patterns possessing a short-range correlation in the position of the holes yield comparable, or even superior, absorption enhancements than periodic (photonic crystal) patterns. This work provides clear evidence that non-deterministic photonic structures represent a viable alternative strategy for photon management in thin-film solar cells, thereby opening the route towards more efficient and potentially cheaper photovoltaic technologies. © 2013 Optical Society of America.

Résumé: We present a new technique based on the self-interference of Supercritical Angle Fluorescence (SAF) emission in order to perform full-field cell membrane imaging. We show that our Point Spread Function (PSF) engineering technique allows us to obtain a 100 nm axial sectioning while conserving the original lateral resolution of the microscope. The images are acquired using an optical module that can be connected to any fluorescent microscope to simultaneously monitor in real time both the cell membrane and in-depth phenomena. © 2013 Copyright SPIE.

Résumé: Circumventing the limit imposed by diffraction is a major issue in the instrumental development to realize finer resolutions in biological samples. With STED microscopy, we exploit the molecular transitions of the fluorescent marker to image well below the Rayleigh criterion. Also in combination with STED, we propose to use an alternative technique for optically sectioning fluorescent emitters close to the water-glass interface by selectively filtering the supercritical emission at the pupil plane. We discuss the instrumental development of such a system and its combination with other imaging techniques. © 2013 Copyright SPIE.

Résumé: A prospective study was performed on neurosurgical samples from 18 patients to evaluate the use of full-field optical coherence tomography (FF-OCT) in brain tumor diagnosis. FF-OCT captures en face slices of tissue samples at 1 μm resolution in 3D to a penetration depth of around 200 μm. A 1 cm 2 specimen is scanned at a single depth and processed in about 5 min. This rapid imaging process is non-invasive and requires neither contrast agent injection nor tissue preparation, which makes it particularly well suited to medical imaging applications. Temporal chronic epileptic parenchyma and brain tumors such as meningiomas, low-grade and high-grade gliomas, and choroid plexus papilloma were imaged. A subpopulation of neurons, myelin fibers and CNS vasculature were clearly identified. Cortex could be discriminated from white matter, but individual glial cells such as astrocytes (normal or reactive) or oligodendrocytes were not observable. This study reports for the first time on the feasibility of using FF-OCT in a real-time manner as a label-free non-invasive imaging technique in an intraoperative neurosurgical clinical setting to assess tumorous glial and epileptic margins. © 2013 The Authors.

Résumé: We report on the experimental and theoretical study of the spatial fluctuations of the local density of states (EM-LDOS) and of the fluorescence intensity in the near-field of a gold nanoantenna. EM-LDOS, fluorescence intensity and topography maps are acquired simultaneously by scanning a fluorescent nanosource grafted on the tip of an atomic force microscope at the surface of the sample. The results are in good quantitative agreement with numerical simulations. This work paves the way for a full near-field characterization of an optical nanoantenna. © 2013 Optical Society of America.

Résumé: Object. This work aimed at evaluating the accuracy of MR-guided high-intensity focused ultrasound (MRgHIFU) brain therapy in human cadaver heads. Methods. Eighteen heads of fresh human cadavers were removed with a dedicated protocol preventing intracerebral air penetration. The MR images allowed determination of the ultrasonic target: a part of the thalamic nucleus ventralis intermedius implicated in essential tremor. Osseous aberrations were corrected with simulation-based time reversal by using CT data from the heads. The ultrasonic session was performed with a 512-element phased-array transducer system operating at 1 MHz under stereotactic conditions with thermometric real-time MR monitoring performed using a 1.5-T imager. Results. Dissection, imaging, targeting, and planning have validated the feasibility of this human cadaver model. The average temperature elevation measured by proton resonance frequency shift was 7.9°C ± 3°C. Based on MRI data, the accuracy of MRgHIFU is 0.4 ± 1 mm along the right/left axis, 0.7 ± 1.2 mm along the dorsal/ventral axis, and 0.5 ± 2.4 mm in the rostral/caudal axis. Conclusions. Despite its limits (temperature, vascularization), the human cadaver model is effective for studying the accuracy of MRgHIFU brain therapy. With the 1-MHz system investigated here, there is millimetric accuracy. ©AANS, 2013.

Résumé: This paper extends the method of particular solutions (MPS) to the computation of eigenfrequencies and eigenmodes of thin plates, in the framework of the Kirchhoff-Love plate theory. Specific approximation schemes are developed, with plane waves (MPS-PW) or Fourier-Bessel functions (MPS-FB). This framework also requires a suitable formulation of the boundary conditions. Numerical tests, on two plates with various boundary conditions, demonstrate that the proposed approach provides competitive results with standard numerical schemes such as the finite element method, at reduced complexity, and with large flexibility in the implementation choices. © 2013 Springer-Verlag Berlin Heidelberg.

Résumé: Peripheral vascular disease is a frequently occurring disease and is most often caused by atherosclerosis and more rarely by anomalies of the collagen or other components of the arterial wall. Arterial stiffness problems form one of the precursor phenomena of peripheral vascular disease, and in the case of atherosclerosis represents an independent risk marker for the occurrence of cardiovascular disease. The first techniques, developed to evaluate arterial stiffness, use indirect measurements such as pulse wave velocity or the analysis of variations in pressure and volume to estimate arterial wall stiffness. Techniques based on the pulse wave lack precision because they assume that arterial stiffness is uniform throughout the path of the pulse wave, and that it is constant throughout the cardiac cycle. Moreover, measuring the velocity of the pulse wave may be less precise in certain pathological situations: metabolic syndrome, obesity, large chest, mega-dolico artery. Techniques based on the analysis of variations in pressure and in volume do not accurately measure blood pressure, which can only be taken externally. In addition, these techniques require dedicated equipment, which is not reimbursed by the French health care system, and which is cumbersome to use (especially for techniques based on variation in pressure) in clinical practice. This explains why these two techniques are not used in clinical practice. Ultrafast echography is a new ultrasound imaging method that can record up to 10,000 images per second. This high temporal resolution makes it possible to measure the velocity of the local pulse wave and arterial wall stiffness thanks to the remote palpation carried out by shear wave. The ease of use and the accuracy of these two techniques suggest that these diagnostic applications will play a significant role in vascular pathology in the future. It is possible in real time, using a traditional vascular ultrasound probe, to make an accurate assessment of local arterial stiffness and of its variation during the cardiac cycle. This technological breakthrough will probably improve phenotype evaluation of patients suffering from vascular diseases, to more effectively evaluate the cardiovascular risk for patients, at primary and secondary prevention level, and to carry out broad epidemiological studies on cardiovascular risks. © 2013.

Résumé: The clinical part of these Guidelines and Recommendations produced under the auspices of the European Federation of Societies for Ultrasound in Medicine and Biology EFSUMB assesses the clinically used applications of all forms of elastography, stressing the evidence from meta-analyses and giving practical advice for their uses and interpretation. Diffuse liver disease forms the largest section, reflecting the wide experience with transient and shear wave elastography. Then follow the breast, thyroid, gastro-intestinal tract, endoscopic elastography, the prostate and the musculo-skeletal system using strain and shear wave elastography as appropriate. The document is intended to form a reference and to guide clinical users in a practical way. © Georg Thieme Verlag KG Stuttgart, New York.

Résumé: Acousto-optic imaging is a technique that maps the optical properties of a thick scattering sample with millimetric resolution. The detection of the acousto-optic signal represents a challenge, because it is very weak among a strong parasitic signal. Various methods based on holography in photorefractive crystals or digital holography have been studied. Here dynamic holography is obtained with the gain medium Nd:YVO4. We study the experimental feasibility of a detection system based on holography in a gain medium and show acousto-optic results obtained in a 5 mm slice of chicken breast. © 2013 Optical Society of America.

Résumé: In this letter, we study metamaterials made out of resonant electric wires arranged on a spatial scale much smaller than the free space wavelength, and we show that they present a hybridization band that is insensible to positional disorder. We experimentally demonstrate defect cavities in disordered and ordered samples and prove that, analogous to those designed in photonic crystals, those cavities can present very high quality factors. In addition, we show that they display mode volumes much smaller than a wavelength cube, owing to the deep subwavelength nature of the unit cell. We underline that this type of structure can be shrunk down to a period close of a few skin depth. Our approach paves the way towards the confinement and manipulation of waves at deep subwavelength scales in both ordered and disordered metamaterials. © 2013 AIP Publishing LLC.

Résumé: Resonances usually result from wave superpositions in cavities where they are due to the wave spatio-temporal folding imposed by the boundaries. These energy accumulations are the signature of the cavity eigenmodes. Here we study a situation in which wave superposition results from the motion of a source emitting sustained overlapping waves. It is found that resonances can be produced in an unbounded space, the boundary conditions being now defined by the trajectory. When periodic trajectories are investigated, it is found that for a discrete subset of orbits, resonant wave modes are excited. Trajectory eigenmodes thus emerge. These modes have three attributes. Their associated resonant wave fields are the Fourier transform of the source's trajectory. They are non-radiative and they satisfy the perimeter Bohr-Sommerfeld quantization rule. © Copyright EPLA, 2013.

Résumé: Focused ultrasound uses an ultrasound beam generated from a large transducer or array of transducers, which is focused to a small focal point (similar to a magnifying glass) to exert an effect in deep tissue, with the intent of sparing surrounding tissues from the effect. Focused ultrasound can also be performed through the skin or natural orifices, usually with the transducer coupled to the surface with fluid. Incisions and needles are not typically needed. Focused ultrasound has many current and potential medical applications ranging from creating transient cellular membrane permeability to thermal heating for tumor and diseased tissue ablation. © 2013 Elsevier Inc. All rights reserved.

Résumé: The surface of thin-film solar cells can be tailored with photonic nanostructures to allow light trapping in the absorbing medium. This in turn increases the optical thickness of the film and thus enhances their absorption. Such a coherent light trapping is generally accomplished with deterministic photonic architectures. Here, we experimentally explore the use of a different nanostructure, a disordered one, for this purpose. We show that the disorder-induced modes in the film allow improvements in the absorption over a broad range of frequencies and impinging angles. © 2013 Optical Society of America.

Résumé: We present a semiconductor-based approach to compensate plasmonic losses. The core idea is to employ an electrically pumped laser diode and to overlap its active region with the evanescent field of a surface plasmon wave. In order to keep the losses at a manageable level, we rely on hybrid waveguide modes stemming from the coupling of a dielectric and a plasmonic mode. The laser device we demonstrate operates-at telecom wavelengths-on such a hybrid plasmonic mode. The device operates by electrical injection at room temperature. The near-field imaging of the laser facet provides evidence of the stimulated emission into the hybrid mode and confirms the prediction of the numerical simulations. © 2013 American Institute of Physics.

Résumé: Surface chemical reactivity is imaged by combining electrochemical activation of a surface transformation process with spatiotemporal ellipsometric microscopy. An imaging ellipsometric microscope is built, allowing ellipsometric images of surfaces with a lateral resolution of ∼1 μm and a thickness sensitivity of ∼0.1 nm in air and 0.4 nm in a liquid. These performances are particularly adapted for using such optical setup as an in situ, real time chemical microscope to observe a chemical surface transformation. This microscope is tested for the monitoring of the electrochemically actuated diazonium grafting of a gold surface. Such reaction is a model system of organic material deposition on a gold surface induced by an electrochemical actuation. Using either plain or physically or chemically structured electrodes, it allows for the characterization of local phenomena associated with the electrografting process. This illustrates its potential to reveal the local (electro)chemical reactivity of surfaces. © 2013 American Chemical Society.

Résumé: As manifested in the similarity relation of diffuse light transport, it is difficult to assess single scattering characteristics from multiply scattered light. We take advantage of the limited validity of the diffusion approximation of light transport and demonstrate, experimentally and numerically, that even deep into the multiple scattering regime, time-resolved detection of transmitted light allows simultaneous assessment of both single scattering anisotropy and scattering mean free path, and therefore also macroscopic parameters like the diffusion constant and the transport mean free path. This is achieved via careful assessment of early light and matching against Monte Carlo simulations of radiative transfer. © 2013 Optical Society of America.

Résumé: The opening and closing of nanocavities in a model soft nanocomposite subjected to cyclic uniaxial tension were directly studied by real-time small-angle X-ray scattering (SAXS). The volume fraction and average shape of the nanocavities have been detected by a pronounced increase in the scattering invariant Q/Q0 and a detailed analysis of the scattering patterns. Cavities appear upon loading past an intrinsic stress σint or intrinsic elongation λint. Upon unloading, nanocavities are progressively closed until the volume void fraction ψvoid reaches 0 for a constant "closure stress" of about 3.5 MPa. As the sample is reloaded, no cavities are observed when the current elongation remains below the maximum elongation of previous cycles λmax(N - 1) (N is the number of the cycles). Above this elongation, the void volume fraction ψvoid of the sample increases again. In contrast with ψvoid, the cumulative void volume fraction ψcum- void appearing in the sample to reach a given maximum historical elongation, λmax, or equivalently maximum historical stress σmax, was found to be independent of loading history. Both results point toward a process of creation of nanovoids in confined rubber domains that have not previously cavitated rather than to the reopening of the previously created cavities. All critical cavitation parameters display a strong memory effect, mostly captured in this uniaxial test by the maximum historical stress or elongation. The closure stress probably results from the Laplace pressure. A mechanism based on the rearrangement of filler agglomerates by strong shear stress after the emergence of nanocavities is proposed to account for the formation and release of the local geometric confinement and the non-reopening of the previously opened nanovoids upon reloading. © 2013 American Chemical Society.

Résumé: The concept of cross density of states characterizes the intrinsic spatial coherence of complex photonic or plasmonic systems, independently of the illumination conditions. Using this tool and the associated intrinsic coherence length, we demonstrate unambiguously the spatial squeezing of eigenmodes on disordered fractal metallic films, thus clarifying a basic issue in plasmonics. © 2013 American Physical Society.

Résumé: Ultrasound propagation in clusters of elliptic (two-dimensional) or ellipsoidal (three-dimensional) scatterers randomly distributed in a fluid is investigated numerically. The essential motivation for the present work is to gain a better understanding of ultrasound propagation in trabecular bone. Bone microstructure exhibits structural anisotropy and multiple wave scattering. Some phenomena remain partially unexplained, such as the propagation of two longitudinal waves. The objective of this study was to shed more light on the occurrence of these two waves, using finite-difference simulations on a model medium simpler than bone. Slabs of anisotropic, scattering media were randomly generated. The coherent wave was obtained through spatial and ensemble-averaging of the transmitted wavefields. When varying relevant medium parameters, four of them appeared to play a significant role for the observation of two waves: (i) the solid fraction, (ii) the direction of propagation relatively to the scatterers orientation, (iii) the ability of scatterers to support shear waves, and (iv) a continuity of the solid matrix along the propagation. These observations are consistent with the hypothesis that fast waves are guided by the locally plate/bar-like solid matrix. If confirmed, this interpretation could significantly help developing approaches for a better understanding of trabecular bone micro-architecture using ultrasound. © 2013 Acoustical Society of America.

Résumé: In this paper, we study the possibility of using lifetime data to estimate the position and orientation of a fluorescent dipole source within a disordered medium. The vector Foldy-Lax equations are employed to calculate the interaction between the fluorescent source and the scatterers that are modeled as point-scatterers. The numerical experiments demonstrate that if good prior knowledge about the positions of the scatterers is available, the position and orientation of the dipole source can be retrieved from its lifetime data with precision. If there is uncertainty about the positions of the scatterers, the dipole source position can be estimated within the same level of uncertainty. © 2013 Optical Society of America.

Résumé: Photorefractive Bi12SiO20 single crystal is used for acousto-optic imaging in thick scattering media in the green part of the spectrum, in an adaptive speckle correlation configuration. Light fields at the output of the scattering sample exhibit typical speckle grains of 1 μm size within the volume of the nonlinear crystal. This heterogeneous illumination induces a complex refractive index structure without applying a reference beam on the crystal, leading to a self-referenced diffraction correlation scheme. We demonstrate that this simple and robust configuration is able to perform axially resolved ultrasound modulated optical tomography of thick scattering media with an improved optical etendue. © 2013 Optical Society of America.

Résumé: We report a demonstration of phase-resolved vibrometry, in which out-of-plane sinusoidal motion is assessed by heterodyne holography. In heterodyne holography, the beam in the reference channel is an optical local oscillator (LO). It is frequency-shifted with respect to the illumination beam to enable frequency conversion within the sensor bandwidth. The proposed scheme introduces a strobe LO, where the reference beam is frequency-shifted and modulated in amplitude, to alleviate the issue of phase retrieval. The strobe LO is both tuned around the first optical modulation sideband at the vibration frequency, and modulated in amplitude to freeze selected mechanical vibration states sequentially. The phase map of the vibration can then be derived from the demodulation of successive vibration states. © 2013 Optical Society of America.

Résumé: The method of the time reversal operator decomposition is usually employed to detect and characterize static targets using the invariants of the time reversal operator. This paper presents a theoretical and experimental investigation into the impact of small displacements of the target on these invariants. To find these invariants, the time reversal operator is built from the multistatic response matrix and then diagonalized. Two methods of recording the multistatic response matrix while the target is moving are studied: Acquisition either element by element or column by column. It is demonstrated that the target displacement generates new significant eigenvalues. Using a perturbation theory, the analytical expressions of the eigenvalues of the time-reversal operator for both acquisition methods are derived. We show that the distribution of the new eigenvalues strongly depends on these two methods. It is also found that for the column by column acquisition, the second eigenvector is simply linked to the scatterer displacements. At last, the implications on the Maximum Likelihood and Multiple Signal Classification detection are also discussed. The theoretical results are in good agreement with numerical and 3.4 MHz ultrasonic experiments. © 2013 Acoustical Society of America.