Articles 2010

Résumé: Thermal conductivity values of several laser materials were determined by photothermal measurements and compared to predicted values. The effect of the ytterbium doping concentration could be modeled using a simple cationic mass difference model. For ytterbium content corresponding to an absorption coefficient of 10 cm-1 (or 85% absorption of the pump power) Yb: GdVO4 (c-axis) and Yb: Gd3 Ga5O12 laser materials present the highest measured thermal conductivity values of 8.1 W m-1 K-1 and 7.7 W m-1 K-1, respectively. Yb: Y3 Al5O12, Yb: Lu 3Al5O12, Yb: CaGdAlO4, and Yb: YAlO3 belong to the 6-7 W m-1 K-1 range while Yb: CaF2 and Yb: Lu2 SiO5 are situated in the 5-6 W m-1 K-1 range. Other matrices such as Yb: SrLaGa3O7, Yb: CaGdAl3O7, Yb: SrGdGa3O7, and Yb: BaLaGa3O7 have thermal conductivity values lower than 5 W m-1 K-1. With the knowledge of the thermal conductivities and the determination of the thermal expansion coefficients, thermal shock parameters are evaluated for several ytterbium doped laser hosts. © 2010 American Institute of Physics.

Résumé: This paper is devoted to mathematical modeling in photoacoustic imaging of small absorbers. We propose a new method for reconstructing small absorbing regions inside a bounded domain from boundary measurements of the induced acoustic signal. We also show the focusing property of the back-propagated acoustic signal. Indeed, we provide two different methods for locating a targeted optical absorber from boundary measurements of the induced acoustic signal. The first method consists of a MUltiple SIgnal Classification (MUSIC)-type algorithm and the second one uses a multifrequency approach. We also show results of computational experiments to demonstrate efficiency of the algorithms. © 2010 Society for Industrial and Applied Mathematics.

Résumé: We demonstrate a compact, integrated device in which surface plasmon polaritons (SPPs) are injected into a passive metal waveguide. We directly excite a SPP mode at a metal-air interface using a room-temperature midinfrared quantum cascade laser which is integrated onto the microchip. The SPP generation relies on end-fire coupling and is demonstrated via both far-field and near-field imaging techniques in the midinfrared. On one hand, a metallic diffraction grating is used to scatter in the far-field a portion of the propagating SPPs, thus allowing their detection with a microbolometer camera. On the other hand, direct images of the generated SPPs in the near-field were collected with a scanning optical microscope. © 2010 American Institute of Physics.

Résumé: Acoustic scattering by an ensemble of scatterers whose positions are close to the positions of a periodic arrangement (with small and random perturbations in the position of each scatterer) is considered in one dimension. Three methods are compared to obtain the effective wavenumber of the coherent field. The first two methods, the quasi-crystalline approximation (QCA) and the coherent potential approximation (CPA), give the exact Floquet solution in the periodic case. However, in a perturbed almost periodic configuration, they give different dispersion relations. These methods are compared to a perturbation approach and confronted with the results obtained from direct numerical calculations. It is shown that CPA is able to get the first correction to the Floquet dispersion relation due to the introduced perturbation, in agreement with the perturbation approach and with direct numerical results, while QCA is unable to get this correction. © 2010 Taylor & Francis.

Résumé: We measure the statistical distribution of the local density of optical states (LDOS) on disordered semicontinuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale. © 2010 The American Physical Society.

Résumé: A droplet bouncing on a liquid bath can self-propel due to its interaction with the waves it generates. The resulting "walker" is a dynamical association where, at a macroscopic scale, a particle (the droplet) is driven by a pilot-wave field. A specificity of this system is that the wave field itself results from the superposition of the waves generated at the points of space recently visited by the particle. It thus contains a memory of the past trajectory of the particle. Here, we investigate the response of this object to forces orthogonal to its motion. We find that the resulting closed orbits present a spontaneous quantization. This is observed only when the memory of the system is long enough for the particle to interact with the wave sources distributed along the whole orbit. An additional force then limits the possible orbits to a discrete set. The wave-sustained path memory is thus demonstrated to generate a quantization of angular momentum. Because a quantum-like uncertainty was also observed recently in these systems, the nonlocality generated by path memory opens new perspectives.

Résumé: The theory of monochromatic time-reversal mirrors (TRM) or equivalently phase conjugate mirrors is developed for electromagnetic waves. We start from the fundamental time-symmetry of the Maxwell's equations. From this symmetry, a differential expression similar to the Lorentz reciprocity theorem is deduced. The radiating conditions on TRM are expressed in terms of 6-dimension Green's functions. To predict the time reversal focusing on antenna arrays, a formalism that involves impedance matrix is developed. We show that antenna coupling can dramatically modify the focal spot. Especially, we observe, that in some circumstances, sub-wavelength focusing on a bi-dimensional array may arise. © 2010 IEEE.

Résumé: We investigate the angular redistribution of light radiated by a single emitter located in the vicinity of dipolar silver nanoparticles. We point out the fundamental role of the phase differences introduced by the optical path difference between the emitter and the particle and demonstrate that the polarizability of the metallic nanoparticle alone cannot predict the emission directionality. In particular, we show that collective or reflective properties of single nanoparticles can be controlled by tuning the distance of a single emitter at a λ/30 scale. These results enable us to design unidirectional and ultracompact nanoantennas composed of just two coupled nanoparticles separated by a distance achievable with biological linkers. © 2010 The American Physical Society.

Résumé: A stand-alone ultrasound shear wave imaging technology has been developed to quantify and visualize Young's modulus distribution by remotely applying ultrasound radiation force and tracking the resulting microvibrations in soft tissues with ultrafast ultrasound imaging. We report the first preliminary data that detected the distribution of local muscle stiffness within and between resting and contracting muscles at different muscle lengths with this technology. This technique may assist clinicians in characterizing muscle injuries or neuromuscular disorders. © 2010 Wiley Periodicals, Inc.

Résumé: We present a study of the optical properties of three-armed square nanospirals made of silver and realized as nanostructured thin films with Glancing Angle Deposition. Calculation of current flows in the nanospirals show excited resonant modes resembling those observed in U-shaped resonators. Four principal resonances were determined: near 200 THz and 480 THz for one polarization and 250 THz and 650 THz for the polarization orthogonal to the first one. In particular, a mode with antiparallel current flow in opposite arms, associated with the observed resonance near 650 THz, indicates the existence of a magnetic-like resonance in the square nanospiral arrays. The robustness of the resonances against variations in the structural parameters of the nanospirals was investigated. This study revealed that the main parameter driving the position of the resonances was the overall dimension of the nanospiral, directly related to the length of their arms. Optical properties of a sample were measured by generalized spectroscopic ellipsometry at near-normal incidence, and evidence conversion between polarization states even for light polarized in the plane containing one of the arms in agreement with the numerical study. The measurements compared favorably to the results of the numerical simulations taking into account the disorder in the sample. © 2010 Optical Society of America.

Résumé: The effective mass density of an inhomogeneous medium is discussed. Random configurations of circular cylindrical scatterers are considered, in various physical contexts: fluid cylinders in another fluid, elastic cylinders in a fluid or in another solid, and movable rigid cylinders in a fluid. In each case, time-harmonic waves are scattered, and an expression for the effective wavenumber due to Linton and Martin [J. Acoust. Soc. Am. 117, 3413-3423 (2005)] is used to derive the effective density in the low frequency limit, correct to second order in the area fraction occupied by the scatterers. Expressions are recovered that agree with either the Ament formula or the effective static mass density, depending upon the physical context. © 2010 Acoustical Society of America.

Résumé: The acoustic guided wave propagation for plate thickness measurement is generally treated in free space to simplify the formal approach. In this paper, propagation in a closed environment is treated by dealing with plates of finite dimensions and arbitrary geometries and linear boundary conditions. The present approach considers the plate Green's function to be composed of two terms. The first term corresponds to the Green's function of an infinite plate. The second term corresponds to a correction term which, in addition to the first term, satisfies all equations. Assuming the boundary conditions to be linear, it is found that the acoustic wave generated by a point source is proportional to that of a circular array of sources centered on it. By measuring the ratio between the two signals, either the plate velocity or plate thickness can be determined. This new method has been successfully applied to isotropic and anisotropic homogeneous plates of different geometries, on inhomogeneous plates, and also in a passive mode, without an active transmitter. © 2010 IEEE.

Résumé: Purpose: To determine the appearance of breast lesions at quantitative ultrasonographic (US) elastography by using supersonic shear imaging (SSI) and to assess the correlation between quantitative values of lesion stiffness and pathologic results, which were used as the reference standard. Materials and Methods: This study was approved by the French National Committee for the Protection of Patients Participating in Biomedical Research Programs. All patients provided written informed consent. Conventional US and SSI quantitative elastography were performed in 46 women (mean age, 57.6 years;age range, 38-71 years) with 48 breast lesions (28 benign, 20 malignant;mean size, 14.7 mm);pathologic results were available in all cases. Quantitative lesion elasticity was measured in terms of the Young modulus (in kilopascals). Sensitivity, specifi city, and area under the curve were obtained by using a receiver operating characteristic curve analysis to assess diagnostic performance. Results: All breast lesions were detected at SSI. Malignant lesions exhibited a mean elasticity value of 146.6 kPa ± 40.05 (standard deviation), whereas benign ones had an elasticity value of 45.3 kPa ± 41.1 (P < .001). Complicated cysts were differentiated from solid lesions because they had elasticity values of 0 kPa (no signal was retrieved from liquid areas). Conclusion: SSI provides quantitative elasticity measurements, thus adding complementary information that potentially could help in breast lesion characterization with B-mode US. © RSNA, 2010.

Résumé: In this study, Silicon-On-Diamond (SOD) micro-structures have been fabricated using either Smart Cut™ or bonded and Etched-Back Silicon On Insulator (BESOI) technology. Thanks to the development of an innovative smoothening process, polycrystalline diamond layers (C*) can be integrated as a buried oxide layer offering new opportunities in terms of thermal management. We describe different technological process flow investigations leading to SOD by bonding C* layer in the stack. As starting material we used poly-crystalline thin diamond films in the 200 nm to 7000 nm range of thickness. The C* is deposited by Chemical Vapour Deposition assisted by Microwave Plasma (MPCVD) onto various 50 mm wafers such as Si, SOI and polycrystalline silicon carbide (pSiC). As the roughness of the diamond layer is not directly compatible with a wafer bonding integration, an innovative smoothening process in 3 steps has been developed and named "DPE" for Deposition, Planarization and Etching. Using the DPE process, the roughness of 5 μm thick diamond layer could be reduced from 50 to 3 nm RMS and down to 1.5 nm RMS for a thin 200 nm layer. In order to demonstrate the feasibility of a GaN on SOD micro-structure design for HEMT applications, layer transfers have been carried out by a bonding and thinning process from C*/Si bulk using oxide bonding layers. From thermal spreading efficiency consideration, new processes of fabrication of SOD/poly-SiC substrate are in progress involving BESOI or Si Smart Cut™ technologies and poly-Si bonding layer starting from C*/poly-SiC. Pure SOD substrate were also fabricated by using C*/SOI and poly-Si bonding layer in a BESOI technology. A thin active silicon layer (70 nm) of 50 mm diameter onto a 140 nm thick diamond BOX layer has been transferred on 200 mm diameter Si substrate for future MOSFET's devices demonstrations. Significant progress has been done in diamond layer integration by wafer bonding. © 2010 Elsevier B.V. All rights reserved.

Résumé: We study the number of propagating Bloch modes NB of an infinite periodic billiard chain. The asymptotic semiclassical behavior of this quantity depends on the phase-space dynamics of the unit cell, growing linearly with the wave number k in systems with a non-null measure of ballistic trajectories and going like ∼√k in diffusive systems. We have calculated numerically NB for a waveguide with cosine-shaped walls exhibiting strongly diffusive dynamics. The semiclassical prediction for diffusive systems is verified to good accuracy and a connection between this result and the universality of the parametric variation of energy levels is presented. © 2010 The American Physical Society.

Résumé: We report laser Doppler ophthalmoscopic fundus imaging in the rat eye with near-IR heterodyne holography. Sequential sampling of the beat of the reflected radiation against a frequency-shifted optical local oscillator is made onto an array detector. Wide-field maps of fluctuation spectra in the 10 Hz to 25 kHz band exhibit angiographic contrasts in the retinal vascular tree without requirement of an exogenous marker. © 2010 Optical Society of America.

Résumé: (Figure Presented) Chemical nanosensors with a submicrometer core-shell composite design, based on a polymer core, a molecularly imprinted polymer (MIP) shell for specific analyte recognition, and an interlayer of gold nanoparticles for signal amplification, are described. SERS measurements on single nanosensors yield detection limits of 10-7 M for the β-blocker propranolol, several orders of magnitude lower than on plain MIP spheres. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Résumé: We study the statistics of the fluorescence decay rate of a dipole emitter embedded in a strongly scattering medium. In the multiple-scattering regime, the probability of observing a decrease in the decay rate is substantial, as predicted by exact numerical simulations. The decrease originates from a reduction of the local density of optical states and is due to collective interactions and interferences. In the strong-scattering regime, signatures of recurrent scattering are visible in the behavior of the average decay rate. © 2010 The American Physical Society.

Résumé: We introduce the resonant metalens, a cluster of coupled subwavelength resonators. Dispersion allows the conversion of subwavelength wave fields into temporal signatures while the Purcell effect permits an efficient radiation of this information in the far field. The study of an array of resonant wires using microwaves provides a physical understanding of the underlying mechanism. We experimentally demonstrate imaging and focusing from the far field with resolutions far below the diffraction limit. This concept is realizable at any frequency where subwavelength resonators can be designed. © 2010 The American Physical Society.

Résumé: At subwavelength distance from the exit surface of a disordered medium, speckle patterns generated by multiple scattering of waves exhibit nonuniversal near-field correlations. A calculation of the field spatial correlation function shows that the correlation length is driven by the microscopic structure of the medium. The averaged speckle spot size can be smaller than the wavelength, even for nonresonant dielectric media. © 2010 The American Physical Society.

Résumé: The decomposition of the time reversal operator, known by the French acronym DORT, is a technique to extract point scatterers monochromatic Greens functions from a medium. It is used to detect, locate, and focus on scatterers in various domains such as underwater acoustics, medical ultrasound, and nondestructive evaluation. A limitation of the method arises from its single-frequency nature, when the signals used in acoustics are often broadband. Reconstruction of the broadband Greens functions from the single-frequency Greens functions can be very difficult when numerous scatterers are present in the medium. Moreover, the method does not take advantage of the axial resolution associated with broadband signals. Time domain methods are investigated here as an answer to these problems. It is shown that the time reversal operator in the time domain takes the form of a tensor. The properties of the invariants are discussed. It is shown they do not have all the expected properties. Another method is proposed that requires a priori information on the medium. © 2010 Acoustical Society of America.

Résumé: A noncontact laser-based ultrasonic technique is proposed for detecting small plate thickness variations caused by corrosion and adhesive disbond between two plates. The method exploits the resonance at the minimum frequency of the S1 Lamb mode dispersion curve. At this minimum frequency, the group velocity vanishes, whereas the phase velocity remains finite. The energy deposited by the laser pulse generates a local resonance of the plate. This vibration is detected at the same point by an optical interferometer. First experiments show the ability to image a 1.5-m deep corroded area on the back side of a 0.5-mm-thick duralumin plate. Because of the finite wavelength of the S1- zero group velocity (ZGV) mode, the spatial resolution is limited to approximately twice the plate thickness. With the same technique we investigate the state of adhesive bonds between duralumin and glass plates. The S 1-Lamb mode resonance is strongly attenuated when plates are rigidly bonded. In the case of thin adhesive layers, we observed other resonances, associated with ZGV modes of the multi-layer structure, whose frequencies and amplitudes vary with adhesive thickness. Experiments were carried out on real automotive adhesively bonded structures and the results were compared with images obtained by X-ray radiography. © 2010 IEEE.

Résumé: The in vivo assessment of the biomechanical properties of the skeletal muscle is a complex issue because the muscle is an anisotropic, viscoelastic and dynamic medium. In this article, these mechanical properties are characterized for the brachialis muscle in vivo using a noninvasive ultrasound-based technique. This supersonic shear imaging technique combines an ultra-fast ultrasonic system and the remote generation of transient mechanical forces into tissue via the radiation force of focused ultrasonic beams. Such an ultrasonic radiation force is induced deep within the muscle by a conventional ultrasonic probe and the resulting shear waves are then imaged with the same probe (5 MHz) at an ultra-fast framerate (up to 5000 frames/s). Local tissue velocity maps are obtained with a conventional speckle tracking technique and provide a full movie of the shear wave propagation through the entire muscle. Shear wave group velocities are then estimated using a time of flight algorithm. This approach provides a complete set of quantitative and in vivo parameters describing the muscle's mechanical properties as a function of active voluntary contraction as well as passive extension of healthy volunteers. Anisotropic properties are also estimated by tilting the probe head with respects to the main muscular fibers direction. Finally, the dispersion of the shear waves is studied for these different configurations and shear modulus and shear viscosity are quantitatively assessed assuming the viscoelastic Voigt's model. © 2010 World Federation for Ultrasound in Medicine & Biology.

Résumé: In this paper, we consider cracks with Dirichlet boundary conditions. We first derive an asymptotic expansion of the boundary perturbations that are due to the presence of a small crack. Based on this formula, we design a noniterative approach for loc ating a collection of small cracks. In order to do so, we construct a response matrix from the boundary measurements. The location and the length of the crack are estimated, respectively, from the pro jection onto the noise space and the first significant singular value of the response matrix. Indeed, the direction of the crack is estimated from the second singular vector. We then consider an extended crack with Dirichlet boundary conditions. We rigorously derive an asymptotic expansion for t he boundary perturbations that are due to a shape deformation of the crack. To reconstruct an extended crack from many boundary measurements, we develop two methods for obtaining a good guess. Several numerical experiments show how the proposed techniques for imaging small cracks as well as those for obtaining good initial guesses toward reconstructing an extended crack behave. © 2010 Society for Industrial and Applied Mathematics.

Résumé: The decomposition of the time reversal operator (DORT method) is applied to electromagnetic waves in order to characterize a dielectric cylinder. It consists in determining the Time Reversal Invariants of the Time Reversal Operator. Here, this matrix is built from the inter-element responses between distinct transmit and receive arrays. In this paper experimental results obtained between 2 and 4 GHz are compared to a theoretical model, developed in an other paper (Minonzio , Theory of the time-reversal operator for a dielectric cylinder using separate transmit and receive arrays, IEEE Trans. Antennas Propag., vol. 57, pp. 23312340, 2009). The DORT method is then applied to an inverse problem to determine the diameter and the permittivity of the cylinder. It is shown experimentally that different experimental parameters can be estimated from the singular values of the time reversal operator. © 2006 IEEE.

Résumé: The excitation of an antisymmetric trapped mode on a symmetric metamaterial resonator is experimentally demonstrated. We use an active electronic device to break the electrical symmetry and therefore to generate this trapped mode on a symmetric spilt ring resonator. Even more, with such a tunable mode coupling resonator, we can precisely tune the resonant mode frequency. In this way, a shift of up to 15 percent is observed.

Résumé: We introduce a method to experimentally measure the monochromatic transmission matrix of a complex medium in optics. This method is based on a spatial phase modulator together with a full-field interferometric measurement on a camera. We determine the transmission matrix of a thick random scattering sample. We show that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. This method might give important insight into the mesoscopic properties of a complex medium. © 2010 The American Physical Society.

Résumé: For fluids, the theoretical investigation of shock wave reflection has a good agreement with experiments when the incident shock Mach number is large. But when it is small, theory predicts that Mach reflections are physically unrealistic, which contradicts experimental evidence. This von Neumann paradox is investigated for shear shock waves in soft elastic solids with theory and simulations. The nonlinear elastic wave equation is approximated by a paraxial wave equation with a cubic nonlinear term. This equation is solved numerically with finite differences and the Godunov scheme. Three reflection regimes are observed. Theory is developed for shock propagation by applying the Rankine-Hugoniot relations and entropic constraints. A characteristic parameter relating diffraction and non-linearity is introduced and its theoretical values are shown to match numerical observations. The numerical solution is then applied to von Neumann reflection, where curved reflected and Mach shocks are observed. Finally, the case of weak von Neumann reflection, where there is no reflected shock, is examined. The smooth but non-monotonic transition between these three reflection regimes, from linear Snell-Descartes to perfect grazing case, provides a solution to the acoustical von Neumann paradox for the shear wave equation. This transition is similar to the quadratic non-linearity in fluids. © 2010 Acoustical Society of America.

Résumé: This paper describes an imaging microscopic technique based on heterodyne digital holography where subwavelength-sized gold colloids can be imaged in cell environments. Surface cellular receptors of 3T3 mouse fibroblasts are labeled with 40 nm gold nanoparticles, and the biological specimen is imaged in a total internal reflection configuration with holographic microscopy. Due to a higher scattering efficiency of the gold nanoparticles versus that of cellular structures, accurate localization of a gold marker is obtained within a 3D mapping of the entire sample's scattered field, with a lateral precision of 5 mn and 100 nm in the x,y and in the z directions respectively, demonstrating the ability of holographic microscopy to locate nanoparticles in living cell environments. © 2010 Optical Society of America.

Résumé: The radiation of Lamb waves by an axisymmetric source on the surface of an anisotropic plate is investigated. An analytical expression of the Green's function, valid in the far field domain, is derived. This approximation shows that the anisotropy of the propagation medium induces a focusing of Lamb modes in some directions, which correspond to minima of the slowness. Numerical simulations and experiments demonstrate that for the fundamental A 0 and S 0 modes, this phenomenon, analog to the phonon focusing effect, can be very strong in materials such as composite fiber-reinforced polymers. This effect due to the plate anisotropy must be correctly taken into account, for example, in order to develop systems for in situ structural health monitoring. The choice of the most appropriate Lamb mode, the excitation frequency, and the design of the array of piezoelectric disks used as transmitters and receivers depends on such considerations. © 2010 Acoustical Society of America.

Résumé: Atoms can scatter light and they can also amplify it by stimulated emission. From this simple starting point, we examine the possibility of realizing a random laser in a cloud of laser-cooled atoms. The answer is not obvious as both processes (elastic scattering and stimulated emission) seem to exclude one another: pumping atoms to make them behave as an amplifier drastically reduces their scattering cross-section. However, we show that even the simplest atom model allows the efficient combination of gain and scattering. Moreover, the supplementary degrees of freedom that atoms offer allow the use of several gain mechanisms, depending on the pumping scheme. We thus first study these different gain mechanisms and show experimentally that they can induce (standard) lasing. We then present how the constraint of combining scattering and gain can be quantified, which leads to an evaluation of the random laser threshold. The results are promising and we draw some prospects for a practical realization of a random laser with cold atoms. © 2010 IOP Publishing Ltd.

Résumé: We study the spontaneous decay rate of a dipóle emitter close to a metallic nanoparticle in the extreme nearfield regime. The metal is modeled using a nonlocal dielectric function that accounts for the microscopic length scales of the free electron gas. We describe quantitatively the crossover between the macroscopic and microscopic regimes and the enhanced nonradiative decay due to microscopic interactions. Our theory is in agreement with results previously established in the asymptotic near- and far-field regimes. © 2010 Optical Society of America.

Résumé: Recent studies have demonstrated the feasibility of transcranial high-intensity focused ultrasound (HIFU) therapy in the brain using adaptive focusing techniques. However, the complexity of the procedures imposes provision of accurate targeting, monitoring and control of this emerging therapeutic modality in order to ensure the safety of the treatment and avoid potential damaging effects of ultrasound on healthy tissues. For these purposes, a complete workflow and setup for HIFU treatment under magnetic resonance (MR) guidance is proposed and implemented in rats. For the first time, tissue displacements induced by the acoustic radiation force are detected in vivo in brain tissues and measured quantitatively using motion-sensitive MR sequences. Such a valuable target control prior to treatment assesses the quality of the focusing pattern in situ and enables us to estimate the acoustic intensity at focus. This MR-acoustic radiation force imaging is then correlated with conventional MR-thermometry sequences which are used to follow the temperature changes during the HIFU therapeutic session. Last, pre- and post-treatment magnetic resonance elastography (MRE) datasets are acquired and evaluated as a new potential way to non-invasively control the stiffness changes due to the presence of thermal necrosis. As a proof of concept, MR-guided HIFU is performed in vitro in turkey breast samples and in vivo in transcranial rat brain experiments. The experiments are conducted using a dedicated MR-compatible HIFU setup in a high-field MRI scanner (7 T). Results obtained on rats confirmed that both the MR localization of the US focal point and the pre- and post-HIFU measurement of the tissue stiffness, together with temperature control during HIFU are feasible and valuable techniques for efficient monitoring of HIFU in the brain. Brain elasticity appears to be more sensitive to the presence of oedema than to tissue necrosis. © 2010 Institute of Physics and Engineering in Medicine.

Résumé: In this article, time reversal is used to generate high power microwave pulses from a low power arbitrary wave generator. We use a reverberation chamber with an aperture on the front face and we take advantage of the pulse compression property of time reversal. High amplitude peaks are generated outside the chamber thanks to the long spreading time of the signals inside. We study the amplitude of this peak and the width of the focal spot with respect to the different experimental parameters. A gain of 18 dB compared to a directive antenna of the same aperture is obtained. © 2010 Académie des sciences.

Résumé: Antenna arrays embedded within a compact Fabry-Perot cavity have to be optimized to limit interactions between elementary antennas. In this article, we propose to use a superstrate metamaterial to design a highly isolated antenna array. This superstrate is based on the microstrip technology. It is made of an inductive and a capacitive grid respectively etched on the top and the bottom of epoxy plate. Metallic vias are then inserted through the superstrate to connect the two etched grids. This superstrate combined with a metallic ground plane leads to the design of a subwavelength cavity. The proposed cavity allows a significant antenna profile reduction and performance enhancement. More importantly, this structure provides a high isolation for an antenna array. © 2010 VSP.