Arnaud TOURIN

Résumé: L'art d'innover au XXIe Siècle transcende la seule approche capitaliste et oblige à repenser le monde en y intégrant tous les défis pendants (effondrement de la biodiversité, transition écologique, réchauffement climatique, déplacements des populations, inégalités etc.). Cette prise de conscience planétaire a permis l'émergence de nouveaux entrepreneurs plus soucieux de l'avenir de l'humanité. Ce livre présente le rôle et la place faite à la recherche publique dans sa propension à innover en créant notamment des startups deeptech pour offrir des solutions aux grands maux de notre époque. Et si les contextes diffèrent d'une zone géographique à une autre, la quasi-totalité des États dans le monde misent sur la science pour se projeter dans l'avenir. En s'appuyant sur l'expérience particulière du laboratoire public « Institut Langevin Ondes et Images », ce livre propose un modèle d'innovation original, capable de répondre tout à la fois aux besoins des laboratoires publics français, aux chercheurs-entrepreneurs voire aux étudiants désireux de se lancer dans une aventure entrepreneuriale. Plus largement, il vise à rappeler à toutes les partie prenantes (politiciens, citoyens etc.) l'importance de conserver une recherche publique de qualité pour se confronter à un monde de plus en plus complexe.

Résumé: Damping is usually associated with irreversibility. Here, we present a counterintuitive concept to achieve time reversal of waves propagating in a lossless medium using a transitory dissipation pulse. Applying a sudden and strong damping in a limited time generates a time-reversed wave. In the limit of a high damping shock, this amounts to "freezing"the initial wave by maintaining the wave amplitude while canceling its time derivative. The initial wave then splits in two counterpropagating waves with half of its amplitude and time evolutions in opposite directions. We implement this damping-based time reversal using phonon waves propagating in a lattice of interacting magnets placed on an air cushion. We show with computer simulations that this concept also applies to broadband time reversal in complex disordered systems.

Résumé: Among the remarkable scattering properties of correlated disordered materials, the origin of pseudogaps and the formation of localized states are some of the most puzzling features. Fundamental differences between scalar and vector waves in both these aspects make their comprehension even more problematic. Here we present an in-depth and comprehensive analysis of the order-to-disorder transition in 2D resonant systems. We show with exact ab initio numerical simulations in finite-size hyperuniform media that localization of 2D vector waves can occur in the presence of correlated disorder, in a regime of moderate density of scatterers. On the contrary, no signature of localization is found for white noise disorder. This is in striking contrast with scalar waves, which localize at high density whatever the amount of correlation. For correlated materials, localization is associated with the formation of pseudogap in the density of states. We develop two complementary models to explain these observations. The first one uses an effective photonic crystal-type framework and the second relies on a diagrammatic treatment of the multiple scattering sequences. We provide explicit theoretical evaluations of the density of states and localization length in good agreement with numerical simulations. In this way, we identify the microscopic processes at the origin of pseudogap formation and clarify the role of the density of states for wave localization in resonant correlated media. The generality of our framework makes possible to apply our predictions for a large variety of scattering systems including dielectric structures with high quality factor, cold atoms, artificial atoms, as well as microwave resonators.

Résumé: Abstract|For the last 20 years, the time-reversal (TR) process has been successfully applied to focus pulses in the microwave frequency range and in complex media. Here we examine the specic conditions to obtain the same results but in the sub-THz frequency range. Because of the stronger attenuation at this much higher frequency, it is more challenging to exploit the TR self-focusing property. The TR of pulses is studied in two kinds of complex media: metallic waveguide and leaky reverberating cavity. For each medium, we propose one or two models to assess the quality of the focusing. For the waveguide, we show that the angle of incidence is an important parameter. Based on these results, we perform TR experiments at 273 GHz with a bandwidth that can be as large as 2 GHz. TR experiments are successfullyrst conducted in a 1m long and 10mm diameter straight hollow cylinder and then in a 5m long and 12mm diameter curved waveguide. Finally, we present results obtained in a cavity of 72 cm3 that leaks through a copper grid. The best focusing is observed with the longer waveguide.

Résumé: Determination of ultrasound scattering and intrinsic attenuations in heterogeneous media is of importance from material characterization to geophysical applications. Here, we present an efficient inverse method within a finite-size scattering medium, where boundary reflection plays a crucial role. To fit the energy profile of scattered coda waves, we solve the acoustic radiative-transfer equation by Monte Carlo simulations for cylinder and slab geometries, under the isotropic scattering approximation. We show that the fit with the simplistic radiative-transfer solution in an infinite medium may result in underestimated values of the scattering mean free path, ls, and absorption, Qi-1, by up to 40%. Our main finding is anomalous transport behavior in thin slab samples, where the ballistic peak and the diffusionlike one are merged into one single peak. This anomalous behavior, related to a wave-focusing effect in the forward direction, can mislead the inverse process and lead to an overestimation of ls by more than 200%. We compare simulated energy profiles with ultrasound envelopes obtained in a polycrystal-like granite slab from the ballistic to the diffusive regime. The ls deduced from off-axis detections agrees with that estimated from the correlation length of the shear-wave velocity by structural imaging analysis.

Résumé: A sound wave travelling in water is scattered by a periodic assembly of air bubbles. The local structure matters even in the low frequency regime. If the bubbles are arranged in a face-centered cubic (fcc) lattice, a total bandgap opens near the Minnaert resonance frequency. If they are arranged in the diamond structure, which one obtains by simply adding a second bubble to the unit cell, one finds an additional branch with a negative slope (optical branch). For a single specific frequency, the medium behaves as if its refractive index (relative to water) is exactly n=−1. We show that a slab of this material can be used to design a three-dimensional flat lens. We also report super-resolution focusing in the near field of the slab and illustrate its potential for imaging in three dimensions.

Résumé: © 2020 Author(s). We propose a phenomenological model, built from results obtained by finite-element numerical simulations, for the transmission and reflection of acoustic waves by a two-dimensional array of cylindrical cavities in a soft elastic medium. We show that the acoustic properties of a cylindrical cavity can be described by two geometrical parameters: its aspect ratio (AR) and the radius of the sphere of equivalent volume. Cylinders with AR close to one are acoustically similar to spheres, whereas flat cylinders exhibit a lower resonance frequency and an increased damping due to their ability to emit shear waves. We provide an example of how our new phenomenological analytical model can help to design thin coatings that can turn strong acoustic reflectors into good absorbers.

Résumé: © 2019, The Author(s). Observing and understanding the motion of an intruder through opaque dense suspensions such as quicksand remains a practical and conceptual challenge. Here we use an ultrasonic probe to monitor the sinking dynamics of a steel ball in a dense glass bead packing (3D) saturated by water. We show that the frictional model developed for dry granular media can be used to describe the ball motion induced by horizontal vibration. From this rheology, we infer the static friction coefficient and effective viscosity that decrease when increasing the vibration intensity. Our main finding is that the vibration-induced reduction of the yield stress and increase of the sinking depth are presumably due to micro-slips induced at the grain contacts but without visible plastic deformation due to macroscopic rearrangements, in contrast to dry granular packings. To explain these results, we propose a mechanism of acoustic lubrication that reduces the inter-particle friction and leads to a decrease of the yield stress. This scenario is different from the mechanism of liquefaction usually invoked in loosely packed quicksands where the vibration-induced compaction increases the pore pressure and decreases the confining pressure on the solid skeleton, thus reducing the granular resistance to external load.

Résumé: © 2017 American Physical Society. Using a multiple scattering theory algorithm, we investigate numerically the transmission of ultrasonic waves through a disordered locally resonant metamaterial containing only monopolar resonators. By comparing the cases of a perfectly random medium with its pair correlated counterpart, we show that the introduction of short range correlation can substantially impact the effective parameters of the sample. We report, notably, the opening of an acoustic transparency window in the region of the hybridization band gap. Interestingly, the transparency window is found to be associated with negative values of both effective compressibility and density. Despite this feature being unexpected for a disordered medium of monopolar resonators, we show that it can be fully described analytically and that it gives rise to negative refraction of waves.

Résumé: © 2017, The Author(s).Time reversal (TR) focusing of ultrasound in granular packings is experimentally investigated. Pulsed elastic waves transmitted from a compressional or shear transducer source are measured by a TR mirror, reversed in time and back-propagated. We find that TR of ballistic coherent waves onto the source position is very robust regardless driving amplitude but provides poor spatial resolution. By contrast, the multiply scattered coda waves offer a finer TR focusing at small amplitude by a lens effect. However, at large amplitude, these TR focusing signals decrease significantly due to the vibration-induced rearrangement of the contact networks, leading to the breakdown of TR invariance. Our observations reveal that granular acoustics is in between particle motion and wave propagation in terms of sensitivity to perturbations. These laboratory experiments are supported by numerical simulations of elastic wave propagation in disordered 2D percolation networks of masses and springs, and should be helpful for source location problems in natural processes.

Résumé: © 2015 AIP Publishing LLC. Gas bubbles in a sound field are submitted to a radiative force, known as the secondary Bjerknes force. We propose an original experimental setup that allows us to investigate in detail this force between two bubbles, as a function of the sonication frequency, as well as the bubbles radii and distance. We report the observation of both attractive and, more interestingly, repulsive Bjerknes force, when the two bubbles are driven in antiphase. Our experiments show the importance of taking multiple scatterings into account, which leads to a strong acoustic coupling of the bubbles when their radii are similar. Our setup demonstrates the accuracy of secondary Bjerknes forces for attracting or repealing a bubble, and could lead to new acoustic tools for noncontact manipulation in microfluidic devices.

Résumé: A bubble metascreen, i.e., a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the metascreen, acoustic superabsorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble metascreens can thus be used as ultrathin coatings for turning acoustic reflectors into perfect absorbers.

Résumé: We demonstrate, based on spatial and frequency resolved wave front shaping of ultrasound with a nonlinear feedback signal, how to achieve optimal spatiotemporal focusing through a complex scattering medium. © 2012 OSA.

Résumé: In this paper, we show how the concept of hybridization band gaps can be utilized to create antennas for MIMO applications. Those strongly decoupled antennas present at the same time a very small form factor and a very low correlation. To that aim, we first explain briefly the concept of hybridization between a resonator and the free space waves continuum. Then we expose the methodology we use to design multi-ports antennas based on that concept. We present numerical and experimental results of 2 ports MIMO antennas at 2.45 GHz, printed on a PCB, whose areas are smaller than 2.6*2.6 cm 2. The two ports display experimentally peak gains of a about 4 dB, efficiencies of 80%, a coupling lower than -30 dB and a correlation lower than 0.1. © 2012 IEEE.

Résumé: We propose a new method for monitoring temporal changes in a complex scattering environment. It is referred to as Time Reversal of the Coda wave Difference (TRECOD). The impulse responses of the probed medium are recorded between all the emitter/receiver pairs of a transceiver array. A Fourier transform of these responses then produces the so-called backscattering transfer matrix at each frequency of the transceiver bandwidth. The matrix acquisition is repeated and the image of the temporal changes occurring between two matrix acquisitions is formed by propagating numerically the significant singular vectors of the matrix difference. Small-scale experiments with ultrasound are used to validate the method and demonstrate its interest for the monitoring of a fluid injection in a porous medium behind a strongly scattering obstacle. © 2012 Copyright Taylor and Francis Group, LLC.

Résumé: In support of the French ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) reactor program, which aims to demonstrate the industrial applicability of sodium fast reactors with an increased level of safety demonstration and availability compared to the past French sodium fast reactors, emphasis is placed on reactor instrumentation. It is in this framework that CEA studies continuous core monitoring to detect as early as possible the onset of sodium boiling. Such a detection system is of particular interest due to the rapid progress and the consequences of a Total Instantaneous Blockage (TIB) at a subassembly inlet, where sodium boiling intervenes in an early phase. In this paper, the authors describe all the particularities which intervene during the different boiling stages and explore possibilities for their detection. © 2011 IEEE.

Résumé: Time reversal is a physical concept that can focus waves both spatially and temporally regardless of the complexity of the propagation medium. Time reversal mirrors have been demonstrated first in acoustics, then with electromagnetic waves, and are being intensively studied in many fields ranging from underwater communications to sensing. In this paper, we will review the principles of time reversal and in particular its ability to focus waves in complex media. We will show that this focusing effect depends on the complexity of the propagation medium rather than on the time reversal mirror itself. A modal approach will be utilized to explain the physical mechanism underlying the concept. A particular focus will be given on the possibility to break the diffraction barrier from the far field using time reversal. We will show that finite size media made out of coupled subwavelength resonators support modes which can radiate efficiently in the far field spatial information of the near field of a source. We will show through various examples that such a process, due to reversibility, permits to beat the diffraction limit using far field time reversal, and especially that this result occurs owing to the broadband inherent nature of time reversal. © 2011 Fabrice Lemoult et al.

Résumé: We report the practical realization of phononic crystals with gas inclusions, using soft lithography techniques. Ultrasonic experiments from 0.3 to 5 MHz confirm the existence of deep and wide minima of transmission through the crystal. We show that the first gap is due to the combined effects of Bragg reflections and bubble resonances. We propose a simple layered model that gives a reasonable prediction of the ultrasonic transmission. © 2009 American Institute of Physics.

Résumé: We experimentally study the effects of correlations in the propagation of ultrasonic waves in water from a multielement source to a multielement detector through a strongly scattering system of randomly placed vertical rods. Due to the strong scattering, the wave transport in the sample is in the diffusive regime. The correlation between the waves is induced when the distance between transducer elements is within the coherence region of the scattered sound. We measure the multichannel transfer matrix H, each element of which represents the signal strength between the m individual transmitters and n receivers. The observed eigenvalue distribution of the matrix H H† clearly shows the effect of correlations between channels and can be interpreted using random matrix theory. These results are of practical importance in many areas, e.g., for evaluating the information transfer capacity of such a complex scattering system. © 2008 The American Physical Society.

Résumé: Coherent backscattering of waves by a random medium is spectacular evidence of interference effects despite disorder and multiple scattering. It manifests itself as a doubling of the wave intensity reflected exactly in the backward direction. This phenomenon has been observed experimentally in optics, acoustics, or seismology. While optical measurements are realized in far-field conditions with a plane wave illumination and a beamwidth much larger than the wavelength, ultrasonic experiments are carried out with wideband controllable arrays of (nearly) pointlike transducers that directly record the wave field, in amplitude and phase. Therefore it is possible to perform beamforming of the incoming and outgoing wave fields before computing the average backscattered intensity. In this paper, the advantages of plane wave beamforming applied to the study of the coherent backscattering effect are shown. Particularly, the angular resolution, the signal-to-noise ratio, as well as the estimation of the enhancement factor can be improved by beamforming. Experimental results are presented with ultrasonic pulses, in the 2.5-3.5 MHz range, propagating in random collections of scatterers. Since the coherent backscattering effect can be taken advantage of to measure diffusive parameters (transport mean free path, diffusion constant), plane-wave beamforming can be applied to the characterization of highly scattering media. © 2007 Acoustical Society of America.

Résumé: Experimental measurements of the coherent wave transmission for ultrasonic waves propagating in water through a random set of scatterers (metallic rods) are presented. Though the densities are moderate (6% and 14%) the experimental results show that the mean-free path deviates from the classical first-order approximation due to the existence of correlations between scatterers. Theoretical results for the mean free path obtained from different approaches are compared to the experimental measurements. The best agreement is obtained with the second-order diagrammatic expansion of the self-energy. © 2006 The American Physical Society.

Résumé: In this letter, time reversal is applied to wideband electromagnetic waves in a reverberant room. To that end a multiantenna time reversal mirror (TRM) has been built. A 150 MHz bandwidth pulse at a central frequency of 2.45 GHz is radiated by a monopolar antenna, spread in time due to reverberation, recorded at the TRM, time reversed, and retransmitted. The time-reversed wave converges back to its source and focus in both time and space. The time compression is studied versus the number of antennas in the TRM and its bandwidth. The focal spot is also measured thanks to an eight-channel receiving array. © 2006 American Institute of Physics.

Résumé: [1] Time reversal (TR) communication in various configurations (single input, single output (SISO); multiple inputs, single output (MISO); or multiple inputs, multiple outputs (MIMO)) is studied. In particular, we report an experimental demonstration of time reversal focusing with electromagnetic waves in a SISO scheme. An antenna transmits a 1 μs electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity. A spatial focusing of the compressed pulse is also shown. This experiment is the first step for a communication scheme based on time reversal. It would be very interesting for ultrawideband communication in complex media since TR would permit compensation for delay spreading. MISO and MIMO TR communications are discussed on the basis of small-scale experiments with ultrasound. In particular, the binary error rate of the method is studied as a function of both data rate and external noise. A simple theoretical approach explains the results. Copyright 2005 by the American Geophysical Union.

Résumé: We present the acoustic analog of resonant tunneling through a double barrier in quantum mechanics. Pairs of identical phononic crystals, in both 2D and 3D, were assembled and separated by a uniform spacer, forming a resonant cavity. The ultrasonic transmission exhibits resonant peaks at frequencies inside the band gaps, where ultrasound tunneling through each phononic crystal occurs. On resonance, the measured group time is large and even predicted to increase exponentially with the thickness of the crystals in the absence of absorption, while off resonance very fast speeds are found. © EDP Sciences.

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 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: We present experimental results on the reversibility of ultrasound in a multiple scattering medium. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through 2D samples with various thickness. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed and sent back into the medium. The time-reversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the second-order moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation of the correlations on a single realisation of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by diffraction. Moreover, successful time-reversal focusing using a single element instead of an array is possible, whereas a one-channel monochromatic phase conjugation fails. The efficiency of broad-band time reversal compared to monochromatic phase conjugation lies in the number of 'information grains' in the frequency bandwidth. © 2002 Elsevier Science B.V. All rights reserved.

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 statistical moments of ultrasonic waves transmitted through a disordered medium with resonant multiple scattering were investigated. An ultrasonic pulsed wave was transmitted from a point source to a 128-element receiving array through two-dimensional samples with various thickness. The results show that a robust estimation on a single realization of disorder is obtained using wide frequency bandwidth.

Résumé: We present a topical review which summarizes the main contributions to `multiple scattering of acoustic and elastic waves' including the most recent advances.The review is divided into five main parts.In the first part, the effects of multiple scattering on ultrasonic propagation are illustrated on the basis of three experimental examples.In the second and third parts, we present the two possible descriptions for the propagation of an acoustic wave in a random medium. The first one is based on the study of the coherent wave, i.e. the wave amplitude averaged over disorder, whereas the second one deals with the propagation of the incoherent intensity, i.e. the intensity averaged over disorder. We especially insist on the microscopic basis for the phenomenological radiative transfer equation and show how it can be solved in the diffusion approximation. The theory is illustrated with experimental results obtained on a two-dimensional multiple-scattering prototype made of thousands of steel rods randomly distributed and immersed in water. In the fourth part, we present experimental evidence that the diffusion equation fails in describing all the aspects of the propagation of an acoustic wave in a random medium: e.g. the coherent backscattering effect recently observed for ultrasonic waves. We show that this effect arises as a consequence of reciprocity. Finally, in the fifth part, we discuss another property which is not taken into account in the radiative transfer theory: the reversibility of an acoustic wave propagating in a disordered medium.

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: 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.