DampingDriven Time Reversal for Waves HidalgoCaballero, S., S. Kottigegollahalli Sreenivas, V. Bacot, S. Wildeman, M. Harazi, X. Jia, A. Tourin, M. Fink, A. Cassinelli, M. Labousse, and E. Fort Physical Review Letters 130, no. 8 (2023)
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 timereversed 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 dampingbased 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.


Passive antenna characterization through impedance correlations in a diffuse field Tamart, M., J. De Rosny, and E. Richalot IEEE Transactions on Antennas and Propagation, 11 (2023)
Résumé: Ambient noise correlations allow the passive recovery of Green’s functions between two probes. Recently, the same approach has been applied to electromagnetism, but by correlating diffuse fields in mode stirred chambers. Until now, only correlation of Sparameters has been studied. However, it has very recently been shown that the result can be difficult to interpret. To overcome this limitation, a new approach is proposed in this paper to directly estimate the self and mutual impedances of two coupled antennas from impedance correlations. The theoretical developments presented are validated experimentally in a reverberation chamber excited by a single antenna where mechanical and source stirring techniques are combined to generate a sufficiently diffuse field environment. It is shown, with antennas of different properties, that this approach allows to reconstruct with a good accuracy the complex impedance matrix between two receiving antennas as well as the transmission coefficient between them. The extracted gain pattern, in good agreement with that measured in an anechoic chamber, shows the good sensitivity of the proposed passive characterization technique.


Computing zerogroupvelocity points in anisotropic elastic waveguides: Globally and locally convergent methods Kiefer, D. A., B. Plestenjak, H. Gravenkamp, and C. Prada The Journal of the Acoustical Society of America 153, no. 2, 13861398 (2023)
Résumé: Dispersion curves of elastic waveguides exhibit points where the group velocity vanishes while the wavenumber remains finite. These are the socalled zerogroupvelocity (ZGV) points. As the elastodynamic energy at these points remains confined close to the source, they are of practical interest for nondestructive testing and quantitative characterization of structures. These applications rely on the correct prediction of the ZGV points. In this contribution, we first model the ZGV resonances in anisotropic plates based on the appearance of an additional modal solution. The resulting governing equation is interpreted as a twoparameter eigenvalue problem. Subsequently, we present three complementary numerical procedures capable of computing ZGV points in arbitrary nondissipative elastic waveguides in the conventional sense that their axial power flux vanishes. The first method is globally convergent and guarantees to find all ZGV points but can only be used for small problems. The second procedure is a very fast, generallyapplicable, Newtontype iteration that is locally convergent and requires initial guesses. The third method combines both kinds of approaches and yields a procedure that is applicable to large problems, does not require initial guesses and is likely to find all ZGV points. The algorithms are implemented in GEW ZGV computation (doi: 10.5281/zenodo.7537442).


Superresolved Imaging Based on Spatiotemporal WaveFront Shaping Noetinger, G., S. Métais, G. Lerosey, M. Fink, S. M. Popoff, and F. Lemoult Physical Review Applied 19, no. 2 (2023)
Résumé: A labelfree approach to improving the performances of confocal scanning imaging is proposed. We experimentally demonstrate its feasibility using acoustic waves. It relies on a way to encode spatial information using the temporal dimension. By moving an emitter, used to insonify an object, along a circular path, we create a temporally modulated wavefield. Because of the symmetries of the problem, the spatiotemporal input field can be decomposed into harmonics corresponding to different spatial vortices. Acquiring the backreflected waves with receivers that are also rotating, multiple images of the same object with different point spread functions are obtained. Not only is the resolution improved compared to a standard confocal configuration, but the accumulation of information also allows the building of images that beat the diffraction limit.


Homogenized transition conditions for plasmonic metasurfaces Lebbe, N., A. Maurel, and K. Pham Physical Review B 107, no. 8 (2023)
Résumé: The present study aims to model the optical response of plasmonic metasurfaces made of a periodic arrangement of metallic particles with arbitrary shape and subwavelength dimensions. By combining homogenization with quasistatic plasmonic eigenmode expansion, the metasurface is replaced by a zerothickness interface associated with frequencydependent effective susceptibilities. The resulting discontinuities of the fields are responsible for strong interaction with the incoming light at the resonances when the complex permittivity of the metal passes close to the real permittivity of an eigenmode. Our modeling provides a physical picture of resonances in plasmonic metasurfaces, and it allows for a huge decrease in the numerical cost of their computations. In addition, comparisons with direct numerics in two dimensions evidence its predictive force at any incidence, particle shape, and arrangement.


Fullfield single molecule localization microscopy with a monodetector Lengauer, M., E. Fort, and S. LévêqueFort Biophysical Journal 122, no. 3S1, 133a (2023)


Comprehensive refractive manipulation of water waves using electrostriction Mouet, V., B. Apffel, and E. Fort Proceedings of the National Academy of Sciences of the United States of America 120, no. 6 (2023)
Résumé: The control of wave propagation based on refraction principles offers unparalleled possibilities as shown by the striking example of optics. This approach is unfortunately limited for water waves as it relies mainly on variations of the liquid depth which, while controlling the wave velocity, also trigger nonlinearities and damping. In this article, we show experimentally that electrostriction allows to implement extensive refractionbased control of water waves in a precise and contactless manner. The setup consists of an electrode under high voltage placed above the grounded conductive water. The waves propagating under the electrode can be slowed down up to approximately half their speed compared to free propagation. We characterize the SnellDescartes laws of refraction and the total internal reflection for the water waves. We implement emblematic refractionbased devices such as electrically tunable focusing lenses, waveguides without obstacles, and beam splitters based on frustrated internal reflection to perform interference experiments.


Superlocalisation of a pointlike emitter in a resonant environment: Correction of the mirage effect Baldassari, L., P. Millien, and A. L. Vanel Inverse Problems and Imaging 17, no. 2, 490506 (2023)
Résumé: In this paper, we show that it is possible to overcome one of the fundamental limitations of superresolution microscopy: the necessity to be in an optically homogeneous environment. Using recent modal approximation results from [10, 7], we show, as a proof of concept, that it is possible to recover the position of a single pointlike emitter in a known resonant environment from farfield measurements, with a precision two orders of magnitude below the classical Rayleigh limit. The procedure does not involve solving any partial differential equation, is computationally light (optimisation in Rd with d of the order of 10) and is therefore suited for the recovery of a very large number of single emitters.


Coherent backscattering of entangled photon pairs Safadi, M., O. Lib, H. C. Lin, C. W. Hsu, A. Goetschy, and Y. Bromberg Nature Physics (2023)
Résumé: Correlations between entangled photons are a key ingredient for testing fundamental aspects of quantum mechanics and an invaluable resource for quantum technologies. However, scattering from a dynamic medium typically scrambles and averages out such correlations. Here we show that multiply scattered entangled photons reflected from a dynamic complex medium remain partially correlated. In experiments and fullwave simulations we observe enhanced correlations, within an angular range determined by the transport mean free path, which prevail over disorder averaging. Theoretical analysis reveals that this enhancement arises from the interference between scattering trajectories, in which the photons leave the sample and are then virtually reinjected back into it. These paths are the quantum counterpart of the paths that lead to the coherent backscattering of classical light. This work points to opportunities for entanglement transport despite dynamic multiple scattering in complex systems.


The Electronic Disorder Landscape of Mixed Halide Perovskites Liu, Y., J. P. Banon, K. Frohna, Y. H. Chiang, G. TumenUlzii, S. D. Stranks, M. Filoche, and R. H. Friend ACS Energy Letters 8, no. 1, 250258 (2023)
Résumé: Band gap tunability of lead mixed halide perovskites makes them promising candidates for various applications in optoelectronics. Here we use the localization landscape theory to reveal that the static disorder due to iodide:bromide compositional alloying contributes at most 3 meV to the Urbach energy. Our modeling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20 nm for the “effective confining potential” for electrons and holes, with shortrange potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to accurately reproduce the experimentally observed absorption spectra of perovskites with halide segregation. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy.


Reconstructing the Spatial Distribution of the Relative Shear Modulus in Quasistatic Ultrasound Elastography: Plane Stress Analysis Seppecher, L., E. Bretin, P. Millien, L. Petrusca, and E. Brusseau Ultrasound in Medicine and Biology (2023)
Résumé: Quasistatic ultrasound elastography (QSUE) is an imaging technique that mainly provides axial strain maps of tissues when the latter are subjected to compression. In this article, a method for reconstructing the relative shear modulus distribution within a linear elastic and isotropic medium, in QSUE, is introduced. More specifically, the plane stress inverse problem is considered. The proposed method is based on the variational formulation of the equilibrium equations and on the choice of adapted discretization spaces, and only requires displacement fields in the analyzed media to be determined. Results from plane stress and 3D numerical simulations, as well as from phantom experiments, showed that the method is able to reconstruct the different regions within a medium, with shear modulus contrasts that unambiguously reveal whether inclusions are stiffer or softer than the surrounding material. More specifically, for the plane stress simulations, inclusiontobackground modulus ratios were found to be very accurately estimated, with an error lower than 3%. For the 3D simulations, for which the plane stress conditions are no longer satisfied, these ratios were, as expected, less accurate, with an error that remained lower than 10% for two of the three cases analyzed but was around 34% for the last case. Concerning the phantom experiments, a comparison with a shear wave elastography technique from a clinical ultrasound scanner was also made. Overall, the inclusiontobackground shear modulus ratios obtained with our approach were found to be closer to those given by the phantom manufacturer than the ratios provided by the clinical system.


Seismic surface wave focal spot imaging: numerical resolution experiments Giammarinaro, B., C. Tsarsitalidou, G. Hillers, J. De Rosny, L. Seydoux, S. Catheline, M. Campillo, and P. Roux Geophysical Journal International 232, no. 1, 201222 (2023)
Résumé: Numerical experiments of seismic wave propagation in a laterally homogeneous layered medium explore subsurface imaging at subwavelength distances for dense seismic arrays. We choose a timereversal approach to simulate fundamental mode Rayleigh surface wavefields that are equivalent to the crosscorrelation results of threecomponent ambient seismic field records. We demonstrate that the synthesized 2D spatial autocorrelation fields in the time domain support local or socalled focal spot imaging. Systematic tests involving clean isotropic surface wavefields but also interfering body wave components and anisotropic incidence assess the accuracy of the phase velocity and dispersion estimates obtained from focal spot properties. The results suggest that data collected within half a wavelength around the origin is usually sufficient to constrain the used Bessel functions models. Generally, the cleaner the surface wavefield the smaller the fitting distances that can be used to accurately estimate the local Rayleigh wave speed. Using models based on isotropic surface wave propagation we find that phase velocity estimates from verticalradial component data are less biased by Pwave energy compared to estimates obtained from verticalvertical component data, that even strong anisotropic surface wave incidence yields phase velocity estimates with an accuracy of 1 per cent or better, and that dispersion can be studied in the presence of noise. Estimates using a model to resolve potential medium anisotropy are significantly biased by anisotropic surface wave incidence. The overall accurate results obtained from nearfield measurements using isotropic medium assumptions imply that dense array seismic Rayleigh wave focal spot imaging can increase the depth sensitivity compared to ambient noise surface wave tomography. The analogy to elastography focal spot medical imaging implies that a high station density and clean surface wavefields support subwavelength resolution of lateral medium variations.


Negative refraction in a singlephase flexural metamaterial with hyperbolic dispersion Marigo, J. J., A. Maurel, and K. Pham Journal of the Mechanics and Physics of Solids 170 (2023)
Résumé: We analyze the band structure of a singlephase metamaterial involving lowfrequency flexural resonances by combining asymptotic homogenization and Bloch–Floquet analysis. We provide the closedform expression of the dispersion relation in the whole Brillouin zone. The dispersion relation involves two effective, frequencydependent, mass densities associated with symmetric and antisymmetric flexural resonances of the beams at the microscopic scale. We demonstrate that our simple locallyresonant structure produces at lowfrequency bandgaps and, in the hyperbolic regions of the dispersion diagram, negative refraction. Our findings are validated by direct numerical calculations.


Diffraction grating with varying slit width: Quasiperiodic homogenization and its numerical implementation Pham, K., N. Lebbe, and A. Maurel Journal of Computational Physics 473, 111727 (2023)
Résumé: We study the diffraction of acoustic waves by thin grating with varying slit width. Using quasiperiodic homogenization, we derive an effective model in which the grating is replaced by effective jump conditions with effective parameters varying along the equivalent interface. The numerical implementations of the actual problem and of its homogenized counterpart are achieved using multimodal methods for a periodic grating with a macroperiod containing many slits with varying widths. The ability of the effective grating to reproduce the scattering properties of the actual one is inspected and discussed.

