FewMolecule Strong Coupling with Dimers of Plasmonic Nanoparticles Assembled on DNA Heintz, J., N. Markešević, E. Y. Gayet, N. Bonod, and S. Bidault ACS Nano 15, no. 9, 1473214743 (2021)
Résumé: Hybrid nanostructures, in which a known number of quantum emitters are strongly coupled to a plasmonic resonator, should feature optical properties at room temperature such as fewphoton nonlinearities or coherent superradiant emission. We demonstrate here that this coupling regime can only be reached with dimers of gold nanoparticles in stringent experimental conditions, when the interparticle spacing falls below 2 nm. Using a short transverse DNA doublestrand, we introduce five dye molecules in the gap between two 40 nm gold particles and actively decrease its length down to sub2 nm values by screening electrostatic repulsion between the particles at high ionic strengths. Singlenanostructure scattering spectroscopy then evidence the observation of a strongcoupling regime in excellent agreement with electrodynamic simulations. Furthermore, we highlight the influence of the planar facets of polycrystalline gold nanoparticles on the probability of observing strongly coupled hybrid nanostructures.
Motsclés: strong coupling; plasmonics; DNA nanotechnology; selfassembly; scattering spectroscopy; dye molecules


Liquid walls and interfaces in arbitrary directions stabilized by vibrations Apffel, B., S. HidalgoCaballero, A. Eddi, and E. Fort Proceedings of the National Academy of Sciences of the United States of America 118, no. 48 (2021)
Résumé: Gravity shapes liquids and plays a crucial role in their internal balance. Creating new equilibrium configurations irrespective of the presence of a gravitational field is challenging with applications on Earth as well as in zerogravity environments. Vibrations are known to alter the shape of liquid interfaces and also to change internal dynamics and stability in depth. Here, we show that vibrations can also create an "artificial gravity" in any direction. We demonstrate that a liquid can maintain an inclined interface when shaken in an arbitrary direction. A necessary condition for the equilibrium to occur is the existence of a velocity gradient determined by dynamical boundary conditions. However, the noslip boundary condition and incompressibility can perturb the required velocity profile, leading to a destabilization of the equilibrium. We show that liquid layers provide a solution, and liquid walls of several centimeters in height can thus be stabilized. We show that the buoyancy equilibrium is not affected by the forcing.


Propagation of scalar waves in dense disordered media exhibiting short And longrange correlations Rohfritsch, A., J. M. Conoir, T. ValierBrasier, R. Pierrat, and R. Marchiano Physical Review E 104, no. 6 (2021)
Résumé: Correlated disorder is at the heart of numerous challenging problematics in physics. In this work we focus on the propagation of acoustic coherent waves in twodimensional dense disordered media exhibiting long and shortrange structural correlations. The media are obtained by inserting elastic cylinders randomly in a stealth hyperuniform medium itself made up of cylinders. The properties of the coherent wave is studied using an original numerical software. In order to understand and discuss the complex physical phenomena occurring in the different media, we also make use of effective media models derived from the quasicrystalline approximation and the theory of Fikioris and Waterman that provides an explicit expression of the effective wave numbers. Our study shows a very good agreement between numerical and homogenization models up to very high concentrations of scatterers. This study shows that media with both short and longrange correlations are of strong interest to design materials with original properties.


Cloaking, trapping and superlensing of lamb waves with negative refraction Legrand, F., B. Gérardin, F. Bruno, J. Laurent, F. Lemoult, C. Prada, and A. Aubry Scientific Reports 11, no. 1 (2021)
Résumé: We report on experimental and numerical implementations of devices based on the negative refraction of elastic guided waves, the socalled Lamb waves. Consisting in plates of varying thickness, these devices rely on the concept of complementary media, where a particular layout of negative index media can cloak an object with its antiobject or trap waves around a negative corner. The diffraction cancellation operated by negative refraction is investigated by means of laser ultrasound experiments. However, unlike original theoretical predictions, these intriguing wave phenomena remain, nevertheless, limited to the propagating component of the wavefield. To go beyond the diffraction limit, negative refraction is combined with the concept of metalens, a device converting the evanescent components of an object into propagating waves. The transport of an evanescent wavefield is then possible from an object plane to a farfield imaging plane. Twenty years after Pendry’s initial proposal, this work thus paves the way towards an elastic superlens.


Partialfield illumination ophthalmoscope: Improving the contrast of a camerabased retinal imager Krafft, L., E. GofasSalas, Y. LaiTim, M. Paques, L. Mugnier, O. Thouvenin, P. Mecê, and S. Meimon Applied Optics 60, no. 31, 99519956 (2021)
Résumé: Effective and accurate in vivo diagnosis of retinal pathologies requires high performance imaging devices, combining a large field of view and the ability to discriminate the ballistic signal from the diffuse background in order to provide a highly contrasted image of the retinal structures. Here, we have implemented the partialfield illumination ophthalmoscope, a patterned illumination modality, integrated to a high pixel rate adaptive optics fullfield microscope. This noninvasive technique enables us to mitigate the low signaltonoise ratio, intrinsic of fullfield ophthalmoscopes, by partially illuminating the retina with complementary patterns to reconstruct a widefield image. This new, to the best of our knowledge, modality provides an image contrast spanning from the fullfield to the confocal contrast, depending on the pattern size. As a result, it offers various tradeoffs in terms of contrast and acquisition speed, guiding the users towards the most efficient system for a particular clinical application.


Diffuse field crosscorrelations: Scattering theory and electromagnetic experiments Davy, M., P. Besnier, P. Del Hougne, J. De Rosny, E. Richalot, F. Sarrazin, D. V. Savin, F. Mortessagne, U. Kuhl, and O. Legrand Physical Review E 104, no. 4 (2021)
Résumé: The passive estimation of impulse responses from ambient noise correlations arouses increasing interest in seismology, acoustics, optics, and electromagnetism. Assuming the equipartition of the noise field, the crosscorrelation function measured with noninvasive receiving probes converges towards the difference of the causal and anticausal Green's functions. Here, we consider the case when the receiving field probes are antennas which are well coupled to a complex medium—a scenario of practical relevance in electromagnetism. We propose a general approach based on the scattering matrix formalism to explore the convergence of the crosscorrelation function. The analytically derived theoretical results for chaotic systems are confirmed in microwave measurements within a modestirred reverberation chamber. This study provides fundamental insight into the Green's function retrieval technique and paves the way for a new technique to characterize electromagnetic antennas.


Effective Model for Elastic Waves in a Substrate Supporting an Array of Plates/Beams with Flexural and Longitudinal Resonances Marigo, J. J., K. Pham, A. Maurel, and S. Guenneau Journal of Elasticity 146, no. 1, 143177 (2021)
Résumé: In a previous study (Marigo et al. in J. Mech. Phys. Solids 143:104029, 2020) we have studied the effect of a periodic array of subwavelength plates or beams over a semiinfinite elastic ground on the propagation of waves hitting the interface. The study was restricted to the low frequency regime where only flexural resonances take place. Here, we present a generalization to higher frequencies which allows us to account for both flexural and longitudinal resonances and to evaluate their interplay. An effective model is obtained using asymptotic analysis and homogenization techniques, which can be expressed in terms of the ground alone with an effective dynamic (frequencydependent) boundary conditions of the Robin’s type. For an inplane wave at oblique incidence, the scattered displacement fields and the reflection coefficients are obtained in closed forms and their effectiveness to reproduce the actual scattering is inspected by comparison with direct numerics in a twodimensional setting.


Full characterization of the transmission properties of a multiplane light converter Boucher, P., A. Goetschy, G. Sorelli, M. Walschaers, and N. Treps Physical Review Research 3, no. 2 (2021)
Résumé: Multiplane light conversion (MPLC) allows to perform arbitrary transformations on a finite set of spatial modes with no theoretical restriction to the quality of the transformation. Even though the number of shaped modes is in general small, the number of modes transmitted by an MPLC system is extremely large. In this paper, we aim to characterize the transmission properties of a multiplane light converter inside and outside the designmodes subspace. We report the construction of the full transmission matrix of such systems. By performing singular value decompositions, we individuate ways to evaluate their efficiency in performing the design transformation. Moreover, we develop an analytical random matrix model that suggests that in the regime of a large number of shaped modes an MPLC system behaves like a random scattering medium with limited number of controlled channels.


Quantitative evaluation of the dynamic activity of HeLa cells in different viability states using dynamic fullfield optical coherence microscopy Park, S., T. Nguyen, E. Benoit, D. L. Sackett, M. GarmendiaCedillos, R. Pursley, C. Boccara, and A. Gandjbakhche Biomedical Optics Express 12, no. 10, 64316441 (2021)
Résumé: Dynamic fullfield optical coherence microscopy (DFFOCM) was used to characterize the intracellular dynamic activities and cytoskeleton of HeLa cells in different viability states. HeLa cell samples were continuously monitored for 24 hours and compared with histological examination to confirm the cell viability states. The averaged mean frequency and magnitude observed in healthy cells were 4.79±0.5 Hz and 2.44±1.06, respectively. In dead cells, the averaged mean frequency was shifted to 8.57±0.71 Hz, whereas the magnitude was significantly decreased to 0.53±0.25. This cell dynamic activity analysis using DFFOCM is expected to replace conventional timeconsuming and biopsiesrequired histological or biochemical methods.


Monte Carlo Simulations of Ultrasound Scattering and Absorption in FiniteSize Heterogeneous Materials Zhou, H., X. Jia, L. Y. Fu, and A. Tourin Physical Review Applied 16, no. 3 (2021)
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 finitesize scattering medium, where boundary reflection plays a crucial role. To fit the energy profile of scattered coda waves, we solve the acoustic radiativetransfer equation by Monte Carlo simulations for cylinder and slab geometries, under the isotropic scattering approximation. We show that the fit with the simplistic radiativetransfer solution in an infinite medium may result in underestimated values of the scattering mean free path, ls, and absorption, Qi1, 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 wavefocusing 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 polycrystallike granite slab from the ballistic to the diffusive regime. The ls deduced from offaxis detections agrees with that estimated from the correlation length of the shearwave velocity by structural imaging analysis.


Retinal blood flow reversal quantitatively monitored in outofplane vessels with laser Doppler holography Puyo, L., M. Paques, and M. Atlan Scientific Reports 11, no. 1 (2021)
Résumé: Laser Doppler holography is a planar blood flow imaging technique recently introduced in ophthalmology to image human retinal and choroidal blood flow noninvasively. Here we present a digital method based on the Doppler spectrum asymmetry that reveals the local direction of blood flow with respect to the optical axis in outofplane vessels. This directional information is overlaid on standard grayscale blood flow images to depict flow moving towards the camera in red and flow moving away from the camera in blue, as in ultrasound color Doppler imaging. We show that thanks to the strong contribution of backscattering to the Doppler spectrum in outofplane vessels, the local axial direction of blood flow can be revealed with a high temporal resolution, which enables us to evidence pathological blood flow reversals. We also demonstrate the use of optical Doppler spectrograms to quantitatively monitor retinal blood flow reversals.


Universal Statistics of Waves in a Random TimeVarying Medium Carminati, R., H. Chen, R. Pierrat, and B. Shapiro Physical Review Letters 127, no. 9 (2021)
Résumé: We study the propagation of waves in a medium in which the wave velocity fluctuates randomly in time. We prove that at long times, the statistical distribution of the wave energy is lognormal, with the average energy growing exponentially. For weak disorder, another regime preexists at shorter times, in which the energy follows a negative exponential distribution, with an average value growing linearly with time. The theory is in perfect agreement with numerical simulations, and applies to different kinds of waves. The existence of such universal statistics bridges the fields of wave propagation in timedisordered and spacedisordered media.


Waveguide efficient directional coupling and decoupling via an integrated plasmonic nanoantenna Blanquer, G., V. Loo, N. Rahbany, C. Couteau, S. Blaize, R. SalasMontiel, Y. De Wilde, and V. Krachmalnicoff Optics Express 29, no. 18, 2903429043 (2021)
Résumé: The development of integrated photonic devices has led to important advancements in the field of lightmatter interaction at the nanoscale. One of the main focal points is the coupling between single photon emitters and optical waveguides aiming to achieve efficient optical confinement and propagation. In this work, we focus on the characterization of a hybrid dielectric/plasmonic waveguide consisting of a gold triangular nanoantenna placed on top of a TiO2 waveguide. The strong directionality of the device is experimentally demonstrated by comparing the intensity scattered by the nanotriangle to the one scattered by a SNOM tip for different illumination geometries. The ability of the plasmonic antenna to generate powerful coupling between a single emitter and the waveguide will also be highlighted through numerical simulations.


Negative Transient Flux in the near Field of a Subwavelength Source Li, X., P. Li, M. H. Lu, M. Fink, and G. Ma Physical Review Applied 16, no. 1 (2021)
Résumé: The emission of waves by a small source is a generic wave problem that has long been thought to be well studied. Here, we report the experimental observation that the energy flux of an outgoing sound wave emitted by a deepsubwavelengthsized source can become negative, which indicates the backward flow of energy. Such a negative transient flux exists, even in a homogenous medium, but is only observable in the time domain and in the extreme near field of the source. By waveimpedance analysis, we show that such a phenomenon is fundamentally rooted in the geometry of the wavefield itself and, hence, is generic. Our findings have implications in the timedependent emission, absorption, and scattering of waves.


Fourier transform acoustooptic imaging with offaxis holographic detection Dutheil, L., M. Bocoum, M. Fink, S. M. Popoff, F. Ramaz, and J. M. Tualle Applied Optics 60, no. 24, 71077112 (2021)
Résumé: Acoustooptic (AO) imaging is an indepth optical imaging technique of highly scattering media. One challenging endapplication for this technique is to perform imaging of living biological tissues. Indeed, because it relies on coherent illumination, AO imaging is sensitive to speckle decorrelation occurring on the millisecond time scale. Camerabased detections are well suited for in vivo imaging provided their integration time is lower than those decorrelation time scales. We present Fourier transform acoustooptic imaging combined with offaxis holography, which relies on plane waves and longduration pulses. We demonstrate, for the first time to the best of our knowledge, a twodimensional imaging system fully compatible with in vivo imaging prerequisites. The method is validated experimentally by performing indepth imaging inside a multiple scattering sample.


Localized modes revealed in random lasers Kumar, B., R. Homri, Priyanka, S. K. Maurya, M. Lebental, and P. Sebbah Optica 8, no. 8, 10331039 (2021)
Résumé: In sufficiently strong scattering media, light transport is suppressed and modes are exponentially localized. Andersonlike localized states have long been recognized as potential candidates for highQ optical modes for lowthreshold, costeffective random lasers. Operating in this regime remains, however, a challenge since Anderson localization is difficult to achieve in optics, and nonlinear mode interaction compromises its observation. Here, we exhibit individually each lasing mode of a lowdimension solidstate random laser by applying a nonuniform optical gain. By undoing gain competition and crosssaturation, we demonstrate that all lasing modes are spatially localized.We find that selective excitation significantly reduces the lasing threshold, while lasing efficiency is greatly improved. We show further how their spatial locations are critical to boost laser power efficiency. By efficiently suppressing the spatial hole burning effect, we can turn on the optimally outcoupled random lasing modes. Our demonstration opens the road to the exploration of linear and nonlinear mode interactions in the presence of gain, as well as disorderengineering for laser applications.


Image Transmission Through a Dynamically Perturbed Multimode Fiber by Deep Learning Resisi, S., S. M. Popoff, and Y. Bromberg Laser & Photonics Reviews, 2000553 (2021)
Résumé: When multimode optical fibers are perturbed, the data that is transmitted through them is scrambled. This presents a major difficulty for many possible applications, such as multimode fiber based telecommunication and endoscopy. To overcome this challenge, a deep learning approach that generalizes over mechanical perturbations is presented. Using this approach, successful reconstruction of the input images from intensityonly measurements of speckle patterns at the output of a 1.5 mlong randomly perturbed multimode fiber is demonstrated. The model's success is explained by hidden correlations in the speckle of random fiber conformations.


Epigenetic rewriting at centromeric DNA repeats leads to increased chromatin accessibility and chromosomal instability Decombe, S., F. Loll, L. Caccianini, K. Affannoukoué, I. Izeddin, J. Mozziconacci, C. Escudé, and J. Lopes Epigenetics and Chromatin 14, no. 1 (2021)
Résumé: Background: Centromeric regions of human chromosomes contain large numbers of tandemly repeated αsatellite sequences. These sequences are covered with constitutive heterochromatin which is enriched in trimethylation of histone H3 on lysine 9 (H3K9me3). Although well studied using artificial chromosomes and global perturbations, the contribution of this epigenetic mark to chromatin structure and genome stability remains poorly known in a more natural context. Results: Using transcriptional activatorlike effectors (TALEs) fused to a histone lysine demethylase (KDM4B), we were able to reduce the level of H3K9me3 on the αsatellites repeats of human chromosome 7. We show that the removal of H3K9me3 affects chromatin structure by increasing the accessibility of DNA repeats to the TALE protein. Tethering TALEdemethylase to centromeric repeats impairs the recruitment of HP1α and proteins of Chromosomal Passenger Complex (CPC) on this specific centromere without affecting CENPA loading. Finally, the epigenetic rewriting by the TALEKDM4B affects specifically the stability of chromosome 7 upon mitosis, highlighting the importance of H3K9me3 in centromere integrity and chromosome stability, mediated by the recruitment of HP1α and the CPC. Conclusion: Our cellular model allows to demonstrate the direct role of pericentromeric H3K9me3 epigenetic mark on centromere integrity and function in a natural context and opens interesting possibilities for further studies regarding the role of the H3K9me3 mark.


Statistical Nonlinear Optical Mapping of Localized and Delocalized Plasmonic Modes in Disordered Gold Metasurfaces Roubaud, G., S. Bidault, S. Gigan, and S. Grésillon ACS Photonics 8, no. 7, 19371943 (2021)
Résumé: Using a statistical analysis of nonlinear luminescence images measured with randomly wavefrontshaped femtosecond excitations, we provide direct insight on both the localized and delocalized plasmonic modes featured by disordered gold metasurfaces. We can independently image areas where farfield wavefront shaping can control the optical properties and areas with strong subwavelength optical hotspots. In practice, the fraction of the disordered plasmonic surface on which wavefront control is feasible depends strongly on the nanoscale morphology of the sample. Close to the percolation threshold, the entire surface is sensitive to wavefront shaping, and we observe the largest densities of delocalized modes as well as the strongest optical hotspots. These results demonstrate how statistical imaging schemes can offset the complexity of disordered nanophotonic systems in order to characterize their optical properties.


Manifestation of aberrations in fullfield optical coherence tomography Barolle, V., J. Scholler, P. Mecê, J. M. Chassot, K. Groux, M. Fink, A. C. Boccara, and A. Aubry Optics Express 29, no. 14, 2204422065 (2021)
Résumé: We report on a theoretical model for image formation in fullfield optical coherence tomography (FFOCT). Because the spatial incoherence of the illumination acts as a virtual confocal pinhole in FFOCT, its imaging performance is equivalent to a scanning timegated coherent confocal microscope. In agreement with optical experiments enabling a precise control of aberrations, FFOCT is shown to have nearly twice the resolution of standard imaging at moderate aberration level. Beyond a rigorous study on the sensitivity of FFOCT with respect to aberrations, this theoretical model paves the way towards an optimized design of adaptive optics and computational tools for highresolution and deep imaging of biological tissues.


Learning and Avoiding Disorder in Multimode Fibers Matthès, M. W., Y. Bromberg, J. De Rosny, and S. M. Popoff Physical Review X 11, no. 2 (2021)
Résumé: Multimode optical fibers (MMFs) have gained renewed interest in the past decade, emerging as a way to boost optical communication data rates in the context of an expected saturation of current singlemode fiberbased networks. They are also attractive for endoscopic applications, offering the possibility to achieve a similar information content as multicore fibers, but with a much smaller footprint, thus reducing the invasiveness of endoscopic procedures. However, these advances are hindered by the unavoidable presence of disorder that affects the propagation of light in MMFs and limits their practical applications. We introduce here a general framework to study and avoid the effect of disorder in wavebased systems and demonstrate its application for multimode fibers. We experimentally find an almost complete set of optical channels that are resilient to disorder induced by strong deformations. These deformation principal modes are obtained by only exploiting measurements for weak perturbations harnessing the generalized WignerSmith operator. We explain this effect by demonstrating that, even for a high level of disorder, the propagation of light in MMFs can be characterized by just a few key properties. These results are made possible thanks to a precise and fast estimation of the modal transmission matrix of the fiber which relies on a modelbased optimization using deep learning frameworks.
Motsclés: multimode fiber; wavefront shaping; disorder; WignerSmith; telecommunications; transmission matrix


Laser Doppler holography of the anterior segment for blood flow imaging, eye tracking, and transparency assessment Puyo, L., C. David, R. Saad, S. Saad, J. Gautier, J. A. Sahel, V. Borderie, M. Paques, and M. Atlan Biomedical Optics Express 12, no. 7, 44784495 (2021)
Résumé: Laser Doppler holography (LDH) is a fullfield blood flow imaging technique able to reveal human retinal and choroidal blood flow with high temporal resolution. We here report on using LDH in the anterior segment of the eye without making changes to the instrument. Blood flow in the bulbar conjunctiva and episclera as well as in corneal neovascularization can be effectively imaged. We additionally demonstrate simultaneous holographic imaging of the anterior and posterior segments by simply adapting the numerical propagation distance to the plane of interest. We used this feature to track the movements of the retina and pupil with high temporal resolution. Finally, we show that the light backscattered by the retina can be used for retroillumination of the anterior segment. Hence digital holography can reveal opacities caused by absorption or diffusion in the cornea and eye lens.


Threedimensional acoustic lensing with a bubbly diamond metamaterial Lanoy, M., F. Lemoult, G. Lerosey, A. Tourin, V. Leroy, and J. H. Page Journal of Applied Physics 129, no. 24, 245107 (2021)
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 facecentered 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 threedimensional flat lens. We also report superresolution focusing in the near field of the slab and illustrate its potential for imaging in three dimensions.


Laserexcited elastic guided waves reveal the complex mechanics of nanoporous silicon Thelen, M., N. Bochud, M. Brinker, C. Prada, and P. Huber Nature Communications 12, no. 1 (2021)


Time domain modelling of a Helmholtz resonator analogue for water waves Euvé, L. P., K. Pham, P. Petitjeans, V. Pagneux, and A. Maurel Journal of Fluid Mechanics 920 (2021)
Résumé: In the context of water waves, we consider a resonator with deep subwavelength resonance, analogue to the Helmholtz resonator in acoustics. In the shallow water regime, using asymptotic analysis, a onedimensional model is derived in which the effect of the resonator is reduced to effective transmission conditions. These conditions clearly highlight two contributions. The first is associated with the dock on its own and it is responsible for a jump of the potential at the free surface. The second is due to the resonant cavity and it is responsible for a jump in the horizontal velocity. It involves as well the uniform amplitude within the resonant cavity with a transient dynamics explicitly given by the equation of a damped oscillator forced by the incident waves. The onedimensional model is validated in the harmonic regime by comparison to direct twodimensional numerics. It is shown to reproduce accurately the scattering coefficients and the amplitude within the resonator; interestingly, this remains broadly true for finite water depths. We further inspect the spatiotemporal behaviour of different types of wave packets interacting with the resonating and radiating cavity.


Open birdcage coil for head imaging at 7T Nikulin, A. V., A. Vignaud, N. I. Avdievich, D. Berrahou, J. De Rosny, and A. Ourir Magnetic Resonance in Medicine (2021)
Résumé: Purpose: To theoretically describe, design, and test the new geometry of the birdcage coil for 7 Tesla anatomical brain imaging, which includes a large window on top, without deliberately jeopardizing its homogeneity and efficiency. This opencage will not only improve patient comfort but also enable the volunteer to follow functional MRI stimuli. This design could also facilitate the tracking of patient compliance and enable better correction of the movement. Methods: Via the transfer matrix approach, a birdcagelike coil with a nonperiodic distribution of rungs is constructed with optimized currents in the coil rungs. Subsequently, the coil is adjusted in fullwave simulations. Then, the coil is assembled, finetuned, and matched on the bench. Finally, these results are confirmed experimentally on a phantom and in vivo. Results: Indeed, the computed isolation of −14.9 dB between the feeding ports of the coil and the symmetry of the circular polarized mode pattern transmit RF magnetic field ((Formula presented.)) showed that the coil was properly optimized. An experimental assessment of the developed coil showed competitive transmit efficiency and coverage compared with the conventional birdcage coil of similar size. Conclusion: The proposed opencage coil can be designed and work without a dramatic drop of performance in terms of the (Formula presented.) field homogeneity, transmit efficiency ((Formula presented.) / (Formula presented.)), peak local specific absorption rate ((Formula presented.)) and SAR efficiency ((Formula presented.) / (Formula presented.)).


Laboratory Landquakes: Insights From Experiments Into the HighFrequency Seismic Signal Generated by Geophysical Granular Flows Arran, M. I., A. Mangeney, J. De Rosny, M. Farin, R. Toussaint, and O. Roche Journal of Geophysical Research: Earth Surface 126, no. 5 (2021)
Résumé: Geophysical granular flows exert basal forces that generate seismic signals, which can be used to better monitor and model these severe natural hazards. A number of empirical relations and existing models link these signals' highfrequency components to a variety of flow properties, many of which are inaccessible by other analyses. However, the range of validity of the empirical relations remains unclear and the models lack validation, owing to the difficulty of adequately controlling and instrumenting fieldscale flows. Here, we present laboratory experiments investigating the normal forces exerted on a basal plate by dense and partially dense flows of spherical glass particles. We measured the power spectra of these forces and inferred predictions for these power spectra from the models for debris flows' seismic signals proposed by Kean et al. (2015, https://doi.org/10.1002/2015GL064811), Lai et al. (2018, https://doi.org/10.1029/2018GL077683), and Farin, Tsai, et al. (2019, https://doi.org/10.1002/esp.4677), using Hertz theory to extend Farin, Tsai, et al. (2019)'s models to higher frequencies. Comparison of our observations to these predictions, and to predictions derived from Bachelet (2018) and Bachelet et al. (2021)'s model for granular flows' seismic signals, shows those of Farin, Tsai, et al. (2019)'s “thinflow” model to be the most accurate, so we examine explanations for this accuracy and discuss its implications for geophysical flows' seismic signals. We also consider the normalization, by the mean force exerted by each flow, of the force's mean squared fluctuations, showing that this ratio varies by 4 orders of magnitude over our experiments, but is determined by the bulk inertial number of the flow.


On the PathLoss of Reconfigurable Intelligent Surfaces: An Approach Based on Green&#x2019;s Theorem Applied to Vector Fields Danufane, F. H., M. Di Renzo, J. De Rosny, and S. Tretyakov IEEE Transactions on Communications, 11 (2021)
Résumé: In this paper, we introduce a physicsconsistent analytical characterization of the freespace pathloss of a wireless link in the presence of a reconfigurable intelligent surface. The proposed approach is based on the vector generalization of Green’s theorem. The obtained pathloss model can be applied to twodimensional homogenized metasurfaces, which are made of subwavelength scattering elements and that operate either in reflection or transmission mode. The pathloss is formulated in terms of a computable integral that depends on the transmission distances, the polarization of the radio waves, the size of the surface, and the desired surface transformation. Closedform expressions are obtained in two asymptotic regimes that are representative of farfield and nearfield deployments. Based on the proposed approach, the impact of several design parameters and operating regimes is unveiled.


A distortion matrix framework for highresolution passive seismic 3D imaging: Application to the San Jacinto fault zone, California Touma, R., T. Blondel, A. Derode, M. Campillo, and A. Aubry Geophysical Journal International 226, no. 2, 780794 (2021)
Résumé: Reflection seismic imaging usually suffers from a loss of resolution and contrast because of the fluctuations of the wave velocities in the Earth's crust. In the literature, phase distortion issues are generally circumvented by means of a background wave velocity model. However, it requires a prior tomography of the wave velocity distribution in the medium, which is often not possible, especially in depth. In this paper, a matrix approach of seismic imaging is developed to retrieve a 3D image of the subsoil, despite a rough knowledge of the background wave velocity. To do so, passive noise crosscorrelations between geophones of a seismic array are investigated under a matrix formalism. They form a reflection matrix that contains all the information available on the medium. A set of matrix operations can then be applied in order to extract the relevant information as a function of the problem considered. On the one hand, the background seismic wave velocity can be estimated and its fluctuations quantified by projecting the reflection matrix in a focused basis. It consists in investigating the response between virtual sources and detectors synthesized at any point in the medium. The minimization of their crosstalk can then be used as a guide star for approaching the actual wave velocity distribution. On the other hand, the detrimental effect of wave velocity fluctuations on imaging is overcome by introducing a novel mathematical object: The distortion matrix. This operator essentially connects any virtual source inside the medium with the distortion that a wavefront, emitted from that point, experiences due to heterogeneities. A time reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each real geophone at the surface and each virtual geophone in depth. Phase distortions can then be compensated for any point of the underground. Applied to passive seismic data recorded along the Clark branch of the San Jacinto fault zone (SJFZ), the present method is shown to provide an image of the fault until a depth of 4 km over the frequency range 1020Hz with an horizontal resolution of 80 m. Strikingly, this resolution is almost one eighth below the diffraction limit imposed by the geophone array aperture. The heterogeneities of the subsoil play the role of a scattering lens and of a transverse waveguide which increase drastically the array aperture. The contrast is also optimized since most of the incoherent noise is eliminated by the iterative time reversal process. Beyond the specific case of the SJFZ, the reported approach can be applied to any scales and areas for which a reflection matrix is available at a spatial sampling satisfying the Nyquist criterion.


Control of the swell by an array of helmholtz resonators Euvé, L. P., N. Piesniewska, A. Maurel, K. Pham, P. Petitjeans, and V. Pagneux Crystals 11, no. 5, 520 (2021)
Résumé: We present a theoretical and experimental study of a resonator of the Helmholtz type for the control of the swell. An experimental demonstration of the shielding effect by a belt made of evenly distributed resonators is given. We then provide indepth analysis of the Fano resonance resulting from the interference between the dock scattering (the background) and the resonant cavity scattering. This is done thanks to spacetime resolved experiments which provides the complexvalued scattering coefficients and amplitude within the resonator. We provide a onedimensional model derived in the shallow water regime owing to asymptotic analysis. The model contains the two ingredients of the Fano resonance and allows us to exhibit the damping due to leakage. When adding heuristically the damping due to losses, it reproduces the main features of the resonance observed experimentally.


Uncorrelated configurations and field uniformity in reverberation chambers stirred by reconfigurable metasurfaces Gros, J. B., G. Lerosey, F. Mortessagne, U. Kuhl, and O. Legrand Applied Physics Letters 118, no. 14, 144101 (2021)
Résumé: Reverberation chambers are currently used to test electromagnetic compatibility as well as to characterize antenna efficiency, wireless devices, and MIMO systems. The related measurements are based on statistical averages and their fluctuations. We introduce a very efficient mode stirring process based on electronically reconfigurable metasurfaces (ERMs). By locally changing the field boundary conditions, the ERMs allow us to generate a humongous number of uncorrelated field realizations even within small reverberation chambers. We fully experimentally characterize this stirring process by determining these uncorrelated realizations via the autocorrelation function of the transmissions. The uniformity criterion parameter σ dB, as defined in the IEC 61000421 standard, is also investigated and reveals the performance of this stirring. The effect of short paths on the two presented quantities is identified. We compare the experimental results on the uniformity criterion parameter with a corresponding model based on random matrix theory and find good agreement, where the only parameter, the modal overlap, is extracted by the quality factor.


Passive imaging of water pipelines using ambient turbulence noise Wang, W., Z. Li, A. Dubey, P. Lee, M. Fink, and R. Murch Mechanical Systems and Signal Processing 160 (2021)
Résumé: Ambient noise generated by flowing water turbulence is harnessed as a signal source for imaging key parameters and fault detection in water pipelines. This approach is important because it can aid in the estimation of wave speed or detection of water pipeline defects such as blockages and leakages. More importantly it overcomes the challenging problem of generating a signal source of sufficient power to provide the necessary signaltonoise ratios for conventional water pipeline imaging and fault detection techniques. In this paper, the expressions of the auto and crosscorrelation functions of the ambient noise between sensors are derived using wave theory. It is shown that the timedomain Green's functions can be extracted from the correlation functions. Experimental and numerical examples are provided for water pipelines to demonstrate that wave speed can be estimated from the timedomain Green's functions. A method for extending the technique, by using straightforward but accurate approximations of the correlation functions, to detect the presence of defects in the profile of the water pipeline is also proposed.


Hybrid modes in a single thermally excited asymmetric dimer antenna AbouHamdan, L., C. Li, R. Haidar, V. Krachmalnicoff, P. Bouchon, and Y. De Wilde Optics Letters 46, no. 5, 981984 (2021)
Résumé: The study of hybrid modes in a single dimer of neighboring antennas is an essential step to optimize the farfield electromagnetic (EM) response of largescale metasurfaces or any complex antenna structure made up of subwavelength building blocks. Here we present farfield infrared spatial modulation spectroscopy (IRSMS) measurements of a single thermally excited asymmetric dimer of square metalinsulatormetal (MIM) antennas separated by a nanometric gap. Through thermal fluctuations, all the EM modes of the antennas are excited, and hybrid bonding and antibonding modes can be observed simultaneously. We study the latter within a plasmon hybridization model, and analyze their effect on the farfield response.


Quantitative Temperature Measurements in Gold Nanorods Using Digital Holography Lalisse, A., A. A. Mohtar, M. C. Nguyen, R. Carminati, J. Plain, and G. Tessier ACS Applied Materials and Interfaces (2021)
Résumé: © Temperature characterization and quantification at the nanoscale remain core challenges in applications based on photoinduced heating of nanoparticles. Here, we propose a new approach to obtain quantitative temperature measurements on individual nanoparticles by combining modulated photothermal stimulation and heterodyne digital holography. From fullfield reconstructed holograms, the temperature is determined with a precision of 0.3 K via a simple approach without requiring any calibration or fitting parameters. As an application, the dependence of temperature on the aspect ratio of gold nanoparticles is investigated. A good agreement with numerical simulation is observed.


Inducing topology in a wire medium based metamaterial [Invited] Yves, S., G. Lerosey, and F. Lemoult Optical Materials Express 11, no. 3, 821841 (2021)
Résumé: © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement We review our attempt to tackle topological photonics based on an experimental platform operating in the microwave frequency range. The latter is based on a resonant metamaterial consisting in a dense collection of finitelength resonant metallic wires, known as the wire medium. Inside, the wave propagation is accurately described by a polariton, which exhibits subwavelength propagating modes as well as a hybridization bandgap. Thanks to a relevant design of the relative lengths of the wires and/or on their spatial positioning, we explore different aspects of topology applied to wave propagation.


A stable, unified model for resonant Faraday cages Delourme, B., E. Lunéville, J. J. Marigo, A. Maurel, J. F. Mercier, and K. Pham Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 477, no. 2245 (2021)
Résumé: © 2021 The Author(s). We study some effective transmission conditions able to reproduce the effect of a periodic array of Dirichlet wires on wave propagation, in particular when the array delimits an acoustic Faraday cage able to resonate. In the study of Hewett & Hewitt (2016 Proc. R. Soc. A 472, 20160062 (doi:10.1098/rspa.2016.0062)) different transmission conditions emerge from the asymptotic analysis whose validity depends on the frequency, specifically the distance to a resonance frequency of the cage. In practice, dealing with such conditions is difficult, especially if the problem is set in the time domain. In the present study, we demonstrate the validity of a simpler unified model derived in Marigo & Maurel (2016 Proc. R. Soc. A 472, 20160068 (doi:10.1098/rspa.2016.0068)), where unified means valid whatever the distance to the resonance frequencies. The effectiveness of the model is discussed in the harmonic regime owing to explicit solutions. It is also exemplified in the time domain, where a formulation guaranteeing the stability of the numerical scheme has been implemented.


Revisiting imperfect interface laws for twodimensional elastodynamics Pham, K., A. Maurel, and J. J. Marigo Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 477, no. 2245 (2021)
Résumé: © 2021 The Authors. We study the interaction of inplane elastic waves with imperfect interfaces composed of a periodic array of voids or cracks. An effective model is derived from highorder asymptotic analysis based on twoscale homogenization and matched asymptotic technique. In twodimensional elasticity, we obtain jump conditions set on the inplane displacements and normal stresses; the jumps involve in addition effective parameters provided by static, elementary problems being the equivalents of the cell problems in classical twoscale homogenization. The derivation of the model is conducted in the transient regime and its stability is guarantied by the positiveness of the effective interfacial energy. Spring models are envisioned as particular cases. It is shown that masslessspring models are recovered in the limit of small void thicknesses and collinear cracks. By contrast, the use of massspring model is justified at normal incidence, otherwise unjustified. We provide quantitative validations of our model and comparison with spring models by means of comparison with direct numerical calculations in the harmonic regime.


A simple novel approach for detecting blood–brain barrier permeability using GPCR internalization Csaba, Z., T. Vitalis, C. CharriautMarlangue, I. Margaill, B. Coqueran, P. L. Leger, I. Parente, A. Jacquens, L. Titomanlio, C. Constans, C. Demene, M. D. Santin, S. Lehericy, N. Perrière, F. Glacial, S. Auvin, M. Tanter, J. F. GhersiEgea, H. AdleBiassette, J. F. Aubry, P. Gressens, and P. Dournaud Neuropathology and Applied Neurobiology 47, no. 2, 297315 (2021)
Résumé: © 2020 British Neuropathological Society Aims: Impairment of blood–brain barrier (BBB) is involved in numerous neurological diseases from developmental to aging stages. Reliable imaging of increased BBB permeability is therefore crucial for basic research and preclinical studies. Today, the analysis of extravasation of exogenous dyes is the principal method to study BBB leakage. However, these procedures are challenging to apply in pups and embryos and may appear difficult to interpret. Here we introduce a novel approach based on agonistinduced internalization of a neuronal G proteincoupled receptor widely distributed in the mammalian brain, the somatostatin receptor type 2 (SST2). Methods: The clinically approved SST2 agonist octreotide (1 kDa), when injected intraperitoneally does not cross an intact BBB. At sites of BBB permeability, however, OCT extravasates and induces SST2 internalization from the neuronal membrane into perinuclear compartments. This allows an unambiguous localization of increased BBB permeability by classical immunohistochemical procedures using specific antibodies against the receptor. Results: We first validated our approach in sensory circumventricular organs which display permissive vascular permeability. Through SST2 internalization, we next monitored BBB opening induced by magnetic resonance imagingguided focused ultrasound in murine cerebral cortex. Finally, we proved that after intraperitoneal agonist injection in pregnant mice, SST2 receptor internalization permits analysis of BBB integrity in embryos during brain development. Conclusions: This approach provides an alternative and simple manner to assess BBB dysfunction and development in different physiological and pathological conditions.


Quantitative Measurement of the Thermal Contact Resistance between a Glass Microsphere and a Plate Doumouro, J., E. Perros, A. Dodu, N. Rahbany, D. Leprat, V. Krachmalnicoff, R. Carminati, W. Poirier, and Y. De Wilde Physical Review Applied 15, no. 1 (2021)
Résumé: © 2021 American Physical Society. Accurate measurements of the thermal resistance between microobjects made of insulating materials are complex because of their small size, low conductivity, and the presence of various illdefined gaps. We address this issue using a modified scanning thermal microscope operating in vacuum and in air. The sphereplate geometry is considered. Under controlled heating power, we measure the temperature on top of a glass microsphere glued to the probe as it approaches a glass plate at room temperature with nanometer accuracy. In vacuum, a jump is observed at contact. From this jump in temperature and the modeling of the thermal resistance of a sphere, the sphereplate contact resistance RK=(1.4±0.18)×107KW1 and effective radius r=36±4 nm are obtained. In air, the temperature on top of the sphere shows a decrease starting from a sphereplate distance of 200μm. A jump is also observed at contact, with a reduced amplitude. The sphereplate coupling out of contact can be described by the resistance shape factor of a sphere in front of a plate in air, placed in a circuit involving a series and a parallel resistance that are determined by fitting the approach curve. The contact resistance in air RK  =(1.2±0.46)×107KW1 is then estimated from the temperature jump. The method is quantitative without requiring any tedious multiplescale numerical simulation, and is versatile to describe the coupling between microobjects from large distances to contact in various environments.


A direct linear inversion for discontinuous elastic parameters recovery from internal displacement information only Ammari, H., E. Bretin, P. Millien, and L. Seppecher Numerische Mathematik 147, no. 1, 189226 (2021)
Résumé: © 2021, The Author(s), under exclusive licence to SpringerVerlag GmbH, DE part of Springer Nature. The aim of this paper is to present and analyze a new direct method for solving the linear elasticity inverse problem. Given measurements of some displacement fields inside a medium, we show that a stable reconstruction of elastic parameters is possible, even for discontinuous parameters and without boundary information. We provide a general approach based on the weak definition of the stiffnesstoforce operator which conduces to see the problem as a linear system. We prove that in the case of shear modulus reconstruction, we have an L2 stability with only one measurement under minimal smoothness assumptions. This stability result is obtained through the proof that the linear operator to invert has closed range. We then describe a direct discretization which provides stable reconstructions of both isotropic and anisotropic stiffness tensors.


Nanometric axial localization of single fluorescent molecules with modulated excitation Jouchet, P., C. Cabriel, N. Bourg, M. Bardou, C. Poüs, E. Fort, and S. LévêqueFort Nature Photonics (2021)
Résumé: © 2021, The Author(s), under exclusive licence to Springer Nature Limited. Distance measurements are commonly performed by phase detection based on a lockin strategy. Superresolution fluorescence microscopy is still striving to perform axial localization but through entirely different strategies. Here we show that an illumination modulation approach can achieve nanometric axial localization precision without compromising the acquisition time, emitter density or lateral localization precision. The excitation pattern is obtained by shifting tilted interference fringes. The molecular localizations are performed by measuring the relative phase between each fluorophore response and the reference modulated excitation pattern. We designed a fast demodulation scheme compatible with the short emission duration of single emitters. This modulated localization microscopy offers a typical axial localization precision of 6.8 nm over the entire field of view and the axial capture range. Furthermore, the interfering pattern being robust to optical aberrations, a nearly uniform axial localization precision enables imaging of biological samples by up to several micrometres in depth.


Bats seek refuge in cluttered environment when exposed to white and red lights at night Barré, K., C. Kerbiriou, R. K. Ing, Y. Bas, C. Azam, I. Le Viol, and K. Spoelstra Movement Ecology 9, no. 1 (2021)
Résumé: © 2021, The Author(s). Background: Artificial light at night is recognized as an increasing threat to biodiversity. However, information on the way highly mobile taxa such as bats spatially respond to light is limited. Following the hypothesis of a behavioural adaptation to the perceived risks of predation, we hypothesised that bats should avoid lit areas by shifting their flight route to less exposed conditions. Methods: Using 3D acoustic localization at four experimentally illuminated sites, we studied how the distance to streetlights emitting white and red light affected the Probability of bats Flying Inside the Forest (PFIF) versus along the forest edge. Results: We show that open, edge, and narrowspace foraging bats strongly change flight patterns by increasing PFIF when getting closer to white and red streetlights placed in the forest edge. These behavioural changes occurred mainly on the streetlight side where light was directed. Conclusions: The results show that bats cope with light exposure by actively seeking refuge in cluttered environment, potentially due to involved predation risks. This is a clear indication that bats make use of landscape structures when reacting to light, and shows the potential of vegetation and streetlight orientation in mitigating effects of light. The study nevertheless calls for preserving darkness as the most efficient way.


Creating Song from Lip and Tongue Videos with a Convolutional Vocoder Zhang, J., P. Roussel, and B. Denby IEEE Access 9 (2021)
Résumé: CCBY A convolutional neural network and deep autoencoder are used to predict Line Spectral Frequencies, F0, and a voiced/unvoiced flag in singing data, using as input only ultrasound images of the tongue and visual images of the lips. A novel convolutional vocoder to transform the learned parameters into an audio signal is also presented. Spectral Distortion of predicted Line Spectral Frequencies is reduced compared to that in an earlier study using handcrafted features and multilayer perceptrons on the same data set; while predicted F0 and voiced/unvoiced flag predictions are found to be highly correlated with their ground truth values. Comparison of the convolutional vocoder to standard vocoders is made. Results can be of interest in the study of singing articulation as well as for silent speech interface research. Sample predicted audio files are available online. Source code: https://github.com/TjuJianyu/SSI_DL.


Fullfield optical coherence tomography: Novel imaging technique for extemporaneous highresolution analysis of mucosal architecture in human gut biopsies Quénéhervé, L., R. Olivier, M. J. Gora, C. Bossard, J. F. Mosnier, E. Benoit A La Guillaume, C. Boccara, C. Brochard, M. Neunlist, and E. Coron Gut 70, no. 1, 68 (2021)
Résumé: © Fullfield optical coherence tomography (FFOCT) is an imaging technique of biological tissue based on tissue light reflectance analysis. We evaluated the feasibility of imaging fresh digestive mucosal biopsies after a quick mounting procedure (5 min) using two distinct modalities of FFOCT. In static FFOCT mode, we gained highresolution images of general gut tissuespecific architecture, such as oesophageal papillae, gastric pits, duodenal villi and colonic crypts. In dynamic FFOCT mode, we imaged individual epithelial cells of the mucosal lining with a cellular or subcellular resolution and identified cellular components of the lamina propria. FFOCT represents a promising dyefree imaging tool for onsite analysis of gut tissue remodelling.


Metamaterial inspired wireless coil for clinical breast imaging Puchnin, V., G. Solomakha, A. Nikulin, A. W. Magill, A. Andreychenko, and A. Shchelokova Journal of Magnetic Resonance 322, 106877 (2021)
Résumé: © 2020 Elsevier Inc. In this work, we propose an application of a metamaterial inspired volumetric wireless coil (WLC) based on coupled splitloop resonators for targeted breast MRI at 1.5 T. Due to strong electromagnetic coupling with the body coil, the metamaterial inspired WLC locally focuses radiofrequency (RF) magnetic flux in the target region, thus improving both transmit and receive performance of the external body coil. This leads to substantial enhancement in local transmit efficiency and improvement of RF safety. Phantom images showed a tenfold increase of signaltonoise ratio (SNR) in the regionofinterest (ROI) and, at the same time, an almost 50fold reduction in transmit power relative to the same body coil used alone.

