Solution to the cocktail party problem: A timereversal active metasurface for multipoint focusing Bourdeloux, C., M. Fink, and F. Lemoult Physical Review Applied 21, no. 5, 054039 (2024)
Résumé: The cocktail party effect is the capability to focus one's auditory attention on particular audio sources while ignoring other audio sources. We propose an experimental strategy to reproduce this ability by designing a timedependent metasurface composed of independent active mirrors. Each active mirror is a programmable acoustic unit cell capable of hearing, computing, and reemitting acoustic signals: each of them acts as a convolution filter. The proper configuration of the metasurface temporal filters allows one to establish an acoustic communication link between groups of individuals immersed in the noisy environment: a multipleuser multipleinput, multipleoutput acoustic system is built.
Motsclés: metasurface;cocktail party;time reversal


Topology optimization for microwave control with reconfigurable intelligent metasurfaces in complex media Karamanos, T. D., M. Fink, and F. Lemoult Physical Review Applied 21, no. 4 (2024)
Résumé: Reconfigurable intelligent metasurfaces have been proposed as an efficient solution for improving wireless telecommunication systems in multiplescattering or reverberating media. Concurrently, topology optimization has been successfully used as an inversedesign technique in many fields, and particularly in electromagnetics. In this work, we apply a gradientbased topology optimization for tuning the binary elements of a metasurface for a focusing goal in a complex environment. First, the metasurface unit cells are approximated as point sources and, then, the optimization problem is formulated. Afterwards, the proposed method is applied to find the optimal parameter sets for three distinct environments of increasing complexity, and the resulting focus for each case is demonstrated via numerical simulations. The combination of a reverberating cavity and a metasurface inside the latter is very powerful since everything can be solved analytically for focusing outside the cavity.


Viscoelastic dynamics of a soft strip subject to a large deformation Delory, A., D. A. Kiefer, M. Lanoy, A. Eddi, Prada C., and F. Lemoult Soft Matter (2024)
Résumé: To produce sounds, we adjust the tension of our vocal folds to shape their properties and control the pitch. This efficient mechanism offers inspiration for designing reconfigurable materials and adaptable soft robots. However, understanding how flexible structures respond to a significant static strain is not straightforward. This complexity also limits the precision of medical imaging when applied to tensioned organs like muscles, tendons, ligaments and blood vessels among others. In this article, we experimentally and theoretically explore the dynamics of a soft strip subject to a substantial static
extension, up to 180%. Our observations reveal a few intriguing effects, such as the resilience of certain vibrational modes to a static deformation. These observations are supported by a model based on the incremental displacement theory. This has promising practical implications for characterizing soft materials but also for scenarios where external actions can be used to tune properties.


A nonlinear delayed resonator for mimicking the hearing haircells Reda, J., M. Fink, and F. Lemoult Europhysics Letters 144, no. 3, 37001 (2023)


Measuring Dirac cones in a brickwall lattice microwave metamaterial Li, B., S. Yves, A. Delory, S. Liu, M. Fink, and F. Lemoult Physical Review B 108, no. 9 (2023)
Résumé: The intriguing discovery of bidimensional structures in solidstate physics has motivated the seeking of their analogs in many fields. In this paper, we propose a general scheme to achieve Dirac cones in the microwave domain. It is based on a bidimensional locally resonant metamaterial ruled by a tightbinding Hamiltonian with asymmetric coupling. By specifically controlling the hopping links between metaatoms, the Dirac cones can be moved in the first Brillouin zone. A proof of this assertion is performed theoretically, numerically, and experimentally using a brickwall lattice of resonant metallic wires. The results directly evidence that the crystalline description of a subwavelengthscaled microwave system provides a really convenient tabletop platform for investigating the tempting challenges offered in solidstate physics.


Guided elastic waves in a highlystretched soft plate Delory, A., F. Lemoult, A. Eddi, and C. Prada Extreme Mechanics Letters, 102018 (2023)


Structurecomposition correspondence in crystalline metamaterials for acoustic valleyHall effect and unidirectional sound guiding Yves, S., G. Lerosey, and F. Lemoult Europhysics Letters 129, no. 4, 44001 (2020)


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.


Soft elastomers: A playground for guided waves Delory, A., F. Lemoult, M. Lanoy, A. Eddi, and C. Prada The Journal of the Acoustical Society of America 151, no. 5, 33433358 (2022)
Résumé: Mechanical waves propagating in soft materials play an important role in physiology. They can be natural, such as the cochlear wave in the inner ear of mammalians, or controlled, such as in elastography in the context of medical imaging. In a recent study, Lanoy, Lemoult, Eddi, and Prada [Proc. Natl. Acad. Sci. U.S.A. 117(48), 3018630190 (2020)] implemented an experimental tabletop platform that allows direct observation of inplane guided waves in a soft strip. Here, a detailed description of the setup and signal processing steps is presented as well as the theoretical framework supporting them. One motivation is to propose a tutorial experiment for visualizing the propagation of guided elastic waves. Last, the versatility of the experimental platform is exploited to illustrate experimentally original features of wave physics, such as backward modes, stationary modes, and Dirac cones.


Unidirectional amplification with acoustic nonHermitian space−time varying metamaterial Wen, X., X. Zhu, A. Fan, W. Y. Tam, J. Zhu, H. W. Wu, F. Lemoult, M. Fink, and J. Li Communications Physics 5, no. 1 (2022)
Résumé: Space−time modulated metamaterials support extraordinary rich applications, such as parametric amplification, frequency conversion, and nonreciprocal transmission. The nonHermitian space−time varying systems combining nonHermiticity and space−time varying capability, have been proposed to realize wave control like unidirectional amplification, while its experimental realization still remains a challenge. Here, based on metamaterials with softwaredefined impulse responses, we experimentally demonstrate nonHermitian space−time varying metamaterials in which the material gain and loss can be dynamically controlled and balanced in the time domain instead of spatial domain, allowing us to suppress scattering at the incident frequency and to increase the efficiency of frequency conversion at the same time. An additional modulation phase delay between different metaatoms results in unidirectional amplification in frequency conversion. The realization of nonHermitian space−time varying metamaterials will offer further opportunities in studying nonHermitian topological physics in dynamic and nonreciprocal systems.


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.


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.


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.


Dirac cones and chiral selection of elastic waves in a soft strip Lanoy, M., F. Lemoult, A. Eddi, and C. Prada Proceedings of the National Academy of Sciences, 202010812 (2020)
Résumé: We study the propagation of inplane elastic waves in a soft thin
strip, a specific geometrical and mechanical hybrid framework
which we expect to exhibit a Diraclike cone. We separate the
low frequencies guided modes (typically 100 Hz for a 1cmwide
strip) and obtain experimentally the full dispersion diagram. Dirac
cones are evidenced together with other remarkable wave phenomena
such as negative wave velocity or pseudozero group
velocity (ZGV). Our measurements are convincingly supported
by a model (and numerical simulation) for both Neumann and
Dirichlet boundary conditions. Finally, we perform oneway chiral
selection by carefully setting the source position and polarization.
Therefore, we show that soft materials support atypical wavebased
phenomena, which is all of the more interesting as they
make most of the biological tissues.


Nonreciprocal Manipulation of Subwavelength Fields in Locally Resonant Metamaterial Crystals ZangenehNejad, F., N. Kaina, S. Yves, F. Lemoult, G. Lerosey, and R. Fleury IEEE Transactions on Antennas and Propagation 68, no. 3, 17261732 (2020)
Résumé: © 2019 IEEE. Locally resonant metamaterial crystals are artificial materials built from small spatially local resonant inclusions periodically arranged at subwavelength scale. Unlike conventional continuous metamaterials, for which spatial dispersion originates mostly (but not exclusively) from the nonlocality of their inclusions, they exhibit large spatially nonlocal effects that emerge solely at the array level because of the periodic structuration of simple spatially local scatterers, often allowing for an intrinsically subwavelength granularity. Herein, we demonstrate the unique relevance of metamaterial crystals to induce nonreciprocal electromagnetic propagation at deep subwavelength scales. This is obtained by combining the breaking of timereversal symmetry, using an externally biased magnetic material, with appropriate spatialdispersion engineering, via subwavelength structural modification of the metamaterial crystal. Interestingly, the material unit cell can be scaled down without affecting this functionality, leading to the exciting possibility of largely enhanced wavematter interaction at deep subwavelength scales. Altogether, our proposal provides an interesting route for transposing the rich physics of nonreciprocal systems down to the subwavelength scale.
Motsclés: Metamaterials; nonreciprocity; paritytime symmetry; subwavelength wave manipulation


Locally polarized wave propagation through crystalline metamaterials Yves, S., T. Berthelot, G. Lerosey, and F. Lemoult Physical Review B 101, no. 3 (2020)
Résumé: © 2020 American Physical Society. Wave propagation control is of fundamental interest in many areas of physics. It can be achieved with wavelengthscaled photonic crystals, hence avoiding lowfrequency applications. By contrast, metamaterials are structured on a deepsubwavelength scale, and therefore usually described through homogenization, neglecting the unitcell structuration. Here, we show with microwaves that, by considering their inherent crystallinity, we can induce wave propagation carrying angular momenta within a subwavelengthscaled collection of wires. Then, inspired by the quantum valley Hall effect in condensedmatter physics, we exploit this bulk circular polarization to create modes propagating along particular interfaces. The latter also carry an edge angular momentum whose conservation during the propagation allows wave routing by design in specific directions. This experimental study not only evidences that crystalline metamaterials are a straightforward tabletop platform to emulate exciting solidstate physics phenomena at the macroscopic scale, but it also opens the door to crystalline polarized subwavelength waveguides.


Mimicking the cochlea with an active acoustic metamaterial Rupin, M., G. Lerosey, J. De Rosny, and F. Lemoult New Journal of Physics 21, no. 9 (2019)
Résumé: © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft. The human ear is a fascinating sensor, capable of detecting pressures over ten octaves of frequency and twelve orders of magnitudes. Here, following a biophysical model, we demonstrate experimentally that the physics of a living cochlea can be emulated by an active onedimensional acoustic metamaterial. The latter solely consists on a set of subwavelength active acoustic resonators, coupled to a main propagating waveguide. By introducing a gradient in the resonators' properties, we establish an experimental setup which mimics the dynamical responses of both the dead and the living cochleae: The cochlear tonotopy as well as the lowAmplitude sound amplifier are reproduced.
Motsclés: Acoustics; Hearing; Inner ear; Metamaterial


Multiple scattering enabled superdirectivity from a subwavelength ensemble of resonators Metais, S., G. Lerosey, and F. Lemoult Physical Review B 100, no. 12 (2019)
Résumé: © 2019 American Physical Society. An ensemble of resonators arranged on a subwavelength scale is usually considered as a bulk effective medium, known as a metamaterial, and can offer unusual macroscopic properties. Here, we take a different approach and limit ourselves to the study of only a few number of such elementary components and demonstrate that they still offer uncommon opportunities. Typically, owing to the multiple scattering and the phase shift that the resonances offer, we observe fields that vary at scales completely independent of the wavelength in free space. By smartly tuning the resonance frequencies, we can design at will the complex current distribution in those resonators. This way, we design a superdirective antenna, i.e., an antenna that is surprisingly more directive than its size would foreshadow. This approach is verified numerically and experimentally in the context of microwaves, but this applies to any wave field where subwavelength resonators exist.


Lefthanded band in an electromagnetic metamaterial induced by subwavelength multiple scattering Yves, S., T. Berthelot, M. Fink, G. Lerosey, and F. Lemoult Applied Physics Letters 114, no. 11 (2019)
Résumé: © 2019 Author(s). Due to the deep subwavelength unit cell in metamaterials, the quasistatic approximation is usually employed to describe the propagation. By making pairs of resonators, we highlight that multiple scattering also occurs at this scale and results in the existence of a dipolar resonance, which leads to a negative index of refraction when we consider several resonators. We experimentally verify the possibility of obtaining a negative index of refraction in periodic metamaterials in two different ways and eventually demonstrate the subwavelength recovery of several point sources in both cases.


Measuring Dirac Cones in a Subwavelength Metamaterial Yves, S., T. Berthelot, M. Fink, G. Lerosey, and F. Lemoult Physical Review Letters 121, no. 26 (2018)
Résumé: © 2018 American Physical Society. The exciting discovery of bidimensional systems in condensed matter physics has triggered the search of their photonic analogues. In this Letter, we describe a general scheme to reproduce some of the systems ruled by a tightbinding Hamiltonian in a locally resonant metamaterial; by specifically controlling the structure and the composition it is possible to engineer the band structure at will. We numerically and experimentally demonstrate this assertion in the microwave domain by reproducing the band structure of graphene, the most famous example of those 2D systems, and by accurately extracting the Dirac cones. This is direct evidence that opting for a crystalline description of those subwavelength scaled systems, as opposed to the usual description in terms of effective parameters, makes them a really convenient tabletop platform to investigate the tantalizing challenges that solidstate physics offer.


Acoustic double negativity induced by position correlations within a disordered set of monopolar resonators Lanoy, M., J. H. Page, G. Lerosey, F. Lemoult, A. Tourin, and V. Leroy Physical Review B 96, no. 22 (2017)
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.


Crystalline Soda Can Metamaterial exhibiting Graphenelike Dispersion at subwavelength scale Yves, S., F. Lemoult, M. Fink, and G. Lerosey Scientific Reports 7, no. 1 (2017)
Résumé: © 2017 The Author(s). Graphene, a honeycomb lattice of carbon atoms ruled by tightbinding interaction, exhibits extraordinary electronic properties due to the presence of Dirac cones within its band structure. These intriguing singularities have naturally motivated the discovery of their classical analogues. In this work, we present a general and direct procedure to reproduce the peculiar physics of graphene within a very simple acoustic metamaterial: a double lattice of soda cans resonating at two different frequencies. The first triangular sublattice generates a bandgap at low frequency, which induces a tightbinding coupling between the resonant defects of the second honeycomb one, hence allowing us to obtain a graphenelike band structure. We prove the relevance of this approach by showing that both numerical and experimental dispersion relations exhibit the requested Dirac cone. We also demonstrate the straightforward monitoring of the coupling strength within the crystal of resonant defects. This work shows that crystalline metamaterials are very promising candidates to investigate tantalizing solidstate physics phenomena with classical waves.


Topological acoustic polaritons: Robust sound manipulation at the subwavelength scale Yves, S., R. Fleury, F. Lemoult, M. Fink, and G. Lerosey New Journal of Physics 19, no. 7 (2017)
Résumé: © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Topological insulators, a hallmark of condensed matter physics, have recently reached the classical realm of acoustic waves. A remarkable property of timereversal invariant topological insulators is the presence of unidirectional spinpolarized propagation along their edges, a property that could lead to a wealth of new opportunities in the ability to guide and manipulate sound. Here, we demonstrate and study the possibility to induce topologically nontrivial acoustic states at the deep subwavelength scale, in a structured twodimensional metamaterial composed of Helmholtz resonators. Radically different from previous designs based on nonresonant sonic crystals, our proposal enables robust sound manipulation on a surface along predefined, subwavelength pathways of arbitrary shapes.
Motsclés: acoustic metamaterials; polaritons; quantum spin Hall effect; topological insulators


Crystalline metamaterials for topological properties at subwavelength scales Yves, S., R. Fleury, T. Berthelot, M. Fink, F. Lemoult, and G. Lerosey Nature Communications 8 (2017)
Résumé: The exciting discovery of topological condensed matter systems has lately triggered a search for their photonic analogues, motivated by the possibility of robust backscatteringimmune light transport. However, topological photonic phases have so far only been observed in photonic crystals and waveguide arrays, which are inherently physically wavelength scaled, hindering their application in compact subwavelength systems. In this letter, we tackle this problem by patterning the deep subwavelength resonant elements of metamaterials onto specific lattices, and create crystalline metamaterials that can develop complex nonlocal properties due to multiple scattering, despite their very subwavelength spatial scale that usually implies to disregard their structure. These spatially dispersive systems can support subwavelength topological phases, as we demonstrate at microwaves by direct field mapping. Our approach gives a straightforward tabletop platform for the study of photonic topological phases, and allows to envision applications benefiting the compactness of metamaterials and the amazing potential of topological insulators.


Spatiotemporal Wave Front Shaping in a Microwave Cavity Del Hougne, P., F. Lemoult, M. Fink, and G. Lerosey Physical Review Letters 117, no. 13 (2016)


Soda cans metamaterial: A subwavelengthscaled phononic crystal Lemoult, F., N. Kaina, M. Fink, and G. Lerosey Crystals 6, no. 7 (2016)
Résumé: © 2016 by the authors; licensee MDPI, Basel, Switzerland.Photonic or phononic crystals and metamaterials, due to their very different typical spatial scales—wavelength and deep subwavelength—and underlying physical mechanisms—Bragg interferences or local resonances—, are often considered to be very different composite media. As such, while the former are commonly used to manipulate and control waves at the scale of the unit cell, i.e., wavelength, the latter are usually considered for their effective properties. Yet we have shown in the last few years that under some approximations, metamaterials can be used as photonic or phononic crystals, with the great advantage that they are much more compact. In this review, we will concentrate on metamaterials made out of soda cans, that is, Helmholtz resonators of deep subwavelength dimensions. We will first show that their properties can be understood, likewise phononic crystals, as resulting from interferences only, through multiple scattering effects and Fano interferences. Then, we will demonstrate that below the resonance frequency of its unit cell, a soda can metamaterial supports a band of subwavelength varying modes, which can be excited coherently using time reversal, in order to beat the diffraction limit from the far field. Above this frequency, the metamaterial supports a band gap, which we will use to demonstrate cavities and waveguides, very similar to those obtained in phononic crystals, albeit of deep subwavelength dimensions. We will finally show that multiple scattering can be taken advantage of in these metamaterials, by correctly structuring them. This allows to turn a metamaterial with a single negative effective property into a negative index metamaterial, which refracts waves negatively, hence acting as a superlens.
Motsclés: Acoustics; Metamaterial; Multiple scattering; Phononic crystals


Exploiting spatiotemporal degrees of freedom for farfield subwavelength focusing using time reversal in fractals Dupré, M., F. Lemoult, M. Fink, and G. Lerosey Physical Review B  Condensed Matter and Materials Physics 93, no. 18 (2016)
Résumé: © 2016 American Physical Society. Materials which possess a high local density of states varying at a subwavelength scale theoretically permit the focusing of waves onto focal spots much smaller than the free space wavelength. To do so, metamaterials  manmade composite media exhibiting properties not available in nature  are usually considered. However, this approach is limited to narrow bandwidths due to their resonant nature. Here, we prove that it is possible to use a fractal resonator alongside time reversal to focus microwaves onto λ/15 subwavelength focal spots from the far field, on extremely wide bandwidths. We first numerically prove that this approach can be realized using a multiplechannel time reversal mirror that utilizes all the degrees of freedom offered by the fractal resonator. Then, we experimentally demonstrate that this approach can be drastically simplified by coupling the fractal resonator to a complex medium, here a cavity, that efficiently converts its spatial degrees of freedom into temporal ones. This makes it possible to achieve deep subwavelength focusing of microwave radiation by time reversing a single channel. Our method can be generalized to other systems coupling complex media and fractal resonators.


Negative refractive index and acoustic superlens from multiple scattering in single negative metamaterials. Kaina, N., F. Lemoult, M. Fink, and G. Lerosey Nature 525, no. 7567, 7781 (2015)


Symmetry issues in the hybridization of multimode waves with resonators: An example with Lamb waves metamaterial Rupin, M., P. Roux, G. Lerosey, and F. Lemoult Scientific Reports 5 (2015)
Résumé: Locally resonant metamaterials derive their effective properties from hybridization between their resonant unit cells and the incoming wave. This phenomenon is well understood in the case of plane waves that propagate in media where the unit cell respects the symmetry of the incident field. However, in many systems, several modes with orthogonal symmetries can coexist at a given frequency, while the resonant unit cells themselves can have asymmetric scattering crosssections. In this paper we are interested in the influence of symmetry breaking on the hybridization of a wave field that includes multiple propagative modes. The A 0 and S 0 Lamb waves that propagate in a thin plate are good candidates for this study, as they are either antisymmetric or symmetric. First we designed an experimental setup with an asymmetric metamaterial made of long rods glued to one side of a metallic plate. We show that the flexural resonances of the rods induce a break of the orthogonality between the A 0/S 0 modes of the freeplate. Finally, based on numerical simulations we show that the orthogonality is preserved in the case of a symmetric metamaterial leading to the presence of two independent polariton curves in the dispersion relation.


WaveField Shaping in Cavities: Waves Trapped in a Box with Controllable Boundaries Dupré, M., P. Del Hougne, M. Fink, F. Lemoult, and G. Lerosey Physical Review Letters 115, 017701 (2015)


Subwavelength focusing in bubbly media using broadband time reversal Lanoy, M., R. Pierrat, F. Lemoult, M. Fink, V. Leroy, and A. Tourin Physical Review B 91, no. 22 (2015)


Superabsorption of acoustic waves with bubble metascreens Leroy, V., A. Strybulevych, M. Lanoy, F. Lemoult, A. Tourin, and J. H. Page Physical Review B 91, 020301 (2015)
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.


Experimental Demonstration of Ordered and Disordered Multiresonant Metamaterials for Lamb Waves Rupin, M., F. Lemoult, G. Lerosey, and P. Roux Physical Review Letters 112, no. 23 (2014)


Ultra small mode volume defect cavities in spatially ordered and disordered metamaterials Kaina, N., F. Lemoult, M. Fink, and G. Lerosey Applied Physics Letters 102, no. 14 (2013)
Résumé: In this letter, we study metamaterials made out of resonant electric wires arranged on a spatial scale much smaller than the free space wavelength, and we show that they present a hybridization band that is insensible to positional disorder. We experimentally demonstrate defect cavities in disordered and ordered samples and prove that, analogous to those designed in photonic crystals, those cavities can present very high quality factors. In addition, we show that they display mode volumes much smaller than a wavelength cube, owing to the deep subwavelength nature of the unit cell. We underline that this type of structure can be shrunk down to a period close of a few skin depth. Our approach paves the way towards the confinement and manipulation of waves at deep subwavelength scales in both ordered and disordered metamaterials. © 2013 AIP Publishing LLC.
Motsclés: Defect cavity; Display modes; Freespace wavelengths; High quality factors; Positional disorder; Spatial scale; Subwavelength; Subwavelength scale; Defects; Metamaterials


Wave propagation control at the deep subwavelength scale in metamaterials Lemoult, F., N. Kaina, M. Fink, and G. Lerosey Nature Physics 9, no. 1, 5560 (2013)
Résumé: The ability to control wave propagation is of fundamental interest in many areas of physics. Photonic crystals proved very useful for this purpose but, because they are based on Bragg interferences, these artificial media require structures with large dimensions. Metamaterials, on the other hand, can exhibit very deep subwavelength spatial scales. In general they are studied for their bulk effective properties that lead to effects such as negative refraction. Here we go beyond this effective medium paradigm and we use a microscopic approach to study metamaterials based on resonant unit cells. We show that we can tailor unit cells locally to shape the flow of waves at deep subwavelength scales. We validate our approach in experiments with both electromagnetic and acoustic waves in the metre range demonstrating cavities, waveguides, corners and splitters with centimetrescale dimensions, an order of magnitude smaller than previous proposals. © 2013 Macmillan Publishers Limited.


Far field subwavelength imaging of magnetic patterns Ourir, A., G. Lerosey, F. Lemoult, M. Fink, and J. De Rosny Applied Physics Letters 101, no. 11 (2012)
Résumé: Far field imaging of subwavelength magnetic objects in real time is a very challenging issue. We propose an original solution based on a planar array of closely spaced split ring resonators. Hybridization between the resonators of such metalens induces subwavelength modes with different frequencies. Thanks to these high Q resonating modes, Purcell like effect allows an evanescent source, close to the metalens, to emit waves that can be collected efficiently in the far field. We present the first microwave experimental demonstration of such metalens to image of a subwavelength magnetic pattern. Numerical simulation shows that this approach is still valid at THz frequencies. © 2012 American Institute of Physics.
Motsclés: Different frequency; Far field; Farfield imaging; Magnetic patterns; MetaLens; Planar arrays; Real time; Split ring resonator; Subwavelength; Subwavelength imaging; THz frequencies; Physical properties; Physics


A polychromatic approach to farfield superlensing at visible wavelengths Lemoult, F., M. Fink, and G. Lerosey Nature Communications 3 (2012)
Résumé: Breaking the diffraction barrier in the visible part of the electromagnetic spectrum is of fundamental importance. Farfield subwavelength focusing of light could, for instance, drastically broaden the possibilities available in nanolithography, lightmatter interactions and sensing at the nanoscale. Similarly, imaging with a nanometric resolution could result in incredible breakthroughs in soft matter and biology. There have been numerous proposals in this regard based on metamaterials, structured illumination methods or diffractive optical components. The common denominator of all these approaches resides in their monochromatic nature. Here we show that using polychromatic light in dispersive metamaterials allows us to circumvent many limitations associated with previous monochromatic approaches. We design a plasmonic metalens based on metallic nanorods that, when used with broadband light fields, can beat the diffraction limit for imaging and focusing from the far field. © 2012 Macmillan Publishers Limited. All rights reserved.
Motsclés: nanomaterial; nanorod; article; diffraction; electromagnetic field; electromagnetic radiation; imaging system; lens; light; polychromatic light; spectral sensitivity


Time reversal in subwavelengthscaled resonant media: Beating the diffraction limit Lemoult, F., A. Ourir, J. De Rosny, A. Tourin, M. Fink, and G. Lerosey International Journal of Microwave Science and Technology (2011)
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.


Revisiting the wire medium: An ideal resonant metalens Lemoult, F., M. Fink, and G. Lerosey Waves in Random and Complex Media 21, no. 4, 591613 (2011)
Résumé: This article is the first one in a series of two dealing with the concept of a 'resonant metalens' we introduced recently. Here, we focus on the physics of a medium with finite dimensions consisting of a square lattice of parallel conducting wires arranged on a subwavelength scale. This medium supports electromagnetic fields that vary much faster than the operating wavelength. We show that such modes are dispersive due to the finiteness of the medium. Their dispersion relation is established in a simple way, a link with designer plasmons is made, and the canalization phenomenon is reinterpreted in the light of our model. We explain how to take advantage of this dispersion in order to code subwavelength wavefields in time. Finally, we show that the resonant nature of the medium ensures an efficient coupling of these modes with free space propagating waves and, thanks to the Purcell effect, with a source placed in the near field of the medium. © 2011 Taylor & Francis.
Motsclés: Conducting wire; Dispersion relations; Efficient coupling; Finite dimensions; Free space; MetaLens; Near fields; Operating wavelength; Purcell effect; Square lattices; Subwavelength; Wavefields; Wire medium; Electromagnetic fields; Wire; Dispersion (waves)


Farfield subwavelength imaging and focusing using a wire medium based resonant metalens Lemoult, F., M. Fink, and G. Lerosey Waves in Random and Complex Media 21, no. 4, 614627 (2011)
Résumé: This is the second article in a series of two dealing with the concept of a 'resonant metalens' we introduced recently. This is a new type of lens capable of coding in time and radiating efficiently in the farfield region subdiffraction information about an object. A proof of the concept of such a lens is performed in the microwave range, using a medium made out of a square lattice of parallel conducting wires with finite length. We investigate a subwavelength focusing scheme with time reversal and demonstrate experimentally spots with focal widths of λ /25. Through a crosscorrelation based imaging procedure we show an image reconstruction with a resolution of λ/80. Eventually we discuss the limitations of such a lens which reside essentially in losses. © 2011 Taylor & Francis.
Motsclés: Conducting wire; Cross correlations; Farfield; Farfield region; Finite length; MetaLens; Square lattices; Subdiffraction; Subwavelength; Time reversal; Wire medium; Image reconstruction; Wire; Focusing


Acoustic resonators for farfield control of sound on a subwavelength scale Lemoult, F., M. Fink, and G. Lerosey Physical Review Letters 107, no. 6 (2011)
Résumé: We prove experimentally that broadband sounds can be controlled and focused at will on a subwavelength scale by using acoustic resonators. We demonstrate our approach in the audible range with soda cans, that is, Helmholtz resonators, and commercial computer speakers. We show that diffractionlimited sound fields convert efficiently into subdiffraction modes in the collection of cans that can be controlled coherently in order to obtain focal spots as thin as 1/25 of a wavelength in air. We establish that subwavelength acoustic pressure spots are responsible for a strong enhancement of the acoustic displacement at focus, which permits us to conclude with a visual experiment exemplifying the interest of our concept for subwavelength sensors and actuators. © 2011 American Physical Society.
Motsclés: Acoustic pressures; Diffraction limited; Farfield; Focal spot; Helmholtz resonators; Sensors and actuators; Strong enhancement; Subdiffraction; Subwavelength; Subwavelength scale; Visual experiments; Acoustic fields; Resonators; Acoustic resonators


Resonant metalenses for breaking the diffraction barrier Lemoult, F., G. Lerosey, J. De Rosny, and M. Fink Physical Review Letters 104, no. 20 (2010)
Résumé: We introduce the resonant metalens, a cluster of coupled subwavelength resonators. Dispersion allows the conversion of subwavelength wave fields into temporal signatures while the Purcell effect permits an efficient radiation of this information in the far field. The study of an array of resonant wires using microwaves provides a physical understanding of the underlying mechanism. We experimentally demonstrate imaging and focusing from the far field with resolutions far below the diffraction limit. This concept is realizable at any frequency where subwavelength resonators can be designed. © 2010 The American Physical Society.
Motsclés: Diffraction barrier; Diffraction limits; Far field; MetaLens; Purcell effect; Subwavelength; Subwavelength resonators; Temporal signatures; Underlying mechanism; Wavefields; Resonators


Manipulating Spatiotemporal Degrees of Freedom of Waves in Random Media Lemoult, F., G. Lerosey, J. De Rosny, and M. Fink Physical Review Letters 103, no. 17 (2009)
Résumé: We show that all the spatiotemporal degrees of freedom available in a complex medium can be harnessed and converted into spatial ones. This is demonstrated experimentally through an instantaneous spatial inversion, using broadband ultrasonic waves in a multiple scattering sample. We show theoretically that the inversion convergence is governed by the total number of degrees of freedom available in the medium for a fixed bandwidth and demonstrate experimentally its use for complex media investigation. We believe our approach has potential in sensing, imagery, focusing, and telecommunication. © 2009 The American Physical Society.
Motsclés: Complex media; Complex medium; Degrees of freedom; Number of degrees of freedom; Waves in random media; Ultrasonics; Mechanics

