Fabrice LEMOULT

Résumé: Reconfigurable intelligent metasurfaces have been proposed as an efficient solution for improving wireless telecommunication systems in multiple-scattering or reverberating media. Concurrently, topology optimization has been successfully used as an inverse-design technique in many fields, and particularly in electromagnetics. In this work, we apply a gradient-based 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.

Résumé: A label-free 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 back-reflected 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.

Résumé: Space−time modulated metamaterials support extraordinary rich applications, such as parametric amplification, frequency conversion, and non-reciprocal transmission. The non-Hermitian space−time varying systems combining non-Hermiticity 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 software-defined impulse responses, we experimentally demonstrate non-Hermitian 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 meta-atoms results in unidirectional amplification in frequency conversion. The realization of non-Hermitian space−time varying metamaterials will offer further opportunities in studying non-Hermitian topological physics in dynamic and nonreciprocal systems.

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

Résumé: We study the propagation of in-plane elastic waves in a soft thin strip, a specific geometrical and mechanical hybrid framework which we expect to exhibit a Dirac-like cone. We separate the low frequencies guided modes (typically 100 Hz for a 1-cm-wide strip) and obtain experimentally the full dispersion diagram. Dirac cones are evidenced together with other remarkable wave phenomena such as negative wave velocity or pseudo-zero group velocity (ZGV). Our measurements are convincingly supported by a model (and numerical simulation) for both Neumann and Dirichlet boundary conditions. Finally, we perform one-way 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.

Résumé: © 2020 American Physical Society. Wave propagation control is of fundamental interest in many areas of physics. It can be achieved with wavelength-scaled photonic crystals, hence avoiding low-frequency applications. By contrast, metamaterials are structured on a deep-subwavelength scale, and therefore usually described through homogenization, neglecting the unit-cell structuration. Here, we show with microwaves that, by considering their inherent crystallinity, we can induce wave propagation carrying angular momenta within a subwavelength-scaled collection of wires. Then, inspired by the quantum valley Hall effect in condensed-matter 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 solid-state physics phenomena at the macroscopic scale, but it also opens the door to crystalline polarized subwavelength waveguides.

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.

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 tight-binding 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 solid-state physics offer.

Résumé: © 2017 The Author(s). Graphene, a honeycomb lattice of carbon atoms ruled by tight-binding 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 sub-lattice generates a bandgap at low frequency, which induces a tight-binding coupling between the resonant defects of the second honeycomb one, hence allowing us to obtain a graphene-like 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 solid-state physics phenomena with classical waves.

Résumé: The exciting discovery of topological condensed matter systems has lately triggered a search for their photonic analogues, motivated by the possibility of robust backscattering-immune 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.

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.

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

Résumé: 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.

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.

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

Résumé: 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 far-field region sub-diffraction 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 sub-wavelength focusing scheme with time reversal and demonstrate experimentally spots with focal widths of λ /25. Through a cross-correlation 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.

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.