Reconfigurable Intelligent Surfaces vs. Relaying: Differences, Similarities, and Performance Comparison
Di Renzo, M., K. Ntontin, J. Song, F. H. Danufane, X. Qian, F. Lazarakis, J. De Rosny, D.-T. Phan-Huy, O. Simeone, R. Zhang, M. Debbah, G. Lerosey, M. Fink, S. Tretyakov, and S. Shamai
IEEE Open Journal of the Communications Society 1, 798-807 (2020)
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Structure-composition correspondence in crystalline metamaterials for acoustic valley-Hall effect and unidirectional sound guiding
Yves, S., G. Lerosey, and F. Lemoult
Europhysics Letters 129, no. 4, 44001 (2020)
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Superresolved Imaging Based on Spatiotemporal Wave-Front Shaping
Noetinger, G., S. Métais, G. Lerosey, M. Fink, S. M. Popoff, and F. Lemoult
Physical Review Applied 19, no. 2 (2023)
Abstract: 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.
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Wavefront Shaping for Wireless Communications in Complex Media: From Time Reversal to Reconfigurable Intelligent Surfaces
Lerosey, G., and M. Fink
Proceedings of the IEEE 110, no. 9, 1210-1226 (2022)
Abstract: Reconfigurable intelligent surfaces (RISs) are gaining huge momentum in the field of wireless communications due to the paradigm shift that they bring. Indeed, they allow making any environment electromagnetically smart and dynamically reconfigurable for more efficient and greener wireless communications. As physicists, we proposed to use electronically tunable metasurfaces to shape the electromagnetic waves carrying our wireless communications in reflection almost ten years ago, inspired by some works that we and colleagues did in the field of wave control in complex media. In this article, we review the seminal works that led us to propose this concept, starting from the original one that is time reversal. Then, we propose a physicist's point of view of RISs using a comparison with phase conjugation. Finally, we highlight what we think are their limitations, relying on both our knowledge of wave control and our study of them over a decade.
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Three-dimensional 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)
Abstract: 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.
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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)
Abstract: 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 61000-4-21 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.
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Inducing topology in a wire medium based metamaterial [Invited]
Yves, S., G. Lerosey, and F. Lemoult
Optical Materials Express 11, no. 3, 821-841 (2021)
Abstract: © 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 finite-length 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.
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Tuning a regular cavity to wave chaos with metasurface-reconfigurable walls
Gros, J. B., P. Del Hougne, and G. Lerosey
Physical Review A 101, no. 6 (2020)
Abstract: © 2020 American Physical Society Wave-chaotic systems underpin a wide range of research activities from fundamental studies of quantum chaos via electromagnetic compatibility up to more recently emerging applications, such as microwave imaging for security screening, antenna characterization, or wave-based analog computation. To implement a wave-chaotic system experimentally, traditionally cavities of elaborate geometries (bow tie shapes, truncated circles, or parallelepipeds with hemispheres) are employed because the geometry dictates the wave field's characteristics. Here, we propose and experimentally verify a conceptually different approach: a cavity of regular geometry but with tunable boundary conditions, experimentally implemented by leveraging a reconfigurable metasurface reflect array. This approach offers an alternative stirring mechanism and enables a fuller study of random matrix theory in connection with wave chaos.
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Nonreciprocal Manipulation of Subwavelength Fields in Locally Resonant Metamaterial Crystals
Zangeneh-Nejad, F., N. Kaina, S. Yves, F. Lemoult, G. Lerosey, and R. Fleury
IEEE Transactions on Antennas and Propagation 68, no. 3, 1726-1732 (2020)
Abstract: © 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 time-reversal symmetry, using an externally biased magnetic material, with appropriate spatial-dispersion 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 wave-matter 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.
Keywords: Metamaterials; nonreciprocity; parity-time symmetry; subwavelength wave manipulation
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Locally polarized wave propagation through crystalline metamaterials
Yves, S., T. Berthelot, G. Lerosey, and F. Lemoult
Physical Review B 101, no. 3 (2020)
Abstract: © 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.
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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)
Abstract: © 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 one-dimensional 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 set-up which mimics the dynamical responses of both the dead and the living cochleae: The cochlear tonotopy as well as the low-Amplitude sound amplifier are reproduced.
Keywords: Acoustics; Hearing; Inner ear; Metamaterial
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Inducing topological effects in locally resonant metamaterials
Yves, S., G. Lerosey, and F. Lemoult
2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)
Abstract: © 2019 IEEE. Metamaterials are artificial media which provide exotic propagation propertiesthanks to their structuration on scales that are much smaller than the free-space wavelength of operation. Consequently, the propagation in these systems is usually described by the use of effective parameters. However, although this description allows to envision metamaterial engineering at a mesoscopic scale, it neglects the effects owing to the spatial patterning at the scale of the unit cell. Yet, it has been shown that the wave physics within a specific class of metamaterial, namely the locally resonant ones, can be accurately explained with the use of band structures [1].
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Multiple scattering enabled superdirectivity from a subwavelength ensemble of resonators
Metais, S., G. Lerosey, and F. Lemoult
Physical Review B 100, no. 12 (2019)
Abstract: © 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.
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WE2.3-performing linear operations using optical complex media (Invited)
Matthes, M., P. Del Hougne, J. De Rosny, G. Lerosey, and S. Popoff
IEEE Photonics Society Summer Topical Meeting Series 2019, SUM 2019 (2019)
Abstract: © 2019 IEEE. We propose using complex media as platforms to build any linear operator using wavefront shaping. We demonstrate that system composed of multimode fiber and spatial light modulator can act like any linear operator and can be reconfigured at will. We experimentally performed several 16×16-single-shot operations.
Keywords: Multimode Fiber; Optical Linear Operations; Wavefront Shaping
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Optical complex media as universal reconfigurable linear operators
Matthès, M. W., P. Del Hougne, J. De Rosny, G. Lerosey, and S. M. Popoff
Optica 6, no. 4, 465-472 (2019)
Abstract: © 2019 Optical Society of America. Performing linear operations using optical devices is a crucial building block in many fields ranging from telecommunications to optical analog computation and machine learning. For many of these applications, key requirements are robustness to fabrication inaccuracies, reconfigurability, and scalability. We propose a way to perform linear operations using complex optical media such as multimode fibers or scattering media as a computational platform driven by wavefront shaping. Given a large random transmission matrix representing light propagation in such a medium, we can extract any desired smaller linear operator by finding suitable input and output projectors. We demonstrate this concept by finding input wavefronts using a spatial light modulator that cause the complex medium to act as a desired complex-valued linear operator on the optical field. We experimentally build several 16 × 16 operators and discuss the fundamental limits of the scalability of our approach. It offers the prospect of reconfigurable, robust, and easy-to-fabricate linear optical analog computation units.
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Smart radio environments empowered by reconfigurable AI meta-surfaces: an idea whose time has come
Renzo, M. D., M. Debbah, D. T. Phan-Huy, A. Zappone, M. S. Alouini, C. Yuen, V. Sciancalepore, G. C. Alexandropoulos, J. Hoydis, H. Gacanin, J. d. Rosny, A. Bounceur, G. Lerosey, and M. Fink
Eurasip Journal on Wireless Communications and Networking 2019, no. 1 (2019)
Abstract: © 2019, The Author(s). Future wireless networks are expected to constitute a distributed intelligent wireless communications, sensing, and computing platform, which will have the challenging requirement of interconnecting the physical and digital worlds in a seamless and sustainable manner. Currently, two main factors prevent wireless network operators from building such networks: (1) the lack of control of the wireless environment, whose impact on the radio waves cannot be customized, and (2) the current operation of wireless radios, which consume a lot of power because new signals are generated whenever data has to be transmitted. In this paper, we challenge the usual “more data needs more power and emission of radio waves” status quo, and motivate that future wireless networks necessitate a smart radio environment: a transformative wireless concept, where the environmental objects are coated with artificial thin films of electromagnetic and reconfigurable material (that are referred to as reconfigurable intelligent meta-surfaces), which are capable of sensing the environment and of applying customized transformations to the radio waves. Smart radio environments have the potential to provide future wireless networks with uninterrupted wireless connectivity, and with the capability of transmitting data without generating new signals but recycling existing radio waves. We will discuss, in particular, two major types of reconfigurable intelligent meta-surfaces applied to wireless networks. The first type of meta-surfaces will be embedded into, e.g., walls, and will be directly controlled by the wireless network operators via a software controller in order to shape the radio waves for, e.g., improving the network coverage. The second type of meta-surfaces will be embedded into objects, e.g., smart t-shirts with sensors for health monitoring, and will backscatter the radio waves generated by cellular base stations in order to report their sensed data to mobile phones. These functionalities will enable wireless network operators to offer new services without the emission of additional radio waves, but by recycling those already existing for other purposes. This paper overviews the current research efforts on smart radio environments, the enabling technologies to realize them in practice, the need of new communication-theoretic models for their analysis and design, and the long-term and open research issues to be solved towards their massive deployment. In a nutshell, this paper is focused on discussing how the availability of reconfigurable intelligent meta-surfaces will allow wireless network operators to redesign common and well-known network communication paradigms.
Keywords: 6G wireless; Environmental AI; Reconfigurable intelligent meta-surfaces; Smart radio environments
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Left-handed band in an electromagnetic metamaterial induced by sub-wavelength multiple scattering
Yves, S., T. Berthelot, M. Fink, G. Lerosey, and F. Lemoult
Applied Physics Letters 114, no. 11 (2019)
Abstract: © 2019 Author(s). Due to the deep sub-wavelength unit cell in metamaterials, the quasi-static 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.
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Optimally diverse communication channels in disordered environments with tuned randomness
Del Hougne, P., M. Fink, and G. Lerosey
Nature Electronics 2, no. 1, 36-41 (2019)
Abstract: © 2019, The Author(s), under exclusive licence to Springer Nature Limited. Multichannel wireless systems have become a standard solution to address our information society’s ever-increasing demand for information transfer. The capacity that such systems can achieve is ultimately limited by the channel diversity in a given propagation medium, and numerous approaches to reduce channel cross-talk by engineering software or hardware details of the signals and antenna arrays have been proposed. Here we show that optimal channel diversity can be achieved by physically shaping the propagation medium itself. Using a reconfigurable metasurface placed inside a random environment, we tune the disorder and impose perfect orthogonality of wireless channels. We report experiments in the microwave domain in which we impose equal weights of the channel matrix eigenvalues for up to 4 × 4 systems, and almost equal weights in larger systems. We also demonstrate enhanced wireless image transmission in an office room in which we augmented the 3 × 3 system’s number of effectively independent channels from two to the optimum of three.
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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)
Abstract: © 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.
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Doubly negative bubbly metamaterials
Lanoy, M., J. H. Page, G. Lerosey, F. Lemoult, V. Leroy, and A. Tourin
2018 12th International Congress on Artificial Materials for Novel Wave Phenomena, METAMATERIALS 2018, 246-248 (2018)
Abstract: © 2018 IEEE. Thanks to their particularly efficient, low frequency Minnaert resonance, air bubbles are known to be excellent candidates for the realization of acoustic metamaterials. Here, we demonstrate that the introduction of pair-wise spatial correlations between the bubbles can result in double negativity. This can occur when the bubble pairs are arranged either in random or periodic configurations. Predictions for both types of structure will be presented and the influence of dissipation on the doubly negative behaviour discussed.
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Leveraging Chaos for Wave-Based Analog Computation: Demonstration with Indoor Wireless Communication Signals
Del Hougne, P., and G. Lerosey
Physical Review X 8, no. 4 (2018)
Abstract: © 2018 authors. Published by the American Physical Society. In sight of fundamental thermal limits on further substantial performance improvements of modern digital computational processing units, wave-based analog computation is becoming an enticing alternative. A wave, as it propagates through a carefully tailored medium, performs the desired computational operation. Yet, the necessary designs are so intricate that experimental demonstrations will necessitate further technological advances. Here, we show that, counterintuitively, the carefully tailored medium can be replaced with a random medium, subject to an appropriate shaping of the incident wave front. Using tunable metasurface reflect-arrays, we demonstrate our concept experimentally in a chaotic microwave cavity. We conclude that off-the-shelf wireless communication infrastructure in combination with a simple reflect-array suffices to perform analog computation with Wi-Fi waves reverberating in a room.
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Layer potential approach for fast eigenvalue characterization of the Helmholtz equation with mixed boundary conditions
Dupré, M., M. Fink, J. Garnier, and G. Lerosey
Computational and Applied Mathematics 37, no. 4, 4675-4685 (2018)
Abstract: © 2018, SBMAC - Sociedade Brasileira de Matemática Aplicada e Computacional. Our goal is to propose an efficient approach to characterize the eigenvalues and eigenfunctions of the Helmholtz equation with mixed (Dirichlet and Neumann) boundary conditions. Our approach is based on layer potentials. We extend the eigenvalue characterization known for Neumann boundary conditions to the case of mixed boundary conditions. The problem is motivated by the need of such methods for real-time wave-field shaping by electronically tunable surfaces.
Keywords: Boundary integral equation; Eigenvalue problem; Layer potential
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Precise Localization of Multiple Noncooperative Objects in a Disordered Cavity by Wave Front Shaping
Del Hougne, P., M. F. Imani, M. Fink, D. R. Smith, and G. Lerosey
Physical Review Letters 121, no. 6 (2018)
Abstract: © 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Complicated multipath trajectories of waves in disordered cavities cause object localization to be very challenging with traditional ray-tracing approaches. Yet it is known that information about the object position is encoded in the Green's function. After a calibration step, traditional time-reversal approaches retrieve a source's location from a broadband impulse response measurement. Here, we show that a nonemitting object's scattering contribution to a reverberant medium suffices to localize the object. We demonstrate our finding in the microwave domain. Then, we further simplify the scheme by replacing the temporal degrees of freedom (d.o.f.) of the broadband measurement with spatial d.o.f. obtained from wave front shaping. A simple electronically reconfigurable reflectarray inside the cavity dynamically modulates parts of the cavity boundaries, thereby providing spatial d.o.f. The demonstrated ability to localize multiple noncooperative objects with a single-frequency scheme may have important applications for sensors in smart homes.
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Dynamic Metasurface Aperture as Smart Around-the-Corner Motion Detector
Del Hougne, P., M. F. Imani, T. Sleasman, J. N. Gollub, M. Fink, G. Lerosey, and D. R. Smith
Scientific Reports 8, no. 1 (2018)
Abstract: © 2018 The Author(s). Detecting and analysing motion is a key feature of Smart Homes and the connected sensor vision they embrace. At present, most motion sen sors operate in line-of-sight Doppler shift schemes. Here, we propose an alternative approach suitable for indoor environments, which effectively constitute disordered cavities for radio frequency (RF) waves; we exploit the fundamental sensitivity of modes of such cavities to perturbations, caused here by moving objects. We establish experimentally three key features of our proposed system: (i) ability to capture the temporal variations of motion and discern information such as periodicity ("smart"), (ii) non line-of-sight motion detection, and (iii) single-frequency operation. Moreover, we explain theoretically and demonstrate experimentally that the use of dynamic metasurface apertures can substantially enhance the performance of RF motion detection. Potential applications include accurately detecting human presence and monitoring inhabitants' vital signs.
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Topological spoof plasmon polaritons based on C6-symmetric crystalline metasurfaces
Fleury, R., S. Yves, T. Berthelot, M. Fink, F. Lemoult, and G. Lerosey
2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena, Metamaterials 2017, 109-111 (2017)
Abstract: © 2017 IEEE. We demonstrate topological surface polaritons that propagate on the surface of a two-dimensional (2D) metamaterial made of a subwavelength periodic arrangement of electromagnetic resonators. Such surface modes are obtained at the boundary between 2D domains of distinct topologies, characterized by non-zero spin-Chern invariants, where a spin degree of freedom is induced by relying on six-fold rotational (C6) crystal symmetry combined with time-reversal symmetry. Experiments are conducted in the microwave regime to corroborate the analytical and numerical predictions. Our proposal enables robust subwavelength guiding of electromagnetic waves on a surface along predefined paths.
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Shaping Microwave Fields Using Nonlinear Unsolicited Feedback: Application to Enhance Energy Harvesting
Del Hougne, P., M. Fink, and G. Lerosey
Physical Review Applied 8, no. 6 (2017)
Abstract: © 2017 American Physical Society. Wave-front shaping has emerged over the past decade as a powerful tool to control wave propagation through complex media, initially in optics and more recently also in the microwave domain with important applications in telecommunication, imaging, and energy transfer. The crux of implementing wave-front shaping concepts in real life is often its need for (direct) feedback, requiring access to the target to focus on. Here, we present the shaping of a microwave field based on indirect, unsolicited, and blind feedback which may be the pivotal step towards practical implementations. With the example of a radio-frequency harvester in a metallic cavity, we demonstrate tenfold enhancement of the harvested power by wave-front shaping based on nonlinear signals detected at an arbitrary position away from the harvesting device.
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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)
Abstract: © 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.
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Multiple scattering limit in optical microscopy
Badon, A., A. C. Boccara, G. Lerosey, M. Fink, and A. Aubry
Optics Express 25, no. 23, 28914-28934 (2017)
Abstract: © 2017 Optical Society of America. Optical microscopy offers a unique insight of biological structures with a sub-micrometer resolution and a minimum invasiveness. However, the inhomogeneities of the specimen itself can induce multiple scattering of light and optical aberrations which limit the observation to depths close to the surface. To predict quantitatively the penetration depth in microscopy, we theoretically derive the single-to-multiple scattering ratio in reflection. From this key quantity, the multiple scattering limit is deduced for various microscopic imaging techniques such as confocal microscopy, optical coherence tomography and related methods.
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Crystalline Soda Can Metamaterial exhibiting Graphene-like Dispersion at subwavelength scale
Yves, S., F. Lemoult, M. Fink, and G. Lerosey
Scientific Reports 7, no. 1 (2017)
Abstract: © 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.
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Slow waves in locally resonant metamaterials line defect waveguides
Kaina, N., A. Causier, Y. Bourlier, M. Fink, T. Berthelot, and G. Lerosey
Scientific Reports 7, no. 1 (2017)
Abstract: © 2017 The Author(s). Many efforts have been devoted to wave slowing, as it is essential, for instance, in analog signal computing and is one prerequisite for increased wave/matter interactions. Despite the interest of many communities, researches have mostly been conducted in optics, where wavelength-scaled structured composite media are promising candidates for compact slow light components. Yet their structural scale prevents them from being transposed to lower frequencies. Here, we propose to overcome this limitation using the deep sub-wavelength scale of locally resonant metamaterials. We experimentally show, in the microwave regime, that introducing coupled resonant defects in such metamaterials creates sub-wavelength waveguides in which wave propagation exhibit reduced group velocities. We qualitatively explain the mechanism underlying this slow wave propagation and demonstrate how it can be used to tune the velocity, achieving group indices as high as 227. We conclude by highlighting the three beneficial consequences of our line defect slow wave waveguides: (1) the sub-wavelength scale making it a compact platform for low frequencies (2) the large group indices that together with the extreme field confinement enables efficient wave/matter interactions and (3) the fact that, contrarily to other approaches, slow wave propagation does not occur at the expense of drastic bandwidth reductions.
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Subwavelength focusing and imaging from the far field using time reversal in subwavelength scaled resonant media
Lemoult, F., M. Dupre, M. Fink, and G. Lerosey
International Conference on Transparent Optical Networks (2017)
Abstract: © 2017 IEEE. In this talk we will show how the use of time dependent and broadband wave fields, in conjunction with media structured at the subwavelength scale and supporting resonant eigenmodes, permits to beat the diffraction limit from the far field for imaging or focusing purposes. Examples will be given in the microwave, the acoustic, and the optical domain.
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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)
Abstract: © 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 time-reversal invariant topological insulators is the presence of unidirectional spin-polarized 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 non-trivial acoustic states at the deep subwavelength scale, in a structured two-dimensional metamaterial composed of Helmholtz resonators. Radically different from previous designs based on non-resonant sonic crystals, our proposal enables robust sound manipulation on a surface along predefined, subwavelength pathways of arbitrary shapes.
Keywords: acoustic metamaterials; polaritons; quantum spin Hall effect; topological insulators
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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)
Abstract: 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.
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Smart optical coherence tomography for ultra-deep imaging through highly scattering media
Badon, A., D. Li, G. Lerosey, A. C. Boccara, M. Fink, and A. Aubry
Science Advances 2, no. 11 (2016)
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Spatio-temporal imaging of light transport in highly scattering media under white light illumination
Badon, A., D. Li, G. Lerosey, A. C. Boccara, M. Fink, and A. Aubry
Optica 3, no. 11, 1160-1166 (2016)
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Spatio-temporal imaging of light transport in strongly scattering media
Badon, A., D. Li, G. Lerosey, C. Boccara, M. Fink, and A. Aubry
2016 URSI International Symposium on Electromagnetic Theory, EMTS 2016, 272-275 (2016)
Abstract: © 2016 IEEE.We report on the passive measurement of time-dependent Green's functions in the optical frequency domain with low-coherence interferometry. Inspired by previous studies in acoustics and seismology, we show how the mutual coherence function of a broadband and incoherent wave-field can directly yield the Green's functions between scatterers of a complex medium. Both the ballistic and multiple scattering components of the Green's function are retrieved. This simple and powerful approach directly yields a wealth of information about the medium under investigation. In particular, it allows to investigate locally the growth of the diffusive halo within the scattering medium. Local measurements of transport parameters can thus be performed and allow to image a strongly scattering layer with a unprecedented resolution of a few transport mean free paths. This constitutes a major breakthrough compared to state-of-the-art techniques such as optical diffuse tomography.
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Intensity-only measurement of partially uncontrollable transmission matrix: demonstration with wave-field shaping in a microwave cavity
Del Hougne, P., B. Rajaei, L. Daudet, and G. Lerosey
Optics Express 24, no. 16, 18631-18641 (2016)
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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)
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Soda cans metamaterial: A subwavelength-scaled phononic crystal
Lemoult, F., N. Kaina, M. Fink, and G. Lerosey
Crystals 6, no. 7 (2016)
Abstract: © 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.
Keywords: Acoustics; Metamaterial; Multiple scattering; Phononic crystals
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Using subwavelength diffraction gratings to design open microwave cavities
Dupre, M., M. Fink, and G. Lerosey
2013 7th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics, METAMATERIALS 2013, 133-135 (2013)
Abstract: Weintroduce an open microwave cavity that has a wall replaced by a sub-wavelength grating. Usually, sub-wavelength gratings show very low transmission. In our experiment, this phenomenon is compensated by the microwave cavity that finally allows all the energy to be transmitted. We study the far field emission of this system and show that coupling the cavity with a sub-wavelength grating gives rise to a zero order emission only at discrete angles and frequencies. We study the relations between angles of emissions and frequencies, the influence of geometric parameters such as the grating fill factor and the behavior of a chaotic cavity. We show that it allows us to make a configurable system that may have many applications in the fields of communications, detection and imaging, and may allow the study of open microwave cavities on a fundamental point of view. © 2013 IEEE.
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Time reversal focusing and the diffraction limit
Fink, M., J. De Rosny, G. Lerosey, and A. Tourin
Proceedings of the International School of Physics "Enrico Fermi" 173, 155-177 (2011)
Abstract: Time reversal mirrors refocus an incident-wave field to the position of the original source, regardless of the complexity of the propagation medium. TRMs have now been implemented in a variety of physical scenarios from GHz Microwaves to MHz Ultrasonics and to hundreds of Hz in ocean acoustics. Common to this broad range of scales is a remarkable robustness exemplified by observations at all scales that the more complex the medium (random or chaotic), the sharper the focus. A TRM acts as an antenna that uses complex environments to appear wider than it is, resulting, for a broad-band pulse, in a refocusing quality that does not depend on the TRM aperture. Moreover, when the complex environment is located in the near field of the source, time reversal focusing opens completely new approaches to super-resolution. We will shown that, for a broad-band source located inside a random metamaterial, a TRM located in the far field radiates a time-reversed wave that interacts with the random medium to regenerate not only the propagating but also the evanescent waves required to refocus below the diffraction limit. © 2011 by Società Italiana di Fisica.
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Exploiting spatiotemporal degrees of freedom for far-field 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)
Abstract: © 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 multiple-channel 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.
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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, 77-81 (2015)
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Symmetry issues in the hybridization of multi-mode waves with resonators: An example with Lamb waves metamaterial
Rupin, M., P. Roux, G. Lerosey, and F. Lemoult
Scientific Reports 5 (2015)
Abstract: 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 cross-sections. 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 anti-symmetric 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 free-plate. 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.
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Wave-Field 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)
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Optical detection and imaging in complex media: How the memory effect can help overcome multiple scattering
Badon, A., D. Li, G. Lerosey, A. C. Boccara, M. Fink, and A. Aubry
CLEO: QELS - Fundamental Science, CLEO_QELS 2015, 1551p (2015)
Abstract: We report on imaging in random scattering media. Our approach is based on the measurement of a reflection matrix between a spatial light modulator and a camera. We take advantage of the memory effect to filter the multiple scattering noise and improve the detection and imaging of objects embedded in scattering media. © 2014 Optical Society of America.
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Overcoming multiple scattering for detection and imaging in strongly scattering media
Badon, A., D. Li, G. Lerosey, C. Boccara, M. Fink, and A. Aubry
Adaptive Optics: Analysis, Methods and Systems, AO 2015, 289 (2015)
Abstract: We report on imaging through thick scattering media based on a matrix approach of wave propagation. We show how to overcome multiple scattering and demonstrate imaging of targets beyond several transport mean free paths. © 2015 OSA.
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Retrieving time-dependent Green's functions in optics with low-coherence interferometry
Badon, A., G. Lerosey, A. C. Boccara, M. Fink, and A. Aubry
CLEO: QELS - Fundamental Science, CLEO_QELS 2015, 1551p (2015)
Abstract: We report on the passive measurement of time-dependent Green's functions in optics with low-coherence interferometry. Inspired by previous studies in acoustics and seismology, we show how the correlations of a broadband and incoherent wave-field can directly yield the Green's functions between scatterers of a complex medium. © 2014 Optical Society of America.
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Image transmission through a scattering medium: Inverse problem and sparsity-based imaging
Gigan, S., S. M. Popoff, A. Liutkus, D. Martina, O. Katz, G. Chardon, R. Carminati, G. Lerosey, M. A. Fink., A. C. Boccara, I. Carron, and L. Daudet
2014 13th Workshop on Information Optics, WIO 2014 (2014)
Abstract: © 2014 IEEE. We demonstrate how to measure accurately the transmission matrix of a complex medium. With this information, we show how to focus light, recover an image, and even perform efficient reconstruction of a sparse object.
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Retrieving time-dependent green's functions in optics with low-coherence interferometry
Badon, A., G. Lerosey, A. C. Boccara, M. Fink, and A. Aubry
Physical Review Letters 114, no. 2 (2015)
Abstract: © 2015 American Physical Society. We report on the passive measurement of time-dependent Green's functions in the optical frequency domain with low-coherence interferometry. Inspired by previous studies in acoustics and seismology, we show how the correlations of a broadband and incoherent wave field can directly yield the Green's functions between scatterers of a complex medium. Both the ballistic and multiple scattering components of the Green's function are retrieved. This approach opens important perspectives for optical imaging and characterization in complex scattering media.
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Time-driven superoscillations with negative refraction
Dubois, M. A., E. Bossy, S. Enoch, S. Guenneau, G. Lerosey, and P. Sebbah
Physical Review Letters 114, no. 1 (2015)
Abstract: © 2015 American Physical Society. The flat-lens concept based on negative refraction proposed by Veselago in 1968 has been mostly investigated in the monochromatic regime. It was recently recognized that time development of the superlensing effect discovered in 2000 by Pendry is yet to be assessed and may spring surprises: Time-dependent illumination could improve the spatial resolution of the focusing. We investigate dynamics of flexural wave focusing by a 45°-tilted square lattice of circular holes drilled in a duralumin plate. Time-resolved experiments reveal that the focused image shrinks with time below the diffraction limit, with a lateral resolution increasing from 0.8λ to 0.35λ, whereas focusing under harmonic excitation remains diffraction limited. Modal analysis reveals the role in pulse reconstruction of radiating lens resonances, which repeatedly self-synchronize at the focal spot to shape a superoscillating field.
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Shaping complex microwave fields in reverberating media with binary tunable metasurfaces.
Kaina, N., M. Dupre, G. Lerosey, and M. Fink
Scientific reports 4, 6693 (2014)
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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)
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Imaging with nature: compressive imaging using a multiply scattering medium.
Liutkus, A., D. Martina, S. Popoff, G. Chardon, O. Katz, G. Lerosey, S. Gigan, L. Daudet, and I. Carron
Scientific reports 4, 5552 (2014)
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Using Subwavelength Diffraction Gratings to Design Open Electromagnetic Cavities
Dupre, M., M. Fink, and G. Lerosey
Physical Review Letters 112, no. 4 (2014)
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Composite media mixing Bragg and local resonances for highly attenuating and broad bandgaps
Kaina, N., M. Fink, and G. Lerosey
Scientific Reports 3 (2013)
Abstract: In this article, we investigate composite media which present both a local resonance and a periodic structure. We numerically and experimentally consider the case of a very academic and simplified system that is a quasi-one dimensional split ring resonator medium. We modify its periodicity to shift the position of the Bragg bandgap relative to the local resonance one. We observe that for a well-chosen lattice constant, the local resonance frequency matches the Bragg frequency thus opening a single bandgap which is at the same time very wide and strongly attenuating. We explain this interesting phenomenon by the dispersive nature of the unit cell of the medium, using an analogy with the concept of white light cavities. Our results provide new ways to design wide and efficient bandgap materials.
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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)
Abstract: 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.
Keywords: Defect cavity; Display modes; Free-space wavelengths; High quality factors; Positional disorder; Spatial scale; Sub-wavelength; Subwavelength scale; Defects; Metamaterials
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Acousto-optic imaging: Merging the best of two worlds
Lerosey, G., and M. Fink
Nature Photonics 7, no. 4, 265-267 (2013)
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Wave propagation control at the deep subwavelength scale in metamaterials
Lemoult, F., N. Kaina, M. Fink, and G. Lerosey
Nature Physics 9, no. 1, 55-60 (2013)
Abstract: 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 centimetre-scale dimensions, an order of magnitude smaller than previous proposals. © 2013 Macmillan Publishers Limited.
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Exploiting the time-reversal operator for adaptive optics, selective focusing and scattering pattern analysis
Popoff, S. M., A. Aubry, G. Lerosey, M. Fink, A. C. Boccara, and S. Gigan
2012 Conference on Lasers and Electro-Optics, CLEO 2012 (2012)
Abstract: We report on the optical measurement of the backscattering matrix in a weakly scattering medium. A decomposition of the time reversal operator allows selective and efficient focusing on individual scatterers, even through an aberrating layer. © 2012 OSA.
Keywords: Backscattering matrix; Decomposition of the time reversal operator; Optical measurement; Scattering medium; Scattering pattern; Time-reversal operator; Lasers; Optical data processing; Scattering
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Dispersion in media containing resonant inclusions: Where does it come from?
Lemoult, F., M. Fink, and G. Lerosey
2012 Conference on Lasers and Electro-Optics, CLEO 2012 (2012)
Abstract: Propagation media containing resonant inclusions have been studied for over a century in acoustics, electromagnetism or solid state physics. There exist some in nature, such as dielectrics, which contain enormous amounts of atoms. To calculate those materials permittivities one considers that each atom "sees" the same electromagnetic field and calculates the average field that takes into account an incoming wave as well as the overall response of the ensemble of atoms [1]. This macroscopic view assumes that there is no variations of the electromagnetic field at the scale of the inter-atomic distance. © 2012 OSA.
Keywords: Average field; Inter-atomic distances; Propagation media; Electromagnetic fields; Lasers; Atoms
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Compact MIMO antenna arrays using metamaterial hybridization band gaps
Lerosey, G., C. Leray, F. Lemoult, J. De Rosny, and A. Tourin
IEEE Antennas and Propagation Society, AP-S International Symposium (Digest), 774-777 (2012)
Abstract: In this talk, we show how the concept of hybridization band gap in metamaterials can be utilized to create antennas for MIMO applications. Those strongly decoupled antennas present at the same time a very small form factor and a very low correlation. To that aim, we first explain briefly the concept of hybridization between a resonator and the free space waves continuum. Then we expose the methodology we use to design multi-ports antennas based on that concept. We present results of several antennas designed using this idea, especially in the wifi bands, and give potential solutions for multi-band compact MIMO antennas for LTE applications. © 2012 IEICE.
Keywords: Free spaces; Low correlation; MIMO antenna; MIMO applications; Multiband; Potential solutions; Small form factors; Antenna arrays; Approximation theory; Energy gap; Metamaterials; Metamaterial antennas
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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)
Abstract: 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.
Keywords: Different frequency; Far field; Far-field imaging; Magnetic patterns; MetaLens; Planar arrays; Real time; Split ring resonator; Sub-wavelength; Subwavelength imaging; THz frequencies; Physical properties; Physics
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A polychromatic approach to far-field superlensing at visible wavelengths
Lemoult, F., M. Fink, and G. Lerosey
Nature Communications 3 (2012)
Abstract: Breaking the diffraction barrier in the visible part of the electromagnetic spectrum is of fundamental importance. Far-field subwavelength focusing of light could, for instance, drastically broaden the possibilities available in nanolithography, light-matter 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.
Keywords: nanomaterial; nanorod; article; diffraction; electromagnetic field; electromagnetic radiation; imaging system; lens; light; polychromatic light; spectral sensitivity
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Hybridization band gap based smart antennas: Deep subwavelength yet directional and strongly decoupled MIMO antennas
Lerosey, G., C. Leray, F. Lemoult, J. De Rosny, A. Tourin, and M. Fink
Proceedings of 6th European Conference on Antennas and Propagation, EuCAP 2012, 2697-2701 (2012)
Abstract: In this paper, we show how the concept of hybridization band gaps can be utilized to create antennas for MIMO applications. Those strongly decoupled antennas present at the same time a very small form factor and a very low correlation. To that aim, we first explain briefly the concept of hybridization between a resonator and the free space waves continuum. Then we expose the methodology we use to design multi-ports antennas based on that concept. We present numerical and experimental results of 2 ports MIMO antennas at 2.45 GHz, printed on a PCB, whose areas are smaller than 2.6*2.6 cm 2. The two ports display experimentally peak gains of a about 4 dB, efficiencies of 80%, a coupling lower than -30 dB and a correlation lower than 0.1. © 2012 IEEE.
Keywords: compact antenna arrays; electromagnetic band gap antennas; metamaterials; MIMO antennas; photonic crystals; Smart antennas; Compact antenna; Electromagnetic band gap antennas; Free spaces; Low correlation; MIMO antenna; MIMO applications; Peak gain; Small form factors; Sub-wavelength; Approximation theory; Energy gap; Metamaterials; Photonic crystals; Smart antennas; Metamaterial antennas
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Controlling waves in space and time for imaging and focusing in complex media
Mosk, A. P., A. Lagendijk, G. Lerosey, and M. Fink
Nature Photonics 6, no. 5, 283-292 (2012)
Abstract: In complex media such as white paint and biological tissue, light encounters nanoscale refractive-index inhomogeneities that cause multiple scattering. Such scattering is usually seen as an impediment to focusing and imaging. However, scientists have recently used strongly scattering materials to focus, shape and compress waves by controlling the many degrees of freedom in the incident waves. This was first demonstrated in the acoustic and microwave domains using time reversal, and is now being performed in the optical realm using spatial light modulators to address the many thousands of spatial degrees of freedom of light. This approach is being used to investigate phenomena such as optical super-resolution and the time reversal of light, thus opening many new avenues for imaging and focusing in turbid media. © 2012 Macmillan Publishers Limited. All rights reserved.
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Exploiting the time-reversal operator for adaptive optics, selective focusing, and scattering pattern analysis
Popoff, S. M., A. Aubry, G. Lerosey, M. Fink, A. C. Boccara, and S. Gigan
Physical Review Letters 107, no. 26 (2011)
Abstract: We report on the experimental measurement of the backscattering matrix of a weakly scattering medium in optics, composed of a few dispersed gold nanobeads. The decomposition of the time-reversal operator is applied to this matrix and we demonstrate selective and efficient focusing on individual scatterers, even through an aberrating layer. Moreover, we show that this approach provides the decomposition of the scattering pattern of a single nanoparticle. These results open important perspectives for optical imaging, characterization, and selective excitation of nanoparticles. © 2011 American Physical Society.
Keywords: Backscattering matrix; Experimental measurements; matrix; Nanobeads; Optical imaging; Scattering medium; Scattering pattern; Selective excitations; Single nanoparticle; Time-reversal operator; Nanoparticles; Scattering
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Controlling light through optical disordered media: Transmission matrix approach
Popoff, S. M., G. Lerosey, M. Fink, A. C. Boccara, and S. Gigan
New Journal of Physics 13 (2011)
Abstract: We experimentally measure the monochromatic transmission matrix (TM) of an optical multiple scattering medium using a spatial light modulator together with a phase-shifting interferometry measurement method. The TM contains all the information needed to shape the scattered output field at will or to detect an image through the medium. We confront theory and experiment for these applications and study the effect of noise on the reconstruction method. We also extracted from the TM information about the statistical properties of the medium and the light transport within it. In particular, we are able to isolate the contributions of the memory effect and measure its attenuation length. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Keywords: Attenuation lengths; Disordered media; Light transport; Memory effects; Multiple-scattering medium; Phase shifting Interferometry; Reconstruction method; Spatial light modulators; Statistical properties; Transmission matrix; Light modulators; Light transmission
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Time reversal in subwavelength-scaled 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)
Abstract: 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.
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Revisiting the wire medium: An ideal resonant metalens
Lemoult, F., M. Fink, and G. Lerosey
Waves in Random and Complex Media 21, no. 4, 591-613 (2011)
Abstract: 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 sub-wavelength 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 sub-wavelength 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.
Keywords: Conducting wire; Dispersion relations; Efficient coupling; Finite dimensions; Free space; MetaLens; Near fields; Operating wavelength; Purcell effect; Square lattices; Sub-wavelength; Wavefields; Wire medium; Electromagnetic fields; Wire; Dispersion (waves)
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Far-field sub-wavelength 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, 614-627 (2011)
Abstract: 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.
Keywords: Conducting wire; Cross correlations; Far-field; Far-field region; Finite length; MetaLens; Square lattices; Sub-diffraction; Sub-wavelength; Time reversal; Wire medium; Image reconstruction; Wire; Focusing
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Transmission matrix in optics: Taking advantage of transmission channels for image transmission in disordered materials
Popoff, S. M., G. Lerosey, R. Carminati, M. Fink, A. C. Boccara, and S. Gigan
2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)
Abstract: Recently, a method has been proposed by I. Vellekoop et al. [1] to focus light through a multiple scattering material, using a spatial light modulator as a tool to shape the incoming beam to obtain a maximal interference on a speckle spot of the output speckle pattern. The result is a bright, diffraction limited, spot which can be several hundred times brighter than the rest of the speckle. © 2011 IEEE.
Keywords: Diffraction limited; Disordered materials; Spatial light modulators; Speckle patterns; Transmission channels; Transmission matrix; Electron optics; Light modulators; Optics; Quantum electronics; Speckle; Light
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Acoustic resonators for far-field control of sound on a subwavelength scale
Lemoult, F., M. Fink, and G. Lerosey
Physical Review Letters 107, no. 6 (2011)
Abstract: 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 diffraction-limited 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.
Keywords: Acoustic pressures; Diffraction limited; Far-field; Focal spot; Helmholtz resonators; Sensors and actuators; Strong enhancement; Sub-diffraction; Sub-wavelength; Subwavelength scale; Visual experiments; Acoustic fields; Resonators; Acoustic resonators
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Image transmission through an opaque material
Popoff, S., G. Lerosey, M. Fink, A. C. Boccara, and S. Gigan
Nature Communications 1, no. 6 (2010)
Abstract: Optical imaging relies on the ability to illuminate an object, collect and analyse the light it scatters or transmits. Propagation through complex media such as biological tissues was so far believed to degrade the attainable depth, as well as the resolution for imaging, because of multiple scattering. This is why such media are usually considered opaque. Recently, we demonstrated that it is possible to measure the complex mesoscopic optical transmission channels that allow light to traverse through such an opaque medium. Here, we show that we can optimally exploit those channels to coherently transmit and recover an arbitrary image with a high fidelity, independently of the complexity of the propagation. © 2010 Macmillan Publishers Limited. All rights reserved.
Keywords: article; imaging system; laser diffraction; light scattering; optical tomography; visual system
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Measuring and exploiting the transmission matrix in optics
Popoff, S. M., G. Lerosey, R. Carminati, M. Fink, A. C. Boceara, and S. Gigan
Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference: 2010 Laser Science to Photonic Applications, CLEO/QELS 2010 (2010)
Abstract: We introduce a method to measure the transmission matrix of a complex medium. This matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. ©2010 IEEE.
Keywords: Complex medium; Light focusing; matrix; Random matrix theory; Random medium; Statistical properties; Transmission matrix
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Theory of electromagnetic time-reversal mirrors
De Rosny, J., G. Lerosey, and M. Fink
IEEE Transactions on Antennas and Propagation 58, no. 10, 3139-3149 (2010)
Abstract: The theory of monochromatic time-reversal mirrors (TRM) or equivalently phase conjugate mirrors is developed for electromagnetic waves. We start from the fundamental time-symmetry of the Maxwell's equations. From this symmetry, a differential expression similar to the Lorentz reciprocity theorem is deduced. The radiating conditions on TRM are expressed in terms of 6-dimension Green's functions. To predict the time reversal focusing on antenna arrays, a formalism that involves impedance matrix is developed. We show that antenna coupling can dramatically modify the focal spot. Especially, we observe, that in some circumstances, sub-wavelength focusing on a bi-dimensional array may arise. © 2010 IEEE.
Keywords: Antenna arrays; diffraction; microwaves; phase conjugate mirrors; plasmonic; sub-wavelength focusing; time-reversal; time-symmetry; Phase conjugate mirrors; Plasmonic; Sub-wavelength; Time-reversal; time-symmetry; Antenna phased arrays; Diffraction; Electromagnetic waves; Electromagnetism; Focusing; Green's function; Maxwell equations; Mirrors; Plasmons; Radio waves; Antennas
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Resonant metalenses for breaking the diffraction barrier
Lemoult, F., G. Lerosey, J. De Rosny, and M. Fink
Physical Review Letters 104, no. 20 (2010)
Abstract: 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.
Keywords: Diffraction barrier; Diffraction limits; Far field; MetaLens; Purcell effect; Sub-wavelength; Sub-wavelength resonators; Temporal signatures; Underlying mechanism; Wavefields; Resonators
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Experimental validation of time reversal ultra wide-band communication system for high data rates
Naqvi, I. H., G. E. Zein, G. Lerosey, J. De Rosny, P. Besnier, A. Tourin, and M. Fink
IET Microwaves, Antennas and Propagation 4, no. 5, 643-650 (2010)
Abstract: An experimental validation of high data rate communication for a time reversal (TR) ultra wide-band (UWB) communication system is performed using binary pulse amplitude modulation (BPAM) in two different dense multipath propagation channels for different data rates (15.62-Mbps≤Rb≤1-Gbps). From the measured received signals, signal, interference and noise contributions are separated. At very high data rates, interference has the most dominant contribution of all. Furthermore, without any processing and equalisation at the receiver, bit error rate (BER) performance is compared for different Rb It is shown that for Rb≤125-Mbps, TR system gives a good BER performance. Finally, the authors introduce a modified TR scheme in which total bandwidth of the TR system is divided into Nsub-bands contributing equal power in the power spectral density (PSD). This technique enables a flat PSD of the TR transmitted signal, reduces inter symbol interference (ISI) and therefore improves the BER performance of the system. © 2010 © The Institution of Engineering and Technology.
Keywords: BER performance; Bit error rate performance; Data rates; Dense multipath; Dominant contributions; Equalisation; Experimental validations; High data rate; High data rate communications; Noise contributions; Propagation channels; Received signals; Time reversal; Transmitted signal; Ultra-wideband communications; Amplitude modulation; Broadband networks; Communication systems; Data flow analysis; Power spectral density; Pulse amplitude modulation; Satellite communication systems; Bit error rate
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Nano-Optics: Yagi-Uda antenna shines bright
Lerosey, G.
Nature Photonics 4, no. 5, 267-268 (2010)
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Nano-Optics: Yagi-Uda antenna shines bright
Lerosey, G.
Nature Photonics (2010)
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Measuring the transmission matrix in optics: An approach to the study and control of light propagation in disordered media
Popoff, S. M., G. Lerosey, R. Carminati, M. Fink, A. C. Boccara, and S. Gigan
Physical Review Letters 104, no. 10 (2010)
Abstract: We introduce a method to experimentally measure the monochromatic transmission matrix of a complex medium in optics. This method is based on a spatial phase modulator together with a full-field interferometric measurement on a camera. We determine the transmission matrix of a thick random scattering sample. We show that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. This method might give important insight into the mesoscopic properties of a complex medium. © 2010 The American Physical Society.
Keywords: Complex medium; Disordered media; Full-field; Interferometric measurement; matrix; Mesoscopic properties; Random matrix theory; Random medium; Random scattering; Spatial phase modulator; Statistical properties; Transmission matrix; Light; Light propagation; Light transmission
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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)
Abstract: 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.
Keywords: Complex media; Complex medium; Degrees of freedom; Number of degrees of freedom; Waves in random media; Ultrasonics; Mechanics
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Time-reversed waves and super-resolution
Fink, M., J. De Rosny, G. Lerosey, and A. Tourin
Comptes Rendus Physique 10, no. 5, 447-463 (2009)
Abstract: Time-reversal mirrors (TRMs) refocus an incident wavefield to the position of the original source regardless of the complexity of the propagation medium. TRMs have now been implemented in a variety of physical scenarios from GHz microwaves to MHz ultrasonics and to hundreds of Hz in ocean acoustics. Common to this broad range of scales is a remarkable robustness exemplified by observations at all scales that the more complex the medium (random or chaotic), the sharper the focus. A TRM acts as an antenna that uses complex environments to appear wider than it is, resulting for a broadband pulse, in a refocusing quality that does not depend on the TRM aperture. Moreover, when the complex environment is located in the near field of the source, time-reversal focusing opens completely new approaches to super-resolution. We will show that, for a broadband source located inside a random metamaterial, a TRM located in the far field radiated a time-reversed wave that interacts with the random medium to regenerate not only the propagating but also the evanescent waves required to refocus below the diffraction limit. This focusing process is very different from that developed with superlenses made of negative index material only valid for narrowband signals. We will emphasize the role of the frequency diversity in time-reversal focusing. To cite this article: M. Fink et al., C. R. Physique 10 (2009). © 2009.
Keywords: Metamaterials; Time-reversal mirror
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Subwavelength dynamic focusing in plasmonic nanostructures using time reversal
Bartal, G., G. Lerosey, and X. Zhang
Physical Review B - Condensed Matter and Materials Physics 79, no. 20 (2009)
Abstract: We employ time reversal for deep subwavelength focusing in plasmonic periodic nanostructures. The strong anisotropy enables propagating modes with very large transverse wave vector and moderate propagation constant, facilitating transformation of diffraction-limited plane waves to high- K Bloch waves in the plasmonic nanostructure. Time reversal is used to excite the waves in the nanostructure at the exact amplitude and phase to focus the incident light to dimensions well below the diffraction limit at any point in the structure, exemplifying a true subdiffractional confinement and resolution. © 2009 The American Physical Society.
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Controlling the phase and amplitude of plasmon sources at a subwavelength scale
Lerosey, G., D. F. P. Pile, P. Mathieu, G. Bartel, and X. Zheng
Nano Letters 9, no. 1, 327-331 (2009)
Abstract: We present a new class of nanoscale plasmonίc sources based on subwavelength dielectric cavities embedded in a metal siab. Exploiting the streng dispersion near the Fabry-Perot resonance in such a resonator, we control the phase and the amplitude of the generated plasmons at the subwavelength scale. As an example, we present a subwavelength unidirectional plasmonic antenna utilizing interference between two plasmonic cavίty sources wίth matched phase and amplitude.© 2009 American Chemcal Society.
Keywords: Dielectric cavities; Fabry-perot resonances; Nano-scale; New class; Sub wavelengths; Subwavelength scale; Optical data storage; Plasmons; nanomaterial; article; chemical model; chemistry; computer simulation; conformation; crystallization; macromolecule; materials testing; methodology; nanotechnology; particle size; refractometry; surface plasmon resonance; surface property; ultrastructure; Computer Simulation; Crystallization; Macromolecular Substances; Materials Testing; Models, Chemical; Mole
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Focusing beyond the diffraction limit with far-field time reversal
Lerosey, G., J. De Rosny, A. Tourin, and M. Fink
Science 315, no. 5815, 1120-1122 (2007)
Abstract: We present an approach for subwavelength focusing of microwaves using both a time-reversal mirror placed in the far field and a random distribution of scatterers placed in the near field of the focusing point. The far-field time-reversal mirror is used to build the time-reversed wave field, which interacts with the random medium to regenerate not only the propagating waves but also the evanescent waves required to refocus below the diffraction limit. Focal spots as small as one-thirtieth of a wavelength are described. We present one example of an application to telecommunications, which shows enhancement of the information transmission rate by a factor of 3.
Keywords: diffraction; microwave radiation; telecommunication; wavelength; acoustics; article; diffraction; lens; lithotripsy; microwave radiation; priority journal; scanning near field optical microscopy; sound transmission; technology; telecommunication; time; ultrasound
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Time reversal telecommunications in complex environments
Tourin, A., G. Lerosey, J. De Rosny, A. Derode, and M. Fink
Comptes Rendus Physique 7, no. 7, 816-822 (2006)
Abstract: The time reversal technique is well known in acoustics and has lead to remarkable applications in ultrasound and underwater acoustics. Here we propose to apply it to MIMO (Multiple Input - Multiple Output) UWB (Ultra Wide Band) communication: in a first 'training' step, the intended user transmits an electromagnetic pulse that propagates in a medium, where it undergoes multiple reflections. The resulting signals are recorded at the base station by one or more antennas, time reversed and used to precode the transmitted symbols. The resulting sequences are sent back by the antennas. The time-reversed wave retraces its former paths and leads to a focus of the message in space and time at the receiver. The equalization step is thus simplified since TR compensates for the reverberation caused by the channel. Furthermore, TR takes advantage of the multipaths to increase the signal strength at the receiver and to improve spatial focusing. To cite this article: A. Tourin et al., C. R. Physique 7 (2006). © 2006 Académie des sciences.
Keywords: MIMO; Multiple scattering; Reverberation; Time reversal; UWB
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Time reversal of wideband microwaves
Lerosey, G., J. De Rosny, A. Tourin, A. Derode, and M. Fink
Applied Physics Letters 88, no. 15 (2006)
Abstract: In this letter, time reversal is applied to wideband electromagnetic waves in a reverberant room. To that end a multiantenna time reversal mirror (TRM) has been built. A 150 MHz bandwidth pulse at a central frequency of 2.45 GHz is radiated by a monopolar antenna, spread in time due to reverberation, recorded at the TRM, time reversed, and retransmitted. The time-reversed wave converges back to its source and focus in both time and space. The time compression is studied versus the number of antennas in the TRM and its bandwidth. The focal spot is also measured thanks to an eight-channel receiving array. © 2006 American Institute of Physics.
Keywords: Arrays; Bandwidth; Microwave antennas; Mirrors; Reverberation; Monopolar antenna; Receiving array; Time reversal mirror (TRM); Microwaves
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Time reversal of electromagnetic waves and telecommunication
Lerosey, G., J. De Rosny, A. Tourin, A. Derode, G. Montaldo, and M. Fink
Radio Science 40, no. 6 (2005)
Abstract: [1] Time reversal (TR) communication in various configurations (single input, single output (SISO); multiple inputs, single output (MISO); or multiple inputs, multiple outputs (MIMO)) is studied. In particular, we report an experimental demonstration of time reversal focusing with electromagnetic waves in a SISO scheme. An antenna transmits a 1 μs electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity. A spatial focusing of the compressed pulse is also shown. This experiment is the first step for a communication scheme based on time reversal. It would be very interesting for ultrawideband communication in complex media since TR would permit compensation for delay spreading. MISO and MIMO TR communications are discussed on the basis of small-scale experiments with ultrasound. In particular, the binary error rate of the method is studied as a function of both data rate and external noise. A simple theoretical approach explains the results. Copyright 2005 by the American Geophysical Union.
Keywords: Antennas; Bandwidth; Error analysis; Functions; Telecommunication systems; Binary error rate; Complex media; Ultrawideband communication; Electromagnetic waves
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Telecommunication in a disordered environment with iterative time reversal
Montaldo, G., G. Lerosey, A. Derode, A. Tourin, J. De Rosny, and M. Fink
Waves Random Media 14, no. 3, 287-302 (2004)
Abstract: We present a method to transmit digital information through a highly scattering medium in a MIMO-MU (multiple input multiple output multiple users) context. It is based on iterations of a time-reversal process, and permits us to focus short pulses, both spatially and temporally, from a base antenna to different users. This iterative technique is shown to be more efficient (lower inter-symbol interference and lower error rate) than classical time-reversal communication, while being computationally light and stable. Experiments are presented: digital information is conveyed from 15 transmitters to 15 receivers by ultrasonic waves propagating through a highly scattering slab. From a theoretical point of view, the iterative technique achieves the inverse filter of propagation in the subspace of non-null singular values of the time-reversal operator. We also investigate the influence of external additive noise, and show that the number of iterations can be optimized to give the lowest error rate. © 2004 IOP Publishing Ltd.
Keywords: Antenna lobes; Antennas; Digital communication systems; Eigenvalues and eigenfunctions; Iterative methods; Matrix algebra; Signal receivers; Transmitters; Ultrasonic propagation; Multiple input multiple output multiple users; Short pulses; Time reversal method; Electromagnetic wave scattering
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Time reversal of electromagnetic waves
Lerosey, G., J. De Rosny, A. Tourin, A. Derode, G. Montaldo, and M. Fink
Physical Review Letters 92, no. 19, 193904-1 (2004)
Abstract: A one-channel electromagnetic time-reversal mirror (TRM) was used for investigating the feasibility of time reversal focusing with electromagnetic waves in the GHz range. Two omnidirectional antennas with a frequency of 2.45 GHz and two transceiver circuit boards were also used for the investigations. The baseband signals were time reversed and the wave carriers were phase conjugated in order to avoid digitizing the radio signals at GHz frequencies. The circuit boards demodulated the radio frequency signal back to the baseband. The frequency bandwidth and the spectral correlations determined the quality of focusing.
Keywords: Analog to digital conversion; Bandwidth; Correlation methods; Demodulation; Light scattering; Light transmission; Low pass filters; Mirrors; Monochromators; Signal receivers; Signal to noise ratio; Transceivers; Ultrasonic effects; Wireless telecommunication systems; Phase conjugation; Quasimonochromatic signals; Spectral correlations; Time reversal mirrors (TRM); Antenna radiation
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