arXiv subjects

Matthieu Davy

Publications and source records attributed to Matthieu Davy.

At least 19 recordsLinked to original sources

arXiv2026

Experimental observation of three-dimensional Anderson localization of electromagnetic waves

A prominent phenomenon in contemporary condensed matter physics is Anderson localization -- suppression of wave propagation in disordered systems as a result of interference effects. Despite being observed with various types of waves over the years, all prior attempts to reach Anderson localization of light in three-dimensional systems have been hampered by experimental artifacts. Here, we report an unambiguous experimental proof of three-dimensional Anderson localization of microwaves in disordered metal aggregates. By studying samples with different metal volume fractions, we show a clear difference between diffusive and localized behaviors, and the latter is confirmed by a scaling analysis of transmitted beam width in excellent agreement with theoretical and numerical results. Our demonstration opens avenues for both fundamental studies and practical applications of this extraordinary phenomenon.

cond-mat.dis-nn↗

arXiv2026

Reflecthernet: Exfiltrating 100BASE-TX Ethernet Traffic Using a Retroreflector Hardware Trojan

Electromagnetic eavesdropping is a well-established attack vector for remotely monitoring a target activity, most notably displays, over considerable ranges. Other targets have been considered resistant to such attacks or do not exhibit sufficient electromagnetic leakage for practical exploitation. Radio-frequency retroreflector attacks (RFRA) were developed to enable covert, active monitoring of a target by implanting a minimal hardware Trojan. These implants, typically implemented using discrete components such as transistors or diodes, do not betray their presence by emitting signals themselves; rather, they modulate the electromagnetic reflectivity of the target depending on the probed signal line data. Prior RFRA work has demonstrated their viability against video links and low-speed peripheral interfaces. In this work, we extend the applicability of RFRA to high-speed targets by presenting a successful attack on the 100BASE-TX Ethernet standard. We describe the design and realization of a compact implant capable of recovering the MLT-3 encoded signaling used in Fast Ethernet, as well as a dedicated demodulation and interpretation pipeline that mitigates errors introduced by the radio channel and maximizes the amount of recovered information. Experimental results validate the feasibility of covertly monitoring Fast Ethernet traffic using RF retroreflection and highlight the viability of such attacks for high-speed links.

cs.CR↗

arXiv2026

Backscatter Communication Through Disordered Media Enabled by a Programmable Leaky Cavity

Retrieving information through disordered scattering systems remains a major challenge in wireless communication and sensing due to the severe distortion and attenuation caused by multiple scattering. Here, we demonstrate robust focusing through random media using a leaky cavity made programmable by reconfigurable metasurfaces. We show that the leaky cavity leverages backscattering from the disordered medium to enhance the degree of control over the transmitted field. We achieve an enhancement in focused intensity by an order of magnitude compared to a horn antenna. We further demonstrate robust communication through the medium using low-power amplitude-modulated signals, and finally implement a backscatter communication scheme in which the same cavity illuminates and decodes data from a passive reflectivity-modulating target embedded within the disorder. Our results open new possibilities for reliable communication and sensing in complex environments.

physics.optics↗

arXiv2025

Single-Antenna Non-Line-of-Sight Matrix Imaging via Reconfigurable Intelligent Surfaces

Modern imaging and sensing in complex environments, ranging from biomedical diagnostics to wireless communication, relies on accurately measuring and then controlling the wave propagation. Conventional approaches demand large arrays of antennas or transducers to reconstruct the full reflection or transmission matrix, enabling advanced protocols such as selective focusing or adaptive wave control. Yet, these arrays are expensive, bulky, and difficult to implement at microwave frequencies. Here, we show that a single transmitting-receiving antenna, when combined with a reconfigurable intelligent surface (RIS), can fully reconstruct the reflection matrix from far-field measurements, effectively transforming the RIS into a programmable synthetic antenna array. This approach allows high-fidelity imaging of complex scenes, selective focusing through clutter, and real-time tracking of moving targets. Our results establish RIS as a versatile, low-cost platform for matrix-based imaging, with broad implications for adaptive wave control, real-time sensing, and imaging in environments previously considered inaccessible.

physics.app-ph↗

arXiv2025

Inverse design of mirror-symmetric disordered systems for broadband perfect transmission

We present a framework for achieving broadband perfect wave transmission in complex systems by optimizing symmetric disordered media via inverse design. We show that leveraging symmetry of complex media reduces the optimization's complexity enabling the incorporation of additional constraints in the parameter space. Starting from a single perfectly transmitting state with predefined input and output wavefronts at a specific frequency, we progressively broaden the bandwidth - from a reflectionless exceptional point with a flattened lineshape to narrowband filters and ultimately to broadband quasi-perfect transmission exhibiting a rainbow effect. Numerical simulations based on the coupled dipole approximation are validated experimentally in a multichannel microwave waveguide with dielectric and metallic scatterers. Finally, we demonstrate broadband enhanced wave transmission through barriers highlighting the potential for advanced wave control applications.

physics.optics↗

arXiv2025

Optimal Targeted Mode Transport in Complex Wave Environments: A Universal Statistical Framework

Recent advances in the field of structured waves have resulted in sophisticated coherent wavefront shaping schemes that provide unprecedented control of waves in various complex settings. These techniques exploit multiple scattering events and the resulting interference of wave paths within these complex environments. Here, we introduce the concept of targeted mode transport (TMT), which enables energy transfer from specific input channels to designated output channels in multimode wave-chaotic cavities by effectively engaging numerous cavity modes. We develop a statistical theory that provides upper bounds on optimal TMT, incorporating operational realities such as losses, coupling strengths and the accessibility of specific interrogating channels. The theoretical predictions for the probability distribution of TMT eigenvalues are validated through experiments with microwave chaotic networks of coaxial cables as well as two-dimensional and three-dimensional complex cavities. These findings have broad implications for applications ranging from indoor wireless communications to imaging and beyond.

physics.optics↗

arXiv2024

Detecting and Focusing on a Nonlinear Target in a Complex Medium

Wavefront shaping techniques allow waves to be focused on a diffraction-limited target deep inside disordered media. To identify the target position, a guidestar is required that typically emits a frequency-shifted signal. Here we present a noninvasive matrix approach operating at a single frequency only, based on the variation of the field scattered by a nonlinear target illuminated at two different incident powers. The local perturbation induced by the nonlinearity serves as a guide for identifying optimal incident wavefronts. We demonstrate maximal focusing on electronic devices embedded in chaotic microwave cavities and extend our approach to temporal signals. Finally, we exploit the programmability offered by reconfigurable smart surfaces to enhance the intensity delivered to a nonlinear target. Our results pave the way for deep imaging protocols that use any type of nonlinearity as feedback, requiring only the measurement of a monochromatic scattering matrix.

physics.app-ph↗

arXiv2023

Over-the-Air Emulation of Electronically Adjustable Rician MIMO Channels in a Programmable-Metasurface-Stirred Reverberation Chamber

We experimentally investigate the feasibility of evaluating multiple-input multiple-output (MIMO) radio equipment under adjustable Rician fading channel conditions in a programmable-metasurface-stirred (PM-stirred) reverberation chamber (RC). Whereas within the "smart radio environment" paradigm PMs offer partial control over the channels to the wireless system, in our use case the PM emulates the uncontrollable fading. We implement a desired Rician K-factor by sweeping a suitably sized subset of all meta-atoms through random configurations. We discover in our setup an upper bound on the accessible K-factors for which the statistics of the channel coefficient distributions closely follow the sought-after Rician distribution. We also discover a lower bound on the accessible K-factors in our setup: there are unstirred paths that never encounter the PM, and paths that encounter the PM are not fully stirred because the average of the meta-atoms' accessible polarizability values is not zero (i.e., the meta-atoms have a non-zero "structural" cross-section). We corroborate these findings with experiments in an anechoic chamber, physics-compliant PhysFad simulations with Lorentzian vs "ideal" meta-atoms, and theoretical analysis. Our work clarifies the scope of applicability of PM-stirred RCs for MIMO Rician channel emulation, as well as electromagnetic compatibility test.

physics.app-ph↗

arXiv2023

Matrix approach for optimal spatio-temporal coherent control of wave scattering

We present and experimentally verify a matrix approach for determining how to optimally sculpt an input wavefront both in space and time for any desired wave-control functionality, irrespective of the complexity of the wave scattering. We leverage a singular value decomposition of the transport matrix that fully captures how both the spatial and temporal degrees of freedom available to shape the input wavefront impact the output wavefront's spatial and temporal form. In our experiments in the microwave domain, we use our formalism to successfully tackle three iconic wave-control tasks in a disordered cavity: (i) reflectionless transient excitation (``virtual perfect absorption''), (ii) optimal energy deposition, and (iii) scattering-invariant time-varying states.

physics.app-ph↗

arXiv2022

Universal and nonuniversal statistics of transmission in thin random layered media

The statistics of transmission through random 1D media are generally presumed to be universal and to depend only upon a single dimensionless parameter-the ratio of the sample length and the mean free path, s = L/l. Here, we show in numerical simulations and optical measurements of random binary systems, and most prominently in systems for which s is less than unity, that the statistics of the logarithm of transmission, ln T, are universal for transmission near the upper cutoff of unity and depend distinctively upon the reflectivity of the layer interfaces and their number near a lower cutoff. The universal segment of the probability distribution function of the logarithm of transmission P (ln T) is manifested with as few as three binary layers. For a given value of s, P (ln T ) evolves towards a universal distribution as the number of layers increases. Optical measurements in stacks of 5 and 20 glass coverslips exhibit statistics at low and moderate values of transmission that are close to those found in simulations for 1D layered media, while differences appear at higher transmission where the transmission time in the medium is longer and the wave explores the transverse nonuniformity of the sample.

cond-mat.dis-nn↗

arXiv2022

Customized anti-reflection structure for perfect transmission through complex media

Getting to grips with the detrimental influence of disordered environments on wave propagation is an interdisciplinary endeavour spanning diverse research areas ranging from telecommunications \cite{basar_wireless_2019} and bio-medical imaging \cite{kubby_wavefront_2019} to seismology \cite{Campillo2003} and material engineering \cite{Chen2010,Molesky2018}. Wavefront shaping techniques are highly promising to overcome the effect of wave scattering as even opaque media feature open channels for which the incident light is fully transmitted \cite{Dorokhov1984,Gerardin2014,Sarma2016,Jeong18}. With this feature being restricted, however, to just a small subset of judiciously engineered states it remains out of reach to render an opaque sample translucent for any incident light field. Here we show that a structureless medium composed of randomly assembled scattering elements can be made fully transmitting to all incoming wavefronts by putting a customized complementary medium in front of it. This special situation is achieved when the reflection matrices of the two media surfaces facing each other satisfy a matrix generalization of the condition for critical coupling. We implement this protocol both numerically and experimentally for the design of electromagnetic waveguides with several dozen scattering elements placed inside of them. The translucent scattering media we introduce here also have the promising property of being able to store incident radiation in their interior for remarkably long times.

physics.optics↗

arXiv2022

Spatio-temporal analysis of electromagnetic field coherence in complex media

We study the coherence in time and space of electromagnetic fields propagated through complex media. Whether for localization, imaging or telecommunication, the development of dedicated numerical techniques is generally based on the exploitation of simplified models considering either coherent or diffuse fields. The optimization of such applications in conditions of partial coherence can therefore be particularly challenging, requiring the development of hybrid algorithms adaptable to prior knowledge on the processed fields. The objective of this work is to provide numerical techniques for decomposing an electromagnetic field into subspaces that can then be filtered according to their level of spatial and temporal coherence. In contrast to the studies carried out on space-space transfer matrices notably used for the calculation of Wigner-Smith operators, these decompositions are carried out on space-time matrices in order to facilitate the study of temporal dispersion. The theory is developed for illustrative purposes using experimental results from a leaky resonant system but seem to be applicable to any scattering and reverberating media capable of transforming localized and coherent excitations into complex and diffuse distributions. To conclude this work, the proposed technique is exploited to improve image reconstruction in a millimeter-wave computational imaging demonstration. In the studied context and from a more general perspective, we propose a technique to select the most suitable subspaces for each application operating under conditions of partial coherence, whether these correspond in the most extreme cases to ballistic paths or to diffuse fields.

physics.comp-ph↗

arXiv2021

Experimental evidence of enhanced broadband transmission in disordered systems with mirror symmetry

We demonstrate in microwave measurements the broadband enhancement of transmission through an opaque barrier due to mirror symmetry. This enhancement relies on constructive interference between mirror scattering paths resulting from strong internal reflections at the left and right interfaces of a multichannel cavity. We observe a strong sensitivity of the conductance to a shift of the barrier from the center of the cavity. Remarkably, the impact of mirror symmetry can be further increased by tuning the degree of disorder within the cavity. We report an additional enhancement of the conductance found by symmetrically placing randomly located scatterers. Our results illuminate the impact of symmetry and disorder correlation on transmission through complex systems.

cond-mat.dis-nn↗

arXiv2021

Exceptional points of PT-symmetric reflectionless states in complex scattering systems

We investigate experimentally and analytically the coalescence of reflectionless (RL) states in symmetric complex wave-scattering systems. We observe RL-exceptional points (EPs), first, with a conventional Fabry-Perot system for which the scattering strength within the system is tuned symmetrically, and then with single- and multi-channel symmetric disordered systems. We identify that an EP of the parity-time (PT)-symmetric RL-operator is obtained when the spacing between central frequencies of two natural resonances of the system is equal to the decay rate into incoming and outgoing channels. Finally, we leverage the transfer functions associated with RL and RL-EPs states to implement first- and second-order analog differentiation.

physics.class-ph↗

arXiv2021

Diffuse field cross-correlations: scattering theory and electromagnetic experiments

The passive estimation of impulse responses from ambient noise correlations arouses increasing interest in seismology, acoustics, optics and electromagnetism. Assuming the equipartition of the noise field, the cross-correlation function measured with non-invasive receiving probes converges towards the difference of the causal and anti-causal Green's functions. Here, we consider the case when the receiving field probes are antennas which are well coupled to a complex medium -- a scenario of practical relevance in electromagnetism. We propose a general approach based on the scattering matrix formalism to explore the convergence of the cross-correlation function. The analytically derived theoretical results for chaotic systems are confirmed in microwave measurements within a mode-stirred reverberation chamber. This study provides new fundamental insights into the Green's function retrieval technique and paves the way for a new technique to characterize electromagnetic antennas.

cond-mat.dis-nn↗

arXiv2020

Mean path length invariance in wave-scattering beyond the diffusive regime

Diffusive random walks feature the surprising property that the average length of all possible random trajectories that enter and exit a finite domain is determined solely by the domain boundary. Changes in the diffusion constant or the mean-free path, that characterize the diffusion process, leave the mean path length unchanged. Here we demonstrate experimentally that this result can be transferred to the scattering of waves, even when wave interference leads to marked deviations from a diffusion process. Using a versatile microwave setup, we establish the mean path length invariance for the crossover to Anderson localization and for the case of a band gap in a photonic crystal. We obtain these results on the mean path length solely based on a transmission matrix measurement through a novel procedure that turns out to be more robust to absorption and incomplete measurement in the localized regime as compared to an assessment based on the full scattering matrix.

cond-mat.soft↗

arXiv2020

Coherent wave control in complex media with arbitrary wavefronts

Wavefront shaping (WFS) has emerged as powerful tool to control the propagation of diverse wave phenomena (light, sound, microwaves, ...) in disordered matter for applications including imaging, communication, energy transfer, micromanipulation, and scattering anomalies. Nonetheless, in practice the necessary coherent control of multiple input channels remains a vexing problem. Here, we overcome this difficulty by doping the disordered medium with programmable meta-atoms in order to adapt it to an imposed arbitrary incoming wavefront. Besides lifting the need for carefully shaped incident wavefronts, our approach also unlocks new opportunities such as sequentially achieving different functionalities with the same arbitrary wavefront. We demonstrate our concept experimentally for electromagnetic waves using programmable metasurfaces in a chaotic cavity, with applications to focusing with the generalized Wigner-Smith operator as well as coherent perfect absorption. We expect our fundamentally new perspective on coherent wave control to facilitate the transition of intricate WFS protocols into real applications for various wave phenomena.

physics.app-ph↗

arXiv2020

On-demand coherent perfect absorption in complex scattering systems: time delay divergence and enhanced sensitivity to perturbations

Non-Hermitian photonic systems capable of perfectly absorbing incident radiation recently attracted much attention both because fundamentally they correspond to an exotic scattering phenomenon (a real-valued scattering matrix zero) and because their extreme sensitivity holds great technological promise. The sharp reflection dip is a hallmark feature underlying many envisioned applications in precision sensing, secure communication and wave filtering. However, a rigorous link between the underlying scattering anomaly and the sensitivity of the system to a perturbation is still missing. Here, we develop a theoretical description in complex scattering systems which quantitatively explains the shape of the reflection dip. We further demonstrate that coherent perfect absorption (CPA) is associated with a phase singularity and we relate the sign of the diverging time delay to the mismatch between excitation rate and intrinsic decay rate. We confirm our theoretical predictions in experiments based on a three-dimensional chaotic cavity excited by eight channels. Rather than relying on operation frequency and attenuation inside the system to be two free parameters, we achieve "on-demand" CPA at an arbitrary frequency by tweaking the chaotic cavity's scattering properties with programmable meta-atom inclusions. Finally, we theoretically prove and experimentally verify the optimal sensitivity of the CPA condition to minute perturbations of the system.

physics.class-ph↗