arXiv subjects

Matthieu Davy

Publications and source records attributed to Matthieu Davy.

30 records · Page 2Linked to original sources

arXiv2020

Experimental realization of optimal energy storage in resonators embedded in scattering media

The ability to enhance light-matter interactions by increasing the energy stored in optical resonators is inherently dependent on their coupling to the incident wavefront. In practice, weak coupling may result from resonators' irregular shapes and/or the scrambling of waves in the surrounding scattering environment. Here, we present a non-invasive wavefront shaping technique providing optimal coupling to resonators. The coherent control of the incident wavefront relies on the lengthening of delay times of waves efficiently exciting the resonator. We demonstrate our concept in microwave experiments by injecting in-situ optimal wavefronts that maximize the energy stored in multiple high-permittivity dielectric scatterers and extended leaky cavities embedded in a complex environment. We expect our framework to find important applications in the enhancement of light-matter interactions in photonic materials as well as to enhance energy harvesting.

physics.optics↗

arXiv2019

Optimized Analog Information Multiplexing Across Custom-Tailored MetaCavity Configurations

Combining tunable metasurfaces with chaotic cavities opens new avenues for finely tailored dynamic control of microwaves with programmable coding metacavities (PCMs). There is currently a strong interest in utilizing PCMs to overcome the notorious difficulty of coherent measurements across large apertures at radiofrequencies, with important applications in imaging and sensing for security screening, medical diagnosis and human-computer interaction. Such approaches rely on multiplexing spatially encoded information across a random sequence of PCM coding patterns for single-port single-frequency acquisition. Here, it is shown that a judiciously tailored rather than random coding sequence is necessary to unlock the full potential of PCMs for analog multiplexing. Specifically, the singular value spectrum of the multiplexing channel matrix is tailored to be perfectly flat - as opposed to downward sloping. In-situ experiments show that thereby the number of necessary measurements to achieve a given reconstruction quality is lowered by a factor of 2.5. Computational imaging and other microwave metrology applications are expected to benefit from the resulting reduction in acquisition time, processing burden and latency. The proposed approach and platform also set the stage for future studies in the emerging field of wave control through engineered wave chaos.

physics.app-ph↗

arXiv2019

Impact of non-Hemiticity on modal strength and correlation in transmission through random open cavities

The nonorthogonality of eigenfunctions over the volume of non-Hermitian systems determines the nature of waves in complex systems. Here, we show in microwave measurements of the transmission matrix that the non-Hermiticity of open random systems leads to enhanced modal excitation and strong correlation between modes. Modal transmission coefficients reach values comparable to the dimensionless conductance which may be much larger than unity. This is accompanied by strong negative correlation between modal speckle patterns ensuring that net transmission is never larger than the incident power.

physics.optics↗

arXiv2018

A Reconfigurable Chaotic Cavity with Fluorescent Lamps for Microwave Computational Imaging

Several computational imaging systems have recently been proposed at microwave and millimeter-wave frequencies enabling a fast and low cost reconstruction of the scattering strength of a scene. The quality of the reconstructed images is directly linked to the degrees of freedom of the system which are the number of uncorrelated radiated patterns that sequentially sample the scene. Frequency diverse antennas such as leaky chaotic cavities and metamaterial apertures take advantage of the spectral decorrelation of transmitted speckle patterns that stems from the reverberation within a medium. We present a reconfigurable chaotic cavity for which the boundary conditions can be tuned by exciting plasma elements, here commercial fluorescent lamps. The interaction of electromagnetic waves with a cold plasma is strongly modified as it is ionized. Instead of being transparent to incident waves, it behaves theoretically as a metallic material. The independent states of the cavity obtained using a differential approach further enhance the degrees of freedom. This relaxes the need of a cavity with a large bandwidth and/or high quality factor. Experimental results validate the use of fluorescent lamps and its limitations are discussed. Images of various metallic objects are provided to illustrate the potentialities of this promising solution.

physics.app-ph↗

arXiv2018

Selectively exciting quasi-normal modes in open disordered systems

Transmission through disordered samples can be controlled by illuminating a sample with waveforms corresponding to the eigenchannels of the transmission matrix. But can the TM be exploited to selectively excite quasi-normal modes and so control the spatial profile and dwell time inside the medium? We show in microwave and numerical studies that spectra of the TM can be analyzed into modal transmission matrices of rank unity. This makes it possible to enhance the energy within a sample by a factor equal to the number of channels. Limits to modal selectivity arise, however, from correlation in the speckle patterns of neighboring modes. In accord with an effective Hamiltonian model, the degree of modal speckle correlation grows with increasing modal spectral overlap and non-orthogonality of the modes of non-Hermitian systems. This is observed when the coupling of a sample to its surroundings increases, as in the crossover from localized to diffusive waves.

physics.class-ph↗

arXiv2015

Statistics and control of waves in disordered media

Fundamental concepts in the quasi-one-dimensional geometry of disordered wires and random waveguides in which ideas of scaling and the transmission matrix were first introduced are reviewed. We discuss the use of the transmission matrix to describe the scaling, fluctuations, delay time, density of states, and control of waves propagating through and within disordered systems. Microwave measurements, random matrix theory calculations, and computer simulations are employed to study the statistics of transmission and focusing in single samples and the scaling of the probability distribution of transmission and transmittance in random ensembles. Finally, we explore the disposition of the energy density of transmission eigenchannels inside random media.

cond-mat.dis-nn↗

arXiv2015

Getting beneath the surface of opaque media: universal structure of transmission eigenchannels

Because the desire to explore opaque materials is ordinarily frustrated by multiple scattering of waves, attention has focused on the transmission matrix of the wave field. This matrix gives the fullest account of transmission and conductance and enables the control of the transmitted flux; however, it cannot address the fundamental issue of the spatial profile of eigenchannels of the transmission matrix inside the sample. Here we obtain a universal expression for the average disposition of energy of transmission eigenchannels for diffusive waves in terms of auxiliary localization lengths determined by the corresponding transmission eigenvalues. The spatial profile of each eigenchannel is shown to be a solution of a generalized diffusion equation. These results reveal the rich structure of transmission eigenchannels and enable the control of wave propagation and the energy distribution inside random media.

physics.optics↗

arXiv2015

Transmission eigenchannels and the densities of states of random media

We show in microwave measurements and computer simulations that the contribution of each eigenchannel of the transmission matrix to the density of states (DOS) is the derivative with angular frequency of a composite phase shift. The accuracy of the measurement of the DOS determined from transmission eigenchannels is confirmed by the agreement with the DOS found from the decomposition of the field into modes. The distribution of the DOS, which underlies the Thouless number, is substantially broadened in the Anderson localization transition. We find a crossover from constant to exponential scaling of fluctuations of the DOS normalized by its average value. These results illuminate the relationships between scattering, stored energy and dynamics in complex media.

physics.optics↗

arXiv2014

Green's function retrieval and fluctuations of cross density of states in multiple scattering media

In this article we derive the average and the variance of the cross-correlation of a noise wavefield. The noise cross-correlation function (NCF) is widely used to passively estimate the Green's function between two probes and is proportional to the cross density of states (CDOS) in photonic and plasmonic systems. We first explain from the ladder approximation how the diffusion halo plays the role of secondary sources to reconstruct the mean Green's function. We then show that fluctuations of NCF are governed by several non-Gaussian correlations. An infinite-range NCF correlation dominates CDOS fluctuations and proves that NCF is not a self averaging quantity with respect to the plurality of noise sources. The link between these correlations and the intensity ones is highlighted. These results are supported by numerical simulations and are of importance for passive imaging applications and material science.

cond-mat.dis-nn↗

arXiv2013

Focusing through random media in space and time: a transmission matrix approach

We exploit the evolution in time of the transmission matrix following pulse excitation of a random medium to focus radiation at a selected time delay t' and position r. The temporal profile of a focused microwave pulse is the same as the incident Gaussian pulse. The contrast in space at time t' of the focused wave is determined by the participation number of transmission eigenvalues M' and the size N' of the measured transmission matrix. The initial rise and subsequent decay in contrast observed reflects the distribution of decay rates of the quasi-normal modes within the sample.

cond-mat.dis-nn↗

arXiv2013

Transmission statistics and focusing in single disordered samples

We show in microwave experiments and random matrix calculations that in samples with a large number of channels the statistics of transmission for different incident channels relative to the average transmission is determined by a single parameter, the participation number of the eigenvalues of the transmission matrix, M. Its inverse, M-1, is equal to the variance of relative total transmission of the sample, while the contrast in maximal focusing is equal to M. The distribution of relative total transmission changes from Gaussian to negative exponential over the range in which M-1 changes from 0 to 1. This provides a framework for transmission and imaging in single samples.

physics.optics↗

arXiv2011

Focusing through random media: eigenchannel participation number and intensity correlation

Using random matrix calculations, we show that, the contrast between maximally focused intensity through random media and the background of the transmitted speckle pattern for diffusive waves is, μ_N =1 +N_{eff}, where N eff is the eigenchannel participation number for the transmission matrix. For diffusive waves, N_{eff} is the inverse of the degree of intensity correlation, κ. The profile of the focused beam relative to the ensemble average intensity is expressed in terms of the square of the normalized spatial field correlation function, F(Δr), and κ. These results are demonstrated in microwaves experiments and provide the parameters for optimal focusing and the limits of imaging.

cond-mat.dis-nn↗