arXiv subjects

Romain Pierrat

Publications and source records attributed to Romain Pierrat.

At least 19 recordsLinked to original sources

arXiv2026

The vanishing distance: a practical range boundary for dynamic wavefront shaping

Wavefront shaping (WFS) is a powerful modern method to control light propagation through scattering media, with applications ranging from biomedical imaging via cryptography to free-space optical communication. While WFS has been highly successful in static or slowly evolving media, its practical relevance in dynamic and extended scattering environments remains difficult to assess. In particular, no simple criterion currently identifies the propagation distance beyond which the residual ballistic component is no longer distinguishable from the scattered background. Here, we introduce the vanishing distance, defined as the propagation length where the ballistic power equals that carried by a single diffuse mode. Unlike the transport mean free path, which characterizes angular randomization of the scattered field, the vanishing distance identifies the loss of the ballistic channel on a modal-power basis. Beyond this distance, transmission or imaging can no longer rely on the ballistic component and must instead exploit the scattered field. We present a simple semi-analytical model, validated against Monte Carlo simulations and experiments in both monodisperse and heterogeneous scattering media, allowing the vanishing distance and the associated minimum source power to be estimated. Across representative scattering environments, the transition is found to occur typically after about 30-40 scattering mean free paths. These results provide a simple guideline for assessing the practical relevance of dynamic wavefront shaping.

physics.optics↗

arXiv2026

Photon thermalization in an open and disordered scattering medium

Thermalization of light, where photons acquire a temperature and chemical potential analogous to a material gas, remains a striking yet experimentally elusive manifestation of quantum statistical physics. To date, it has been realized only in carefully engineered photonic environments that enforce repeated absorption-emission cycles. Here we show that such thermalization can emerge in a radically simpler setting: an open scattering medium. Using a pumped fluorescent dye solution doped with colloidal particles, we demonstrate that multiple scattering alone suffices to trap photons long enough to drive them to thermal equilibrium. The emitted radiation follows a Bose-Einstein distribution with a finite chemical potential, independently tunable via optical pumping, while its temperature is set by the host medium. A clear spectroscopic signature of thermalization is observed as a plateau at the sample temperature over a finite spectral range. Our results establish disordered scattering media as a generic platform for photon thermalization, extending this fundamental phenomenon beyond resonant cavities and opening new routes towards cavity-free photonic thermodynamics and thermal light sources operating under ambient conditions.

physics.optics↗

arXiv2025

Acoustic transparency and absorption in dense granular suspensions

We demonstrate the existence of a frequency band exhibiting acoustic transparency in 2D and 3D dense granular suspensions, enabling the transmission of a low-frequency ballistic wave excited by a high-frequency broadband ultrasound pulse. This phenomenon is attributed to spatial correlations in the structural disorder of the medium. To support this interpretation, we use an existing model that incorporates such correlations via the structure factor. Its predictions are shown to agree well with those of the Generalized Coherent Potential Approximation (GCPA) model, which is known to apply at high volume fractions, including the close packing limit, but does not explicitly account for disorder correlation. Within the transparency band, attenuation is found to be dominated by absorption rather than scattering. Measurements of the frequency dependence of the absorption coefficient reveal significant deviations from conventional models, challenging the current understanding of acoustic absorption in dense granular media.

cond-mat.soft↗

arXiv2024

Open and trapping channels in complex resonant media

We present a statistical study of the transmission and dwell-time matrices in disordered media composed of resonators, focusing on how frequency detuning influences their eigenvalue distributions. Our analysis reveals that the distribution of transmission eigenvalues undergoes a transition from a monomodal to a bimodal profile, and back to monomodal, as the frequency approaches the resonant frequency of the particles. Moreover, the distribution of dwell-time eigenvalues broadens significantly near resonance, with the longest lifetimes exceeding the median by several orders of magnitude. These results are explained by examining how frequency $\omega$ affects the transport mean free path of light, $\ell(\omega)$, and the energy transport velocity, $v_E(\omega)$, which in turn shape the observed distributions. We demonstrate the strong potential of wavefront shaping to enhance both transmission and energy storage in resonant disordered media. In the diffusive regime, where the system thickness $L$ exceeds the mean free path, both transmission and dwell time can be enhanced by a factor $\varpropto L/\ell(\omega) \gg 1$ when using wavefronts associated with the largest eigenvalues instead of plane waves. In the localized regime, the enhancements become $\varpropto Ne^{2L/\xi}$ for transmission and $\varpropto N\xi /L$ for dwell time, where $\xi$ is the localization length and $N$ is the number of controlled scattering channels. Finally, we show that employing high-$Q$ resonators instead of low-$Q$ ones increases energy storage within the medium by a factor of $\varpropto Q/k\ell(\omega)$, in both the diffusive and localized regimes.

physics.optics↗

arXiv2024

Causality and instability in wave propagation in random time-varying media

We develop a theoretical model to investigate wave propagation in media with random time-varying properties, where temporal fluctuations lead to complex scattering dynamics. Focusing on the ensemble-averaged field, we derive an exact expression for the average Green's function in the presence of finite temporal disorder, and extend the analysis to the thermodynamic limit. In contrast to spatial disorder, causality prevents recurrent scattering, allowing us to achieve a non-perturbative solution. We introduce an effective medium description providing a simple analysis of the propagation regimes. Our findings offer new insights into wave dynamics in temporally disordered media, with potential applications in time-varying metamaterials, dynamic sensing, and imaging in turbulent or chaotic environments.

physics.optics↗

arXiv2024

Mesoscopic light transport in nonlinear disordered media

Nonlinear disordered media uniquely combine multiple scattering and second-harmonic generation. Here, we investigate the statistical properties of the nonlinear light generated within such media. We report super-Rayleigh statistics of the second-harmonic speckle intensity, and demonstrate that it is caused by the mesoscopic correlations arising in extreme scattering conditions. The measured conductance is the lowest ever observed in an isotropically scattering 3D medium, with applications in broadband second-harmonic generation, wavefront shaping in nonlinear disordered media, and photonic computing.

physics.optics↗

arXiv2024

Multiple scattering theory in one dimensional space and time dependent disorder: Average field

We theoretically study the propagation of light in one-dimensional space- and time-dependent disorder. The disorder is described by a fluctuating permittivity $\epsilon(x,t)$ exhibiting short-range correlations in space and time, without cross correlation between them. Depending on the illumination conditions, we show that the intensity of the average field decays exponentially in space or in time, with characteristic length or time defining the scattering mean-free path $\ell_s$ and the scattering mean-free time $\tau_s$. In the weak scattering regime, we provide explicit expressions for $\ell_s$ and $\tau_s$, that are checked against rigorous numerical simulations.

physics.optics↗

arXiv2023

Photon diffusion in space and time in a second-order nonlinear disordered medium

We report experimental and theoretical investigations on photon diffusion in a second-order nonlinear disordered medium under conditions of strong nonlinearity. Experimentally, photons at the fundamental wavelength ($λ=1064$ nm) are launched into the structure in the form of a cylindrical pellet, and the second-harmonic ($λ=532$ nm) photons are temporally analyzed in transmission. For comparison, separate experiments are carried out with incident green light at $λ=532$ nm. We observe that the second harmonic light peaks earlier compared to the incident green photons. Next, the sideways spatial scattering of the fundamental as well as second-harmonic photons is recorded. The spatial diffusion profiles of second-harmonic photons are seen to peak deeper inside the medium in comparison to both the fundamental and incident green photons. In order to give more physical insights into the experimental results, a theoretical model is derived from first principles. It is based on the coupling of transport equations. Solved numerically using a Monte Carlo algorithm and experimentally estimated transport parameters at both wavelengths, it gives excellent semi-quantitative agreement with the experiments for both fundamental and second-harmonic light.

physics.optics↗

arXiv2023

Light in correlated disordered media

The optics of correlated disordered media is a fascinating research topic emerging at the interface between the physics of waves in complex media and nanophotonics. Inspired by photonic structures in nature and enabled by advances in nanofabrication processes, recent investigations have unveiled how the design of structural correlations down to the subwavelength scale could be exploited to control the scattering, transport and localization of light in matter. From optical transparency to superdiffusive light transport to photonic gaps, the optics of correlated disordered media challenges our physical intuition and offers new perspectives for applications. This article reviews the theoretical foundations, state-of-the-art experimental techniques and major achievements in the study of light interaction with correlated disorder, covering a wide range of systems -- from short-range correlated photonic liquids, to Lévy glasses containing fractal heterogeneities, to hyperuniform disordered photonic materials. The mechanisms underlying light scattering and transport phenomena are elucidated on the basis of rigorous theoretical arguments. We overview the exciting ongoing research on mesoscopic phenomena, such as transport phase transitions and speckle statistics, and the current development of disorder engineering for applications such as light-energy management and visual appearance design. Special efforts are finally made to identify the main theoretical and experimental challenges to address in the near future.

physics.optics↗

arXiv2022

Speckle decorrelation in fundamental and second-harmonic light scattered from nonlinear disorder

Speckle patterns generated in a disordered medium carry a lot of information despite the apparent complete randomness in the intensity pattern. When the medium possesses $χ^{(2)}$ nonlinearity, the speckle is sensitive to the phase of the incident fundamental light, as well as the light generated within. Here, we examine the speckle decorrelation in the fundamental and second-harmonic transmitted light as a function of varying power in the fundamental beam. At low incident powers, the speckle patterns produced by successive pulses exhibit strong correlations, that decrease with increasing power. The average correlation in the second-harmonic speckle decays faster than in the fundamental speckle. Next, we construct a theoretical model, backed up by numerical computations, to obtain deeper physical insights on the faster decorrelations in the second-harmonic light. Whilst providing excellent qualitative agreement with the experiments, the model sheds important light on the contribution of two effects in the correlations, namely, the generation of second-harmonic light, and the propagation thereof.

physics.optics↗

arXiv2020

Absorption of scalar waves in correlated disordered media and its maximization using stealth hyperuniformity

We develop a multiple scattering theory for the absorption of waves in disordered media. Based on a general expression of the average absorbed power, we discuss the possibility to maximize absorption by using structural correlations of disorder as a degree of freedom. In a model system made of absorbing scatterers in a transparent background, we show that a stealth hyperuniform distribution of the scatterers allows the average absorbed power to reach its maximum value. This study provides a theoretical framework for the design of efficient non-resonant absorbers made of dilute disordered materials, for broadband and omnidirectional light, and other kinds of waves.

physics.optics↗

arXiv2019

Enhanced Absorption of Waves in Stealth Hyperuniform Disordered Media

We study the propagation of waves in a set of absorbing subwavelength scatterers positioned on a stealth hyperuniform point pattern. We show that spatial correlations in the disorder substantially enhance absorption compared to a fully disordered structure with the same density of scatterers. The non-resonant nature of the mechanism provides broad angular and spectral robustness. These results demonstrate the possibility to design low-density materials with blackbody-like absorption.

physics.optics↗

arXiv2019

Coherent light propagation through cold atomic clouds beyond the independent scattering approximation

We calculate the relative permittivity of a cold atomic gas under weak probe illumination, up to second order in the density. Within the framework of a diagrammatic representation method, we identify all the second order diagrams that enter into the description of the relative permittivity for coherent light transmission. These diagrams originate from pairwise position correlation and recurrent scattering. Using coupled dipole equations, we numerically simulate the coherent transmission with scalar and vector waves, and find good agreement with the perturbative calculations. We applied this perturbative expansion approach to a classical gas at rest, but the method is extendable to thermal gas with finite atomic motion and to quantum gases where non-trivial pair correlations can be naturally included.

physics.atom-ph↗

arXiv2019

Cross density of states and mode connectivity: Probing wave localization in complex media

We introduce the mode connectivity as a measure of the number of eigenmodes of a wave equation connecting two points at a given frequency. Based on numerical simulations of scattering of electromagnetic waves in disordered media, we show that the connectivity discriminates between the diffusive and the Anderson localized regimes. For practical measurements, the connectivity is encoded in the second-order coherence function characterizing the intensity emitted by two incoherent classical or quantum dipole sources. The analysis applies to all processes in which spatially localized modes build up, and to all kinds of waves.

physics.optics↗

arXiv2018

Blind Ghost Imaging

Ghost imaging is an unconventional optical imaging technique that reconstructs the shape of an object combining the measurement of two signals: one that interacted with the object, but without any spatial information, the other containing spatial information, but that never interacted with the object. Ghost imaging is a very flexible technique, that has been generalized to the single-photon regime, to the time domain, to infrared and terahertz frequencies, and many more conditions. Here we demonstrate that ghost imaging can be performed without ever knowing the patterns illuminating the object, but using patterns correlated with them, doesn't matter how weakly. As an experimental proof we exploit the recently discovered correlation between the reflected and transmitted light from a scattering layer, and reconstruct the image of an object hidden behind a scattering layer using only the reflected light, which never interacts with the object. This method opens new perspectives for non-invasive imaging behind or within turbid media.

physics.optics↗

arXiv2018

Photon echoes in strongly scattering media: a diagrammatic approach

We study photon echo generation in disordered media with the help of multiple scattering theory based on diagrammatic approach and numerical simulations. We show that a strong correlation exists between the driving fields at the origin of the echo and the echo beam. Opening the way to a better understanding of non-linear wave propagation in complex materials, this work supports recent experimental results with applications to the measurement of the optical dipole lifetime $T_2$ in powders.

physics.optics↗

arXiv2017

Optimizing Hyperuniformity in Self-Assembled Bidisperse Emulsions

We study long range density fluctuations (hyperuniformity) in two-dimensional jammed packings of bidisperse droplets. Taking advantage of microfluidics, we systematically span a large range of size and concentration ratios of the two droplet populations. We identify various defects increasing long range density fluctuations mainly due to organization of local particle environment. By choosing an appropriate bidispersity, we fabricate materials with a high level of hyperuniformity. Interesting transparency properties of these optimized materials are established based on numerical simulations.

cond-mat.soft↗

arXiv2017

Mean path length invariance in multiple light scattering

Our everyday experience teaches us that the structure of a medium strongly influences how light propagates through it. A disordered medium, e.g., appears transparent or opaque, depending on whether its structure features a mean free path that is larger or smaller than the medium thickness. While the microstructure of the medium uniquely determines the shape of all penetrating light paths, recent theoretical insights indicate that the mean length of these paths is entirely independent of any structural medium property and thus also invariant with respect to a change in the mean free path. Here, we report an experiment that demonstrates this surprising property explicitly. Using colloidal solutions with varying concentration and particle size, we establish an invariance of the mean path length spanning nearly two orders of magnitude in scattering strength, from almost transparent to very opaque media. This very general, fundamental and counterintuitive result can be extended to a wide range of systems, however ordered, correlated or disordered, and has important consequences for many fields, including light trapping and harvesting for solar cells and more generally in photonic structure design.

physics.optics↗