Propagation of scalar waves in dense disordered media exhibiting short- and long-range correlations.

Correlated disorder is at the heart of numerous challenging problematics in physics. In this work we focus on the propagation of acoustic coherent waves in two-dimensional dense disordered media exhibiting long- and short-range structural correlations. The media are obtained by inserting elastic cylinders randomly in a stealth hyperuniform medium itself made up of cylinders.

Measuring cell displacements in opaque tissues: dynamic light scattering in the multiple scattering regime.

Coherent light scattered by tissues brings structural and dynamic information, at depth, that standard imaging techniques cannot reach. Dynamics of cells or sub-cellular elements can be measured thanks to dynamic light scattering in thin samples (single scattering regime) or thanks to diffusive wave spectroscopy in thick samples (diffusion regime). Here, we address the intermediate regime and provide an analytical relationship between scattered light fluctuations and the distribution of cell displacements as a function of time.

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.

Modeling of an active terahertz imaging system in brownout conditions.

We present a theoretical evaluation of a subterahertz (subTHz) system to image through a scattering medium composed of scatterers of sizes close to the wavelength. We specifically study the case of sand grain clouds created by helicopter rotor airflow during landing in arid areas. The different powers received by one pixel of a matrix made of subTHz sensors are identified.

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.

Observation of mean path length invariance in light-scattering media.

The microstructure of a medium strongly influences how light propagates through it. The amount of disorder it contains determines whether the medium is transparent or opaque. Theory predicts that exciting such a medium homogeneously and isotropically makes some of its optical properties depend only on the medium's outer geometry.

Radiative transfer of acoustic waves in continuous complex media: Beyond the Helmholtz equation.

Heterogeneity can be accounted for by a random potential in the wave equation. For acoustic waves in a fluid with fluctuations of both density and compressibility (as well as for electromagnetic waves in a medium with fluctuation of both permittivity and permeability) the random potential entails a scalar and an operator contribution. For simplicity, the latter is usually overlooked in multiple scattering theory: whatever the type of waves, this simplification amounts to considering the Helmholtz equation with a sound speed c depending on position r.

Scattering mean free path in continuous complex media: beyond the Helmholtz equation.

We present theoretical calculations of the ensemble-averaged (or effective or coherent) wave field propagating in a heterogeneous medium considered as one realization of a random process. In the literature, it is usually assumed that heterogeneity can be accounted for by a random scalar function of the space coordinates, termed the potential. Physically, this amounts to replacing the constant wave speed in Helmholtz' equation by a space-dependent speed.

Invariance property of wave scattering through disordered media.

A fundamental insight in the theory of diffusive random walks is that the mean length of trajectories traversing a finite open system is independent of the details of the diffusion process. Instead, the mean trajectory length depends only on the system's boundary geometry and is thus unaffected by the value of the mean free path. Here we show that this result is rooted on a much deeper level than that of a random walk, which allows us to extend the reach of this universal invariance property beyond the diffusion approximation.

Transport equation for the time correlation function of scattered field in dynamic turbid media.

I derive a transport equation for the time correlation function in transmission and reflexion and inside a turbid medium. This equation goes beyond the diffusion approximation that is at the root of the well-established diffusing-wave spectroscopy technique. It takes into account all the transport regimes from ballistic to diffusive and the relaxation in direction at each scattering event.

Light propagation in multilayered scattering media beyond the diffusive regime.

We describe a method to solve the radiative transfer equation (RTE) in multilayered geometry with index mismatch and demonstrate its potential for modeling light propagation in biological systems. The method is compared to Monte Carlo simulations with high accuracy but is much more efficient in terms of computer time. We illustrate the potential of the method by studying a multilayered system containing a weakly scattering layer surrounded by highly scattering layers, with anisotropic scattering and index mismatched interfaces.

Photon diffusion coefficient in scattering and absorbing media.

We present a unified derivation of the photon diffusion coefficient for both steady-state and time-dependent transport in disordered absorbing media. The derivation is based on a modal analysis of the time-dependent radiative transfer equation. This approach confirms that the dynamic diffusion coefficient is given by the random-walk result D = cl(*)/3, where l(*) is the transport mean free path and c is the energy velocity, independent of the level of absorption.

Spatial coherence in strongly scattering media.

We study the spatial coherence of an optical beam in a strongly scattering medium confined in a slab geometry. Using the radiative transfer equation, we study numerically the behavior of the transverse spatial coherence length in the different transport regimes. Transitions from the ballistic to the diffusive regimes are clearly identified.