Leader : Michel Claire
At the mesoscopic level, wave transport in a disordered medium is characterized by multiple scattering. As soon as the disorder becomes large enough, interferences turn out to dominate the scattering phenomenon. It is the strong localization regime, more known as Anderson localization regime. Until now, most of the experimental studies have been conducted considering only the linear regime. However, experiments that involve a coupling between Anderson localization and Bose-Einstein condensates have lead the community to consider the interaction between nonlinear effects and multiple scattering (speckle instability, random lasers, etc.).
With the aim of confronting nonlinear effects and disorder, guided optics offers an ideal experimental support in which the non-linearities are well known. A multimode nonlinear optical fiber turns out to be a “model system” to study the evolution of the spatial structure of a wave in a nonlinear disordered medium : it is technologically possible to manufacture an optical fiber with an index profile invariant in the propagation direction which presents a transverse disorder in order to ensure the existence of localized modes. We thus have a simple tool allowing a direct visualization of the spatial distribution of the wave in a nonlinear highly diffusive medium.
Beyond the idea of a “model experiment”, the experimental setup opens perspectives for the preliminary studies we already performed concerning the optical thermalization phenomenon in the perturbative nonlinear regime.
Incoherent nonlinear optics is being spotlighted since few years, as witnessed by actual subjects as incoherent solitons, “white lasers”, or extreme events. In particular, thermalization and condensation of optical waves give rise to a growing interest. Based on the weak turbulence kinetic theory, a kinetic approach of statistical nonlinear optics has been developed : based on a simple analogy with thermodynamics, wave mixing can be seen as a system of classically colliding particles (gas). A wave propagating in a nonlinear medium thus undergoes a thermalization process. It means that its spectrum inevitably evolves towards a thermodynamic equilibrium.
A prominent example is the optical wave condensation. The thermodynamic equilibrium spectrum of a wave propagating in a nonlinear medium exhibits a divergence for the fundamental mode that, by analogy with the Bose-Einstein transition in quantum systems, gives rise to a condensation process. Two major challenges clearly emerge : 1. The first one directly concerns the disordered systems community : having a “simple” experimental system with a well-known non-linearity (optical Kerr effect), coupled with a spatial disorder ensuring the existence of localized modes. This system will allow (i) to determine the major effects associated to the nonlinear propagation of a wave in a disordered system, (ii) to highlight them by a direct visualization of the wave, 2. The second combines wave propagation in scattering media and incoherent nonlinear optics : answering the fundamental question of the asymptotic evolution of an incoherent nonlinear optical wave subjected to a disordered potential and realizing the first experimental study to bring out classical waves thermalization in a random potential.
Numerical studies are in progress, and we plan an experimental development based on the use of a chalcogenide glass micro-structured optical fiber. The microstructure will ensure the optical index disorder in the transverse direction and the invariance in the propagation direction. Chalcogenide glasses present the non-negligible advantage of having a high nonlinear coefficient, in comparison with silica glasses usually used in optical fibers.
MOSAIQ, Physique Mésoscopique, Photonique
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