MERCADIER , Nicolas

In this thesis, we consider the problem of light transport in non-linear disordered media, where frequency redistribution can occur during scattering. Atomic vapors, that provide efficient resonance radiation trapping, are used as a model system to investigate the properties of two specific transport phenomena: random lasing action and Lévy...

We investigate multiple scattering of near-resonant light in a Doppler-broadened atomic vapor. We experimentally characterize the length distribution of the steps between successive scattering events. The obtained power law is characteristic of a superdiffusive behavior, where rare but very long steps (Lévy flights) dominate the transport properties.

The multiple scattering of photons in a hot, resonant, atomic vapor is investigated and shown to exhibit a Lévy Flight-like behavior. Monte Carlo simulations give insights into the frequency redistribution process that originates the long steps characteristic of this class of random walk phenomena.

Properties of random and fluctuating systems are often studied through the use of Gaussian distributions. However, in a number of situations, rare events have drastic consequences, which can not be explained by Gaussian statistics. Considerable efforts have thus been devoted to the study of non Gaussian fluctuations such as Lévy statistics,...

We address the problem of achieving a random laser with a cloud of cold atoms, in which gain and scattering are provided by the same atoms. In this system, the elastic scattering cross-section is related to the complex atomic polarizability. As a consequence, the random laser threshold is expressed as a function of this polarizability, which...

In conventional lasers optical cavities are used to provide feedback to gain media. Mirrorless lasers can be built by using disordered structures to induce multiple scattering, which increases the path length in the medium, providing the necessary feedback. Interestingly, light or microwave amplification by stimulated emission also occurs...

Atoms can scatter light and they can also amplify it by stimulated emission. From this simple starting point, we examine the possibility of realizing a random laser in a cloud of laser-cooled atoms. The answer is not obvious as both processes (elastic scattering and stimulated emission) seem to exclude one another: pumping atoms to make them...