Wave propagation in random media: diffusion vs. localization

16.2.2009: Dr. Thomas Wellens (Physikalisches Institut, Univ. Freiburg): As it is well known, waves are fundamentally distinct from (classical) particles in their ability to display interference. However, in presence of disorder, interferences tend to be washed out. In this case, wave propagation reduces to a simple diffusion process - like a random walk of a classical particle ("soccer ball in the forest"). But under appropriate circumstances, some interferences may also survive the disorder average and induce interesting effects, for example turn a metal into an insulator (Anderson localization) or increase the brightness of saturn's rings (coherent backscattering). After a general introduction into the physics of multiple scattering, the second part of the talk deals with propagation of waves in nonlinear random media. Generally, a nonlinearity arises whenever the wave interacts with itself or with the scattering medium in such a way that properties like refractive index, mean free path, etc., are not constant, but depend on the wave intensity. The question is now: how do these nonlinearities affect the multiple scattering interferences? To tackle this problem, I will develop a diagrammatic theory for performing disorder averages with nonlinear wave equations. As main result, I show how the coherent backscattering interference is either diminished or amplified, depending on the type of nonlinearity.