Experimental Evidence for Recurrent Multiple Scattering Events of Light in Disordered Media

The interest in the field of multiple light scattering in disordered media has gone through an enormous revival after it was recognized that interference effects can be important, even after many scattering events. Several interference phenomena like weak localization [1] and short and long range spatial correlations in intensity fluctuations are the focus of today’s research [2]. The phenomenon of weak localization is sometimes seen as a precursor to strong or Anderson localization of light [3], which would be the light counterpart of Anderson localization of electrons. The approach to a strong localization transition in a medium with strong disorder manifests itself as a reduction of the diffusion coefficient, eventually to zero. Localization in three dimensions of microwaves has been reported [4]. It is still a challenge to find experimental evidence for strong localization of light. In all multiple scattering phenomena that have been observed so far, recurrent scattering of light waves can be disregarded. Recurrent events are events in which a wave is scattered more than once by the same scatterer. The neglect of recurrent scattering will be referred to as the self-avoiding multiple scattering (SAMS) approximation, and is valid in the weak scattering limit. However, for strong scattering recurrent events can become important. They could play a role in strong localization of light. In order to find evidence for the existence of recurrent scattering we studied coherent backscattering or weak localization of light. Coherent backscattering manifests itself in the form of an enhancement of the intensity in the back direction for the light scattered from a disordered sample. This enhancement originates from constructive interference between multiply scattered amplitudes and their time reversed counterparts. Moving away from the exact backscattering direction, phase differences develop that average out this interference effect. The result is a “cone” of enhanced backscattering on top of the diffuse background, which has a width of the order of sk,d 21 where , is the (transport) mean free path of the light in