Experimental observation of speckle instability in Kerr random media

Large intensity fluctuations of light diffusing through random media originates from the interference of the multiply scattered waves. The resulting speckle pattern can be viewed as a fingerprint of the disorder configuration of the medium and therefore is highly sensitive to scatterer motion. In the presence of a non linearity, the speckle may become instable as a result of the positive feedback provided by the scattering medium, even in absence of scatterer motion. This paper demonstrates speckle instability in a scattering two dimensional nonlinear disorder system. A photorefractive liquid crystal light valve (LCLV), which combines a nematic liquid crystal (LC) layer with a thin monocrystalline photorefractive crystal (BSO) in the form of a cell wall, is considered. A controlled random orientation of the LC layer is obtained by projecting the computer-generated random pattern of a spatial light modulator (SLM) onto the BSO film. The nonlinearity is provided by the reorientational Kerr effect of the LC. The existence of a threshold is confirmed and the dynamical behavior of wave in nonlinear random media below and above threshold in the particular regime of very strong disorder, where interference effects may result in Anderson localization, is investigated.