Experimental observation of speckle instability in Kerr random media

In a disordered medium with Kerr-type nonlinearity, the transmitted speckle pattern was predicted to become unstable, as a result of the positive feedback between the intensity fluctuations and the nonlinear dependence of the local refractive index. We present the first experimental evidence of speckle instability of light transversally scattered in a liquid crystal cell. A two-dimensional controlled disorder is imprinted in the medium through a suitable illumination of a photoconductive wall of the cell, whereas the nonlinear response is obtained through optical reorientation of the liquid crystal molecules. Speckle patterns with oscillating intensity are observed above a critical threshold, which is found to depend on the scattering mean free path, as predicted by theory.