  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.
