Thermodiffusive and Electrostrictive Optical Nonlinearities in Critical Microemulsions

We have studied the propagation of a Gaussian laser beam in a critical microemulsion, in the weakly self-focusing regime. The experiment is carried out as a function of both temperature and beam diameter. We find that the previously observed very large nonlinearities in microemulsions are a mixture of electrostriction, thermal expansion and thermodiffusion and we estimate the amplitudes of these contributions. The critical behaviour of the electrostrictive part closely matches that of the osmotic compressibility of the mixture.