Light-induced dielectric structures and enhanced self-focusing in critical photorefractive ferroelectrics.

We consider the nonlinear dynamics occurring when an optical beam couples to dielectric material polarization in an unbiased photorefractive crystal undergoing a ferroelectric phase transition. The polarization profile produced by the light-induced electric field is evaluated by means of the Landau-Ginzburg approach and is found to manifest new thermodynamical states with their own specific nonlinear optical effects. We show that a temperature T(C), lower than the critical one, exists such that (a) if T>T(C) the optical beam experiences an increasing self-focusing for decreasing temperatures and (b) if T<T(C) the optical beam allows the existence of thermodynamically metastable states associated with an optical ultra-self-focusing effect.