Solutions for vectorial beam coupling under ac field in cubic photorefractive crystals.

We develop a theory of vectorial wave coupling in cubic photorefractive crystals placed in an alternating ac field to enhance the nonlinear response. It is proven in the general case that despite essential differences between the diffusion and the ac nonlocal responses, the latter keeps the light interference fringes straight during the interaction. This fundamental feature allows, under certain restrictions, to reduce the nonlinear problem of vectorial coupling to the known linear problem of vectorial Bragg diffraction from a spatially uniform grating, which admits an exact solution. As a result, the nonlinear vectorial problem can be effectively solved for a number of practically important cases. These cases include nontrivial polarization effects and also the high-contrast effects. The presence of conservation laws involving the polarization degrees of freedom is shown. A number of particular cases relevant to experiments with BTO crystals are considered.