Counterpropagating beams in biased photorefractive crystals: Anisotropic theory

We formulate an anisotropic nonlocal theory of the space charge field induced by. the coherent counterpropagating beams in biased photorefractive crystals. We establish that the competition between the drift and diffusion terms has to be taken into account when the crystal (c) over cap axis is tilted with respect to the propagation direction of the beams. We demonstrate that this configuration combines the features of both spatial soliton formation without energy exchange and two-wave mixing with energy exchange leading to pattern formation.