Enhanced Photorefractive Transient Response

A physical model for the photorefractive effect consists of considering two laser beams which are allowed to cross in a photorefractive crystal, producing an interference pattern. This interference pattern, or alternating light and dark regions in the crystal, results in the selective excitation of free-carriers from impurity trapped sites in the illuminated regions into the conduction band. In the conduction band, there is a subsequent diffusion of these electrons to neighboring dark regions where they are re-trapped at empty impurity sites. This transport of charge develops a space-charge field which, at equilibrium, produces a restoring force that exactly balances the diffusive force experienced by the excited free-carriers.