A theoretical and experimental analysis was made of photo-refractive 2-beam coupling in undoped material as a function of temperature. The theoretical treatment included the metastable state of the EL2 defect and its optical properties (optical generation and recovery) in the photo-refractive model. Among the features which were predicted by the model were a change in the sign of the photo-refractive beam coupling gain around 150K, an enhancement of the space-charge field by a factor of 2 as compared with the diffusion field and the appearance of a strong peak which was due to an absorption around 150K. All of these features were observed experimentally, and there was a good correlation between the experimental data and theoretical simulations.

P.Delaye, B.Sugg: Physical Review B, 1994, 50[23], 16973-84