SHALLOW-TRAP-INDUCED POSITIVE ABSORPTIVE 2-BEAM COUPLING GAIN AND LIGHT-INDUCED TRANSPARENCY IN NOMINALLY UNDOPED BARIUM-TITANATE

We use the asymmetry of beam coupling with respect to the orientation of the polar axis in a nominally undoped barium titanate crystal to determine the electro-optic and absorptive "gain" in the usual beam-coupling geometry. For small grating wave vectors, the electro-optic coupling vanishes but the absorptive coupling remains finite and positive. Positive absorptive coupling at small grating wave vectors is correlated with the light-induced transparency of the crystal described herein. The intensity and grating wave vector dependence of the electro-optic and absorptive coupling, and the light-induced transparency are consistent with a model incorporating deep and shallow levels.