Thermal self-focusing and stimulated scattering in photorefractive crystals

Thermal self-focusing in photorefractive crystals differs from thermal effects in other crystals or glasses. Thermal lensing occurs when a refractive-index gradient is produced thermally by local heating with an intense laser beam. In photorefractive crystals, this index gradient creates a new phase matching condition, and light is preferentially scattered off photorefractive gratings. A significant difference between this and other stimulated scattering effects is that the beam divergence angle is intensity dependent. The diffraction angle of an incident probe beam can be significantly increased by the stimulated scattering phenomenon. We present the intensity dependence of the divergence angle for SBN, BaTiO3, and BSO. Divergence half-angles of up to 6° have been observed. We also describe the effects of polarization and orientation on beam divergence.