Self-focusing in photorefractive two-wave-mixing in drift- and diffusion-type material

Summary form only given.We report experimental results on self-focusing and self-defocusing of light in two-wave-mixing. The complex coupling coefficient for two-wave-mixing and the corresponding first-order (linear) and nonlinear (third-order) susceptibility were derived. Amplification and self-focusing were measured as functions of the frequency shift between signal and pump beam in two-wave-mixing in BSO and BaTiO/sub 3/. The frequency dependence of the self-focusing is found to have a dispersion shape as is plausible from the analogy of the photorefractive and the laser amplification. In a drift-type material, the self-focusing in two-wave-mixing has to be distinguished from photorefractive self-focusing. This is possible because two-wave-mixing self-focusing requires coherent fields, while photorefractive self-focusing does not. We were able to observe pure two wave-mixing self-focusing by using pump/signal ratios above 10, at which value the photorefractive self-focusing is suppressed. The self-focusing in two-wave-mixing in diffusion-type material (BaTiO/sub 3/) is isotropic in the transverse plane, whereas in the case of drift-type material (BSO) we found that the amount of self-focusing in two-wave-mixing is larger by a factor of about three along the direction of the external field than along the direction perpendicular to the external field. This anisotropy can be explained by pump depletion in BSO. Consequences of gain saturation for the measurements of self-focusing in BaTiO/sub 3/ are discussed. In general two-wave-mixing self-focusing can be changed to defocusing by a change of frequency shift between signal and pump beam and by a change of orientation of the crystal.