Beam self-trapping in a BCT crystal

We present some aspects of wave self-focusing and self-defocusing in a photorefractive Ba0.77Ca0.23TiO3 (BCT) crystal without external electric field and without background illumination. The effects depend on the cross-section of the input beam. We show that by decreasing of the diameter of the input beam from 730 mu m the fanning effect disappears at 150 pm. A symmetrical self-focusing is observed for input diameters from 150 gm down to 40 mu m and a symmetrical self-defocusing for input diameters from 40 mu m down to 20 pm. The ID self-trapping is detected at 65 pin in BCT. Light power and wavelength are correspondingly 3 mW and 633 nm. The experimental results are supplemented with numerical calculations based on both photovoltaic model and model of screening soliton.