ANOMALOUS PHOTOREFRACTIVE RESPONSE IN 2-BEAM COUPLING WITH FOCUSED BEAMS IN PHOTOREFRACTIVE CRYSTALS AND WAVE-GUIDES

We report on anomalous two-beam coupling results observed in photorefractive BaTiO3 crystals and ion-implanted waveguides when focussed beams have been used. The two-beam coupling gain, investigated as a function of the input beam ratio, showed an enhanced magnitude, when compared with the standard theory, and exhibited a strong beam ratio dependence. In order to explain the significant difference we observed between theory and experiment, we discuss several models, based on intensity-dependent processes, and show that only a modification with the enhanced photoinduced space-charge field being a nonlinear function of modulation, has given an agreement with the experimentally determined results.