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 photorefr active BaTiO3 crystals and ion-implanted waveguides when focussed beam s have been used. The two-beam coupling gain, investigated as a functi on of the input beam ratio, showed an enhanced magnitude, when compare d with the standard theory, and exhibited a strong beam ratio dependen ce. 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 enhanc ed photoinduced space-charge field being a nonlinear function of modul ation, has given an agreement with the experimentally determined resul ts.