We report on anomalous two-beam coupling results observed in photorefractive BaTiO3 crystals and ion-implanted wave-guides 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.
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