The inherent nonlinearity of the charge transport processes in photorefractive materials give rise to some new phenomena, such as generation of harmonics or grating cross talk, that have been extensively investigated for non-fixed gratings. Here, we investigate the corresponding phenomena taking place during the thermal fixing process. We study in detail the kinetics of the thermal fixing process for high light modulation initial recording that results to be itself nonlinear. For single grating recording we obtain the amplitude of the first and second harmonic proton gratings and the final developed amplitude for the first harmonic. In turn, for multiple recording we focus our attention on combinational components. It is obtained that the key parameters that control nonlinear fixing are the proton concentration and grating K-vectors. Some preliminary experimental data are also presented.
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