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Two-wave mixing in photorefractive InP:Fe crystals is revisited in terms of an externally applied dc field combined with the moving-grating technique. We show that the disagreement between theory and experiment that was previously observed in the literature can be reconciled by an examination of the influence of both the absorption along the propagation axis and the nonlinearities that occur at relatively large fringe modulations. A finite-difference method is used to demonstrate the dependence of these nonlinearities on the average incident intensity.

A comparative study of the temperature dependence of the two-wave mixing gain is performed at three wavelengths (i.e., 1.06, 1.32, and 1.54 µm) for the fringe spacing values Λ=1.6 µm and 5 µm. With the latter the gain sign changes at low temperature for λ=1.06 and λ=1.54 µm, but not for λ=1.32 µm. The gain measurements are performed both with and without an external electric field. The theoretical model must fit all the experiments. In our case the energy level of the second defect is found to be near the valence band. We demonstrate that the agreement between the theory and the experiments performed at room temperature is improved by incorporating the indirect transitions (involving the excited state of iron) into the two-defect model. The result of this approach is presented as the extended two-defect model.