We present a new model (including both temperature and electron-hole effects) of two-beam coupling in photorefractive semiconductors under an external dc field E0. This model predicts that the exponential gain Γ can exhibit an intensity-dependent resonant behavior, yielding a π/2 phase shift of the space-charge field with respect to the incident fringe pattern. This optimum intensity strongly depends on crystal temperature but it is practically independent of the grating period. As an illustration this model is applied to InP:Fe. In this case the resonance occurs when the hole photoionization rate and the electron thermal emission rate are equal. Values of Γ as high as 20 cm−1 at 1.06 μm, for a fringe spacing of 15 μm and E0 =10 kV/cm, are predicted. The comparison between theory and experimental data requires taking into account the variation of the pump intensity throughout the sample (due to optical absorption), which reduces the volume where the resonance condition is fulfilled; in this way a satisfactory fit of the experimental gain versus intensity curve has been obtained. The characteristics of this new resonance mechanism suggest several gain enhancement techniques which are briefly discussed. In particular, an increase of Γ by using an additional incoherent backward propagating beam is predicted and experimentally demonstrated.

We present a new model, which incorporates both temperature and electron-hole effects, for two-beam coupling in photorefractive semiconductors under an external dc field E(0). We show that the exponential gain ? exhibits an intensity-dependent resonance. The application of this model to InP:Fe allows us to predict a value of ? near 20 cm(-1) for a thin sample at 1.06 microm with E(0) = 10 kV/cm.

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A finite difference method is used to analyze the behavior of photorefractive InP:Fe at high fringe contrast under externally applied field. The evolution with increasing fringe contrast m of the electron-hole resonance, which occurs in the presence of a continuous field, is studied. As m increases, the field tends to concentrate in a small zone and reaches very large local values. On the other hand, the resonance loses height and widens. For illuminations closed to the resonance, the two-wave mixing gain presents the usual aspect of a decreasing function of m. However, for other illuminations, it can be larger at a small pump-to-probe ratio than at a large one. When an ac field technique is used, nonlinearities drastically reduce the two-wave mixing gain, even at small fringe contrast. As m increases, the space-charge field tends to take a square shape. Even for rather small m values (0.1), the gain presents a maximum near the dc field resonant illumination, which is not predicted by the linear theory. Finally, when drift is the dominant process for the grating formation, the applied field limits the amplitude of the space-charge field, independently of an optimized dopant concentration which allows high gain in the small m approximation.

After recalling the principal results of the theory of two-wave mixing gain enhancement in photorefractive InP:Fe crystals under a dc field, we report on experiments performed with several samples manufactured at CNET. For the first time, we believe, a maximum gain of 11.4 cm(-1) has been obtained under an applied dc field of 10 kV/cm in a thermally stablized crystal (T = 290 K) at lambda = 1.06 microm. In spite of a decrease in the factor n(3)r(eff)/lambda high gains (Gamma >/= 5 cm(-1)) have been achieved at lambda = 1.32microm in thick crystals. These experimental results are in good agreement with theoretical predictions.