NUMERICAL-ANALYSIS OF PHOTOREFRACTIVE INP-FE AT LARGE FRINGE CONTRAST

A finite difference method is used to analyze the behavior of photoref ractive 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 stud ied. As m increases, the field tends to concentrate in a small zone an d reaches very large local values. On the other hand, the resonance lo ses height and widens. For illuminations closed to the resonance, the two-wave mixing gain presents the usual aspect of a decreasing functio n 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, eve n at small fringe contrast. As m increases, the space-charge field ten ds to take a square shape. Even for rather small m values (0.1), the g ain presents a maximum near the de field resonant illumination, which is not predicted by the linear theory. Finally, when drift is the domi nant process for the grating formation, the applied field limits the a mplitude of the space-charge field, independently of an optimized dopa nt concentration which allows high gain in the small m approximation. (C) 1995 American Institute of Physics.