A theoretical study was made of the characteristics of low-frequency space-charge waves in semi-insulating crystals. The calculations were based upon published data on the EL2 center. They allowed for the competition of electrons and holes that were photo-excited in the spectral region (0.8 to 1.6) and for a non-linearity of electron transport due to a field- increased capture cross-section. The resultant characteristics defined the enhancement factor of the photo-refractive response as a function of the applied field and the fundamental grating period, and specified the conditions for the generation of spatial sub-harmonics. The main conclusion was that, in the case of n-type material, pronounced space-charge wave effects were expected in moderate applied fields (less than 1kV/cm).

B.I.Sturman, M.Aguilar, F.Agulló-López: Physical Review B, 1996, 54[19], 13737-43