Photorefraction Without the Electrooptic Effect

Photorefractive crystals can couple optical beams in more than one way. In the usual photorefractive effect, light redistributes charge between empty and full trapping sites in the crystal. The electric field of the charge then alters the refractive index of the crystal by the electrooptic effect. However, the trapping sites themselves produce a variation in the local polarizability of the crystal. If the incident intensity pattern is periodic in space, then the resulting periodic redistribution of trapping sites will form a grating that can scatter light, even if the electrooptic effect is zero in the crystal. This "trap" grating is always 90° out of phase with the electrooptic grating, due to the spatial derivative in Gauss's law.