MULTIPLE GRATINGS IN SEMIINSULATING GAAS CRYSTAL

Analyzing anisotropic diffraction properties of photorefractive gratin gs in GaAs crystals and the symmetry difference of energy exchange bet ween index-of-refraction and amplitude gratings, we identify three dif ferent kind of gratings, namely a photorefractive, a non-photorefracti ve index-of-refraction and a photochromic grating simultaneously occur ring in semiinsulating GaAs crystals. We show that the latter two grat ings probably originate from electric polarizability changes arising f rom spatial modulation of photoelectron traps occurring during photore fractive recording. Experiments are carried out using a continuous pha se modulation technique in a two-wave mixing setup at a wavelength of 1.06 mu m.