Relaxation of the optical density of glass modulated with bichromatic radiation

Exposure of oxide glass to two coherent light beams with different frequencies omega and 2 omega leads to the buildup of spatial index gratings in the glass. The kinetics of the writing and relaxation of the gratings is investigated. Illumination with one beam or a background illumination of the sample accelerates the relaxation process; this acceleration is due to the appreciable photoconductivity of the glass. The diffraction is found to reach a residual self-maintaining level when a grating is illuminated continuously with a beam at the fundamental frequency. The results are analyzed on the basis of a model that includes the coherent photogalvanic current, the accumulation of a grating of nonuniform charges under the action of this current, and the back-effect of the static field, via electrooptic effects, on the propagation of the beams. It is shown that an optical instability of the photoinduced index gratings exists in the glass. (C) 1996 American Institute of Physics.