RELAXATION OF THE OPTICAL-DENSITY OF GLASS MODULATED WITH BICHROMATICRADIATION

Exposure of oxide glass to two coherent light beams with different fre quencies omega and 2 omega leads to the buildup of spatial index grati ngs in the glass. The kinetics of the writing and relaxation of the gr atings is investigated. Illumination with one beam or a background ill umination of the sample accelerates the relaxation process; this accel eration 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 fr equency. The results are analyzed on the basis of a model that include s the coherent photogalvanic current, the accumulation of a grating of nonuniform charges under the action of this current, and the back-eff ect of the static field, via electrooptic effects, on the propagation of the beams. It is shown that an optical instability of the photoindu ced index gratings exists in the glass. (C) 1996 American Institute of Physics.