Photoelectric instability in oxide glass

The spatially periodic modulation of optical anisotropy (MOA) induced in oxide glass by mutually coherent light beams with different frequencies (omega and 2 omega) is unstable under illumination with monochromatic light with frequency omega. Disturbances with small amplitudes intensify and disturbances with large amplitudes relax. Irrespective of its initial degree, the MOA reaches the same steady-state level, which depends on the illumination intensity. Intensification of MOA is accompanied by the appearance of second-harmonic radiation whose intensity grows in time to a steady-state level. The instability of the anisotropy is due to degenerate three-wave mixing and to feedback arising as a result of the coherent photogalvanic effect. A hypothesis that takes into account the observed giant growth (by three orders of magnitude) of light absorption in the MOA region is proposed to explain the stabilization of the instability and the formation of stationary periodic refractive-index gratings. (C) 1997 American Institute of Physics. [S0021-3640(97)00524-0].