HIGHER HARMONIC GRATINGS IN PHOTOREFRACTIVE MATERIALS AT LARGE MODULATION WITH MOVING FRINGES

The nonlinear differential equations describing the response of photorefractive material to a moving sinusoidal intensity pattern that permits large modulation depth are solved by a numerical technique under steady-state condition. It is found that, at large modulation, the higher harmonics of the variables (space charge field, electron density, and ionized donor density) have appreciable amplitudes that can be further enhanced by moving the fringes. It is also shown that the optimized amplitudes of the higher harmonics of the space charge field cam be strong and comparable with the fundamental and that the dependence of these higher harmonics of the space charge field on the spatial frequency is much weaker at large modulation. © 1990 Optical Society of America.