Abstract

Wave mixing experiments in photorefractive crystals(PRC) have resulted in the observation of large number of interesting phenomena. Communications about spatial subharmonic generation were published in [1,2]. Subharmonic gratings with a wave vector q/n (n - integer) have been occuring during the illumination of PRC Bi₁₂SiO₂₀ by moving interference pattern with a wave vector q (Fig.1). Authors of [3] suggested the mechanism of generation of subharmonic at the expense of noise component amplification in the field of two pump waves. Amplification is caused by small off-Bragg vector Δq (see Fig.1). In this work we consider another possible mechanism of spatial subharmonic generation. By our opinion the particular mechanism which may lead to subharmonic generation is the following. Under illumination of PRC by interference pattern with intensity distribution I(x,t)=I_O(1+0.5mexp[i(qx-Ωt)]+c.c) photoelectron concentration grating is generated there. That grating moves with the phase velocity v=Ω/q. Here I_O is the average light intensity, m is a contrast of interference pattern, Ω is the frequency detuning of interacting beams. Fluctuation of space charge field ~E_l/2exp[i(qx/2-Ωt/2)]+c.c. causes the generation of photocurrent wave ~j_1/2exp[i(qx/2-Ωt/2)]+c.c. That photocurrent wave has the same space-time structure as a space charge field wave and consequently may intensify the space charge separation under suitable phase relation between those two waves. If that intensification exceeds the Maxwell relaxation, the space charge field amplitude of subharmonic will increase. The optimum condition for such instability is realised when the photocurrent wave is under resonance with trap charge exchange wave (TCEW) [4].

© 1991 Optical Society of America