Dynamics of transverse waves and zigzag instabilities in photorefractive two-wave mixing with a feedback mirror

We report on the appearance of running transverse waves and zigzag instabil ities in photorefractive counter-propagating two-wave mixing with a single external feedback mirror. These secondary structures are caused by the slug gish temporal response of the crystal through the buildup of a reflection g rating. We derive a Landau-type equation for the critical mode amplitude ab ove the primary instability threshold and compare it with the results from numerical simulations in the case of low and high optical aspect ratios. Th e effect of intensity-dependent relaxation time on the formation of the spa tiotemporal structures is discussed. (C) 2001 Optical Society of America.