FUNDAMENTALS OF THE NONLINEAR-THEORY OF PHOTOREFRACTIVE SUBHARMONICS

We formulate the fundamentals of the nonlinear theory of low-frequency space-charge waves in semiinsulating photorefractive crystals. This i ncludes an analysis of dispersion relations for the waves, and of thei r parametric excitation by a running light pattern, a description of v arious stationary states (split and unsplit subharmonics) beyond the t hreshold of the parametric instability, and a study of the stability o f those nonlinear steady states against small perturbations. Nonlinear eigenfrequency and mutual frequency shifts for strong waves and renor malization of the coupling coefficients for weak waves are important e lements of the theory. An investigation of the stability of the subhar monics also incorporates their phase relations as well as certain spec ial features of space-charge waves. One of the consequences of the the ory is the modulational instability of the main subharmonic, character ized by doubling of the period of the primary light pattern. Finally w e discuss further development and applications of the theory.