Ac square-wave field-induced subharmonics in photorefractive sillenite: threshold for excitation by inclusion of higher harmonics

We investigated both analytically and numerically the simultaneous influenc e of higher-harmonic gratings and subharmonic gratings on the threshold for generation of the subharmonic K/2 grating. The higher-harmonic grating cau ses feedback to the fundamental grating, thus leading to a nonlinear correc tion in dissipation, and the threshold for subharmonic generation is substa ntially modified by the presence of more subharmonics. The numerical soluti on shows that the inclusion of five higher-harmonic components and four sub harmonic components are sufficient to cover the entire spatial region. The discrepancy between the analytical and the numerical solution increases wit h increased electric-field amplitude. (C) 1999 Optical Society of America [ S0740-3224(99)02301-2] OCIS codes: 090.7330, 160.5320, 190.4410, 190.5330.