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 influence of higher-harmonic gratings and subharmonic gratings on the threshold for generation of the subharmonic K/2 grating. The higher-harmonic grating causes feedback to the fundamental grating, thus leading to a nonlinear correction in dissipation, and the threshold for subharmonic generation is substantially modified by the presence of more subharmonics. The numerical solution shows that the inclusion of five higher-harmonic components and four subharmonic components are sufficient to cover the entire spatial region. The discrepancy between the analytical and the numerical solution increases with 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.