Dependence of the Bragg Condition on an External Electric Field for a Polymeric Photorefractive Material

We investigated the effect of an applied electric field on the Bragg condition of degenerate four-wave mixing in a polymeric photorefractive material with a low glass-transition temperature. For a polymeric photorefractive material the application of an external electric field as is necessary for photorefractivity leads to birefringence of the material by poling of the nonlinear optical chromophore. Because the propagation vectors of the pumping and reading beams inside the material are influenced by the refractive index of the material, the Bragg condition depends on the magnitude of the external field. Using an oriented gas model and the-coupled-mode theory, we numerically analyzed the Bragg-mismatch effect that causes a reduction in diffraction efficiency as a function of an external field. We present the boundary conditions for sample thickness and grating spacing for which the Bragg-mismatch effect must be taken into account.