Propagation of low-intensity Gaussian fields in photorefractive media with real and imaginary intensity-dependent refractive index components

A theoretical analysis of a low power Gaussian field propagating in unbiased self-defocussing photorefractive media that includes both real and imaginary components of the intensity-dependent refractive index is presented. The analysis, which is in excellent agreement with experimental observations, shows that the imaginary component of the intensity-dependent refractive index can have a focussing effect independent of the focussing or defocussing effect of the real component of the intensity-dependent refractive index. These findings suggest that the imaginary component of the intensity-dependent refractive index is the cause of the previously observed apparent self-focussing in unbiased self-defocussing photorefractive media.