Photorefractive ac response beyond the low-contrast limit

We show that a photorefractive crystal, placed in an ac field and exposed to nonuniform light, exhibits singularities of the induced space charge and discontinuities of the corresponding space-charge field. For a symmetric beam, the singularity appears at the intensity maximum when the light contrast exceeds a certain (rather low) threshold value. Also, we analyze a diffraction model to understand the effect of the singular ac response on the propagation of a Gaussian light beam.