The photorefractive effect is a special holographic recording mechanism, in which nonuniform illumination leads to formation of a spatially modulated space-charge field, which in turn modulates the refractive index.O. Ostroverkhova and W. E. Moerner, Chem. Rev. 104, 3267-3314 (2004) If nonuniform illumination is created by two interfering light beams, a phase grating (hologram) that can diffract light is produced in a PR (photorefractive) material. Since such a hologram can typically be erased by uniform optical illumination, PR holograms are dynamic, that is, they may be erased and rewritten many times, one of many properties that distinguish photorefractivity from other mechanisms for hologram formation.