For many years, considerable research efforts have been directed toward exploring the interaction between light and matter for the prospective replacement of electronic devices with faster, more sensitive, and more reliable optical devices. Therefore, materials whose optical properties are sensitive to light, including nonlinear optical (NLO) materials, have attracted attention. An important subset of NLO materials are materials that exhibit the photorefractive (PR) effect, or PR materials. The PR effect refers to spatial modulation of the index of refraction under nonuniform illumination via space-charge-field formation and electro-optic nonlinearity. The effect arises when charge carriers, photogenerated by a spatially modulated light intensity, separate by drift and/or diffusion processes and become trapped to produce a nonuniform space-charge distribution. The resulting internal space-charge electric field then modulates the refractive index to create a phase grating, or hologram, which can diffract a light beam. Because such a hologram can typically be erased by uniform optical illumination, PR holograms are dynamic, that is, they may be erased and rewritten, one of many properties that distinguish PR materials from other mechanisms for hologram formation. 1 The PR effect was first observed in 1966 in a LiNbO3 crystal, 2 a discovery that launched extensive studies of the effect in inorganics. Until 1990, all PR materi-