ABSTRACT
One of the more interesting phenomena that can be produced in certain glasses is the ability to permanently change the refractive index by exposure to light; if not directly, then with a subsequent thermal treatment. Patterns of the refractive index in glasses and crystals provide important optical devices such as diffraction grating, holograms, and gradient index lenses, to name the important ones; see Borelli1 for a review. Unfortunately, there is an effect in certain ferroelectric crystals also called photorefractive, which has an entirely different origin stemming from an electronic effect that cannot exist in glass because it is an insulator. The refractive index of a material is defined by the polarization that is induced in the material in response to the electric field of the incident light.
