Abstract

Tailoring of photorefractive crystals and optimization for certain applications requires detailed knowledge on microscopic origins of the photorefractive effect. Illumination generates movable charge carriers that migrate in the crystal and are trapped at different sites; a charge redistribution happens and a space charge field appears that modulates the refractive index via the electrooptic effect. For transition metal doped LiNbO₃ and moderate light intensities sources and traps of the charge carriers have been identified, e.g., Fe²⁺ and Fe³⁺ ions in LiNbO₃:Fe. However, up to now in many important photorefractive materials, e.g., in BaTiO₃, KNbO₃, Sr_1-xBa_xNb₂O₆ (SBN), KTa_1-xNb_xO₃ (KTN) and Bi₁₂TiO₂₀ (BTO), the photorefractive centers are not known. Several experimental hints (light-induced absorption changes, nonlinear dependence of photoconductivity on light intensity, dark buildup of holograms, activation of crystals for infrared recording by green illumination, transient dark conductivity phenomena) indicate that at least two different interacting centers contribute simultaneously, each of them occurring in different valence states.

© 1994 IEEE