Photorefractive crystals - The role of defects

Abstract

Defects are of crucial importance for the light-induced build-up of space charge fields in photorefractive crystals. The one-center model is explained which describes the charge transport properties of transition metal-doped LiNbO3 and LiTaO3 at usual cw laser intensities. One impurity center occurs in two states, e.g. Fe as Fe2+ and Fe3+ in LiNbO3:Fe. Then we discuss the two-center model which has to be applied to other photorefractive crystals. Two interacting centers have to be taken into account, each of them occuring in two different states. Experimental observations supporting this model are presented for BaTiO3 and KNbO3. Furthermore, we demonstrate that the two-center model has to be used for doped LiNbO3 crystals at high light intensities, too.