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 LiNbO 3 and LiTaO 3 at usual cw laser intensities. One impurity center occurs in two states, e.g. Fe as Fe 2+ and Fe 3+ in LiNbO 3 :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 BaTiO 3 and KNbO 3 . Furthermore, we demonstrate that the two-center model has to be used for doped LiNbO 3 crystals at high light intensities, too.
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