PHOTOREFRACTIVE EFFECTS IN LINBO3-CR INDUCED BY 2-STEP EXCITATION

With the help of a Q-switched ruby laser elementary volume phase holograms are recorded in LiNbO//3:Cr crystals via stepwise two-photon excitations. Readout experiments are performed with lower intensity and unwanted erasure effects are largely reduced. The light induced charge transport is investigated and the following model is proposed: The crystals contain essentially Cr**3** plus ions and presumably a small number of Cr**4** plus ions. By two-step processes electrons of Cr**3** plus ions are excited to the conduction band. These electrons are then captured by Cr**4** plus ions froming Cr**3** plus ions again or by other unknown traps forming color centers. The model describes the principal experimental results.