PHOTOREFRACTIVE CHARGE COMPENSATION AT ELEVATED-TEMPERATURES AND APPLICATION TO KNBO3

We treat the photorefractive effect in oxide crystals at elevated temperatures where charge compensation occurs in the absence of photoexcitation of the compensating species. These species can be either mobile ions or holes in the valence band. Two models are presented that take into account particularities of ion and hole transport. In the small-modulation approximation, solutions for the steady state and the dynamic evolution of the photorefractive effect are given. The maximum space-charge field E(q) that can be reached depends on the effective number of electron traps in the crystal. However, in the steady state, while the component of the space-charge field that is due to electrons and the one that is due to the compensating carriers both approach the value E(q), an almost complete compensation of these two components occurs. The speed of compensation is slower for larger grating spacings than for smaller grating spacings and can be increased by applying an electric field. Applying an external electric field also produces a phase shift between the two gratings, therefore increasing the total space-charge field. Experiments performed in KNbO3 confirm the theoretical predictions and indicate that the ionic model is more appropriate for this crystal. Implications of these compensation effects for quasipermanent hologram storage are discussed.