Anomalous photorefractive behaviors in doped KNSBN crystals

Anomalous photorefractive behaviors that cannot be explained by previous models are observed in doped KNSBN crystals. The space-charge field increases to its maximum value and then gradually decreases as the incident light intensity increases. The superlinear dependence of the response rate on intensity, i.e. (tau) -1 varies direct as Ix (x greater than 1), are obtained in a Co:KNSBN crystal. The nearly linear dependence of response rate on intensity is measured in heavy reduced Co:KNSBN crystal, in which the dark decay of space-charge field is much faster and the total amount of dark decay is also larger than another one. A new model of two types of carriers with two deep and two shallow traps is proposed to explain these results.