Abstract
Large values of the exponential gain coefficient are obtained (Γ = 8-20 cm-1) in photorefractive potassium sodium barium strontium niobate crystals doped with transition metal elements: Co, Cu, Fe, Cr and Ni by using two-wave coupling experiments with argon ion laser (λ = 488 nm). The response time is measured to be 60-1000 ms at an intensity of 5W/cm2 for various samples. We find that in all the crystals electrons are the dominant photorefractive charge carriers, and the typical density of carriers is 1016-1017 cm-3.