Spectroscopic study of light-induced charge transfer processes in pure KNSBN, KNSBN:Ce and KNSBN:Cu crystals

Photorefractive potassium sodium strontium barium niobate (KNSBN) crystals show two light-induced absorption bands under illumination at low temperature. The first absorption lies in the NIR (around 0.72 eV) and can be assigned to Nb4+ polarons. The second light-induced absorption appears as a broad band in the VIS spectral region (around 2.0 eV). The temperature dependence and the decay kinetics of the two centers are investigated for pure KNSBN, KNSBN:Ce and KNSBN:Cu crystals. Pure KNSBN crystals show a photoluminescence emission band around 765 nm, whereas Ce or Cu doping quenches the luminescence of KNSBN crystals. Two thermoluminescence peaks are observed at 80 and 228 K, respectively, with the same spectral distribution in pure KNSBN, confirming the existence of two electron trapping centers induced by illumination. The related light-induced charge transfer processes and the roles of Ce and Cu in KNSBN crystals are discussed and compared with similar centers in SBN crystals.