Light-induced VIS-absorption and light-induced charge transfer in pure anddoped SBN crystals

Under illumination with Ar+-, Kr+-laser or UV light, SrxBa1-xNb2O6 (SBN):Ce , SBN:Cr and pure SBN crystals show a light-induced absorption band around 2.1 eV (600 nm) with a considerable dichroism. This light-induced VIS-absor ption exhibits nonlinear intensity dependence with saturation at high illum ination strength. The temperature dependence of the light-induced absorptio n of the VIS-center for a specific illumination intensity shows for tempera tures T < 200 K a nearly constant value, which however decreases rapidly at higher temperatures T > 200 K. The decay of the light-induced VIS-centers after the illumination is switched off is non-monoexponential and strongly depends on temperature. The decay rate of the VIS-centers is nearly zero at low temperatures T < 100 K, increasing rapidly above T > 140 K. The absorp tion and therefore the concentration of Ce3+- or Cr3+-centers in SBN crysta ls have been found to decrease with increasing illumination intensity at T = 2 K. At low temperatures T < 140 K under illumination (with Ar+-, Kr+-las er, UV light), in addition to the VIS-center absorption, a strong absorptio n band because of Nb4+ polarons [1], similar to absorption bands observed i n reduced SBN crystals [2], appears in the NIR spectral legion (lambda(max) about 1.6 mu m). The bandshape of this Nb4+ polaron absorption extends wit h its tail into the visible region, overlapping with the absorption of the VIS centers. The Nb4+ polaron absorption decays rapidly even at low tempera tures, and above T > 140 K no NIR absorption of these centers could be dete cted any more. A light-induced charge transfer model is proposed to explain the experimental results of the light-induced NIR and VIS absorption and t he possible origin of the light-induced VIS-centers. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.