Light-induced charge transfer and kinetics of the NIR absorption of Nb 4+ polarons in SBN crystals at low temperatures

Abstract

Sr1-xBaxNb2O6 (SBN) crystals with open tungsten-bronze structure show enhanced photorefractive properties with doping of impurities such as Ce, Cr, Rh etc. Under illumination with Kr+ laser (647 nm) or Ar+ laser light (488 nm or 514 nm) or UV light at low temperature, pure and doped SBN crystals show a broad polaron absorption band around 0.7 eV (6000 cm-1). The first step of a theoretical model involves the excitation of electrons by illumination from Cr3+/Ce3+ to higher excited states or the conduction band. The excited electrons can then be trapped by Nb5+ to form Nb4+ polarons and further on can directly tunnel through or hop over the potential barrier (with a value Δ≈0.15±0.02 eV) to recombine with Cr4+/Ce4+ ions. The experimental intensity dependence, temperature dependence, and decay process of the light-induced Nb4+ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assumption of different recombination rates of polarons at distinct distances from the Cr4+/Ce4+ recombination centers and therefore many parallel decay channels are active where each decay channel obeys a monoexponential decay law. A stretched exponential decay function is employed to fit in this case the decay process of the Nb4+ polarons at different temperatures and under illumination with different intensities. Due to the high dielectric constant value (e33 and e11 have values in the 102-103 range) at low temperature, the long range Coulomb attraction (to Ce3+ Sr/Ba) or repulsion (from Cr3+ Nb) of the electronic polaron is suppressed. The leading role in the attraction and the following trapping of the electronic Jahn–Teller polaron, both on Cr3+ Nb and Ce3+ Sr/Ba centers, is played by the indirect dipole–dipole interaction via the soft TO-mode.