Light-induced charge transfer and kinetics of the NIR absorption of Nb4+ polarons in SBN crystals at low temperatures

Sr1-xBaxNb2O6 (SBN) crystals with open tungsten-bronze structure show enhan ced 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 sho w a broad polaron absorption band around 0.7 eV (6000 cm(-1)). The first st ep of a theoretical model involves the excitation of electrons by illuminat ion from Cr3+/Ce3+ to higher excited states or the conduction band. The exc ited electrons can then be trapped by Nb5+ to form Nb4+ polarons and furthe r on can directly tunnel through or hop over the potential barrier (with a value Delta approximate to 0.15 +/- 0.02 eV) to recombine with Cr4+/Ce4+ io ns. The experimental intensity dependence, temperature dependence, and deca y process of the light-induced Nb4+ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assu mption of different recombination rates of polarons at distinct distances f rom the Cr4+/Ce4+ recombination centers and therefore many parallel decay c hannels an active where each decay channel obeys a monoexponential decay la w. A stretched exponential decay function is employed to fit in this case t he decay process of the Nb4+ polarons at different temperatures and under i llumination with different intensities. Due to the high dielectric constant value (epsilon(33) and epsilon(11) have values in the 10(2) - 10(3) range) at low temperature, the long range Coulomb attraction (to Ce-Sr/Ba(3+)) or repulsion (from Cr-Nb(3+)) of the electronic polaron is suppressed. The le ading role in the attraction and the following trapping of the electronic J ahn-Teller polaron, both on Cr-Nb(3+) and Ce-Sr/Ba(3+) centers, is played b y the indirect dipole-dipole interaction via the soft TO-mode.