Lifetime of small polarons in iron-doped lithium-niobate crystals

Pulsed illumination of lithium-niobate crystals with green light excites el ectrons from deep traps into the intrinsic defect Nb-Li(5+) (Nb on Li site in the valence state 5+) and creates Nb-Li(4+) centers (small polarons). Th e electrons trapped in this more shallow center increase the light absorpti on in the red and near infrared. The dark decay of the polaron concentratio n is observed by monitoring the relaxation of these absorption changes. Iro n-doped lithium-niobate crystals with different concentrations of Nb-Li are investigated for various illumination conditions and temperatures. The rel axation shows a stretched-exponential behavior which is in disagreement wit h the predictions of the standard rate-equation-based model. The observed l ifetimes of the polarons range from tens of nanoseconds to some millisecond s. Computer simulations reveal that all results can be explained considerin g distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to the next available dee p electron trap. Based on the new insights, tailoring of lithium-niobate cr ystals for nonvolatile holographic storage becomes possible. (C) 2000 Ameri can Institute of Physics. [S0021-8979(00)03303-X].