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

Pulsed illumination of lithium-niobate crystals with green light excites electrons 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). The electrons trapped in this more shallow center increase the light absorption in the red and near infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron-doped lithium-niobate crystals with different concentrations of Nb-Li are investigated for various illumination conditions and temperatures. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation-based model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to the next available deep electron trap. Based on the new insights, tailoring of lithium-niobate crystals for nonvolatile holographic storage becomes possible. (C) 2000 American Institute of Physics. [S0021-8979(00)03303-X].