Role of iron in lithium-niobate crystals for the dark-storage time of holograms

The dark decay of holograms stored in iron-doped photorefractive lithium-ni obate crystals is studied for samples containing up to 0.25 wt% Fe2O3 (iron concentration 71 x 10(18) cm(-3)). The oxidation/reduction state of the cr ystals, i.e., the concentration ratio of Fe2+ and Fe3+ ions, is changed in a wide range by thermal annealing. The dark decay is attributed to two effe cts: An ionic dark conductivity arising from mobile protons and an electron ic dark conductivity caused by tunneling of electrons between iron sites. T he latter is proportional to the effective trap density, i.e., to the densi ty of charge carriers which can be moved between the iron sites. The propor tionality factor is the specific dark conductivity which increases exponent ially with the third root of the entire iron concentration. (C) 2000 Americ an Institute of Physics. [S0021-8979(00)09519-0].