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Transient light-induced absorption in LiNbO3 is observed in the blue-green spectral range after pulsed illumination with 532 nm. Its buildup and decay in Fe-doped LiNbO3 is satisfactorily described by a sum of two stretched exponential functions. For undoped LiNbO3, however, only one stretched exponential decay is observed. These experimental results are explained by the formation of both small Nb(Li)4+ electron polarons and O- hole polarons. The mechanism is discussed on the basis of a proposed band scheme.

The dark decay of holograms stored in iron-doped photorefractive lithium-niobate crystals is studied for samples containing up to 0.25 wt% Fe2O3 (iron concentration 71×1018 cm−3). The oxidation/reduction state of the crystals, 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 effects: An ionic dark conductivity arising from mobile protons and an electronic dark conductivity caused by tunneling of electrons between iron sites. The latter is proportional to the effective trap density, i.e., to the density of charge carriers which can be moved between the iron sites. The proportionality factor is the specific dark conductivity which increases exponentially with the third root of the entire iron concentration.

The temporal behavior of polarons generated by pulsed illumination of LiNbO 3 crystals doped with Fe, Fe+ Ti, and Cu is examined by detecting the light-induced absorption ␣ li in a pump-probe experiment. The decay of the polarons is satisfactorily described by a stretched exponential function over a temporal range of six decades. This fact is attributed to the dependence of the polaron lifetime on the distance to the nearest deep traps or to the disorder in LiNbO 3 . The dependence of the amplitude and dynamics of the light-induced absorption on the pump intensity are explained within the framework of these models. The wavelength dependence of ␣ li displays a maximum at about 740 nm, revealing the optical excitation of small bound Nb Li 4+ polarons. Temperature-dependent measurements in the range 293-473 K reveal an Arrhenius behavior with an activation energy of about E A = 0.38 eV.

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