Two-photon- and x-ray-induced Nb4+ and O− small polarons in LiNbO3

ESR investigations show that two-photon excitation and x irradiation of nominally pure LiNbO3 crystals at 20 K create electrons, which are self-trapped as Nb4+, and holes, which are captured at acceptor-type defects as O−. The radiation-induced absorption at low temperatures is attributed to these centers. The trapped electrons and holes play an important role in the two-photon-induced photorefractive process.