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Q: Quantenoptik und Photonik

Q 60: Laserspektroskopie I

Q 60.3: Talk

Thursday, March 16, 2006, 11:40–11:55, HII

Competing photochromic properties of photoexcited small electron and hole polarons in LiNbO₃ — •C. Merschjann, M. Imlau und M. Wöhlecke — Fachbereich Physik, Universität Osnabrück, Barbarastraße 7, D-49069 Osnabrück

Chemically reduced congruently melting lithium niobate possesses Nb_Li⁴⁺:Nb_Nb⁴⁺ - bipolarons. These bipolarons can be dissociated either thermally or via illumination with intense light in the blue-green spectral range yielding both bound (Nb_Li⁴⁺) and free (Nb_Nb⁴⁺) small polarons. In addition to this optical dissociation, small (O⁻) hole polarons are present in LiNbO₃ due to multi-photon-excitation. Small electron and hole polarons exhibit characteristic absorption bands, thus their simultaneous excitation and decay can be studied using transient absorption spectroscopy. With this method we have investigated the competition of the photochromic properties of the respective polarons in reduced LiNbO₃ crystals upon excitation with intense light pulses at λ = 532 nm. We found that each particular polaron can be studied separately by choosing adequate wavelengths of the probing beam. The photochromic behavior is determined in differently reduced samples as a function of pump intensity and temperature. The results of our systematic study can be successfully interpreted within an energy band model taking the polaronic contributions into account. As a remarkable feature we found that the transient photochromic absorption can vanish completely in the blue-green spectral range due to the competing photochromic properties of the polarons involved. Supported by the Deutsche Forschungsgemeinschaft (Projects IM 37/2-1 and TFB 13-04).