Berlin 2015 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 11: Small Polarons in LiNbO₃

DF 11.1: Talk

Wednesday, March 18, 2015, 09:30–09:50, EB 133C

Two-photon absorption in presence of small polaron formation in lithiumniobate — •Mirco Imlau, Holger Badorreck, Stefan Nolte, Felix Freytag, and Javid Shirdel — Department of Physics, Osnabrueck University, D-49069 Osnabrueck

Two-photon absorption (TPA) appears in nominally undoped lithiumniobate with photon energies below the band gap energy of 3.8 eV and intensities of a few PW/m²; it is commonly studied by means of the z-scan technique. However, using ultrashort laser pulses with durations below 100 fs, the transmission signal is additionally affected by optically excited free carriers (FCA) and subsequently formed small polarons that have rise times of 50-100 fs (Nb_Nb⁴⁺ [Qiu et al. phys. stat. sol. c 2, 232 (2005)]) and below 400 fs (Nb_Li⁴⁺ [Beyer et al. Appl. Phys. B 83, 527 (2006)]). We applied the z-scan technique with 100 fs laser pulses at 2.5 eV to study the composed transmission loss that is given by TPA, FCA, and small polaron absorption. For data analysis, the well-known theory of Sheik-Bahae (IEEE J. Quantum Electron. 26, 760 (1990)) is modified by contributions from absorption centers within the band gap [Shanmugavelu, J. Appl. Phys. 114, 243103 (2013)]. The results of our analysis is experimentally verified by systematic z-scan studies using stretched pulses with durations up to 1.000 fs. We discuss the possibility to determine risetime and absorption cross section of optically excited hot carriers and present a refined model for the small polaron formation path in lithiumniobate. Financial support by the DFG (IM37/5-2, INST 190/165-1 FUGG) is gratefully acknowledged.