Regensburg 2010 – scientific programme
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DF: Fachverband Dielektrische Festkörper
DF 2: Optical and nonlinear optical properties, photonic I
DF 2.1: Talk
Monday, March 22, 2010, 11:00–11:20, H11
Relaxation dynamics of mixed phase and amplitude gratings attributed to optically excited small polarons in reduced LiNbO3 — •Hauke Bruening, Raphael-Sung Hardt, Bettina Schoke, and Mirco Imlau — Department of Physics, University of Osnabrück, Barbarastr. 7, D-49069 Osnabrück, Germany
The relaxation dynamics of elementary holographic gratings recorded with intense ns-laser pulses (λ = 532 nm, I = 350 GW/m2) is studied in thermally reduced, nominally undoped LiNbO3 in the time regime of 10−8 s to 102 s at 785 nm. A stretched exponential decay of the diffraction efficiency is uncovered showing lifetimes in the ms-regime and a stretching coefficient 0,7 < β < 0,9. By temperature dependent measurements, we can attribute the grating signal to probe light diffraction at a spatial density modulation of small bound NbLi4+-polarons. Taking the presence of stable NbLi4+:NbNb4+-bipolarons in the ground state into account, the density modulation is recorded via the optical gating mechanism. Remarkably, the initial value of the diffraction efficiency exceeds the expectation value for a pure c(NbLi4+/5+)-amplitude grating, i.e., the polaron density modulation is accompanied with a spatial modulation of the index of refraction. We discuss the origin of the phase grating in the frame of a microscopic model for polaron absorption and the presence of intrinsic photovoltaic currents.
*Financial support by the DFG (Projects IM37/5-1 and GRK 695) is gratefully acknowledged.