Formation time of a small electron polaron in LiNbO 3 : measurements and interpretation

Abstract

Infrared optical absorption attributed to the electron polaron on a non-defective site in LiNbO3 and KNbO3 has previously been observed using pulsed electron and laser techniques. With subpicosecond laser excitation and spectroscopy, it is possible to measure a rise time of the infrared absorption, which may be interpreted as the time for a band-state conduction electron to cool by phonon scattering, collapse its wavefunction around a site made attractive by thermal disorder, and relax vibrationally to a small polaron. This is a process which is of fundamental interest, involving dynamics of self-localization from band states and vibrational relaxation of a localized electron in an otherwise non-defective lattice. For example, Gavartin and Shluger have recently performed calculations on the role of thermal fluctuations in self-trapping of holes in MgO. We report initial measurements on the rise time of infrared absorption at 0.95 eV (Mg-perturbed polaron) in LiNbO3:Mg to be τR ≈ 230 fs at T = 20 K and τR ≈ 110 fs at T = 296 K. We discuss 2 stages that together may account for the delay and its temperature dependence: free-electron cooling and vibrational relaxation of a “defect” (small polaron) in a host. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)