Picosecond Near-to-mid-infrared Absorption of Pulse-Injected Small Polarons in Magnesium Doped Lithium Niobate

Femtosecond-pulse-induced (E-pump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO3:Mg. We find a non-instantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width approximate to 0.5 eV), for t > 1 ps. The modelling succeeds by considering small Nb-Nb(4+) electron polaron formation along the sequence: (i) two-photon injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electron-phonon-coupling. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement