Picosecond near-to-mid-infrared absorption of pulse-injected small polarons in magnesium doped lithium niobate

Femtosecond-pulse-induced (Epump = 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 noninstantaneous 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 ≈ 0:5 eV), for t > 1 ps. The modelling succeeds by considering small Nb4+ Nb electron polaron formation along the sequence: (i) twophoton 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 electronphonon- coupling.