Reduction-induced polarons and optical response of Mg-doped LiNbO3 crystals

We studied the visible and IR dispersion of absorption coefficient and refractive index for congruent LiNbO3 and Mg:LiNbO3 crystals before and after chemical reduction at different annealing temperatures. The concentration of Mg in Mg:LiNbO3 samples was just below or above the photorefractive threshold. The reduction-induced changes in the absorption coefficient reveal the formation of polarons typical for doped LiNbO3 crystals. It was shown that the polaron concentration is maximal when the Mg concentration is just below the photorefractive threshold and the annealing temperature is near 500 degreesC. This temperature is optimal for the most efficient polaron formation at all considered concentrations of Mg. The fitting of the experimental absorption dispersion curves indicates that intermediate polarons are formed in LiNbO3:Mg crystals preferably. The spectral dependence of transmission for samples of lithium niobate of various thicknesses was studied. The results indicate that there are spatial regions with much greater absorption than that of bulk crystals. We assume that, in general, polarons are localized in thin near-surface regions. The spectral dependence of the refractive index in the vicinity of the phonon absorption edge indicates some essential changes of the phonon subsystem taking place after reduction. The infrared contribution into the dispersion of the dielectric function real part increases considerably after reduction.