Influence of small polarons on the optical properties ofMg:LiNbO3crystals

Abstract

We have measured refractive indices and absorption coefficients for ${\mathrm{M}\mathrm{g}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}$ single crystals of different Mg contents (0--7 mol %) in the visible and IR ranges up to the phonon absorption edge. The obtained dependencies of optical characteristics on Mg concentration in an IR region of 4--5 \ensuremath{\mu}m indicate the four-step character of structural changes under subsequent Mg doping. Spectral dependencies of optical characteristics reveal the existence of two polaron resonance bands, centered at 1.3 and 3.2 \ensuremath{\mu}m, which are not observed in the pure ${\mathrm{LiNbO}}_{3}.$ The theory approximation of the band at 1.3 \ensuremath{\mu}m, enhanced after chemical reduction, enabled us to determine the main polaron parameters for the 1.3-\ensuremath{\mu}m band, and to make a possible assignment of the 3.2 \ensuremath{\mu}m band. We assign this to direct transitions between energy levels of polarons trapped at ${\mathrm{Nb}}_{\mathrm{Nb}}$ sites and those of polarons trapped at ${\mathrm{Nb}}_{\mathrm{Li}}$ antisites in a lattice. A number of peculiarities are revealed in the absorption behavior at the low-energy edge from 1000 to 3000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The values of the uninduced part of absorption are sufficiently less than those predicted by a phonon oscillator model. Absorption spectral dependencies are modulated at a rather stable period, being of the order of frequencies of the lowest-energy longitudinal optical phonons, excited at a room temperature.