Influence of small polarons on the optical properties of Mg : LiNbO3 crystals - art. no. 054304

摘要

We have measured refractive indices and absorption coefficients for Mg:LiNbO3 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 mum 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 mum, which are not observed in the pure LiNbO3. The theory approximation of the band at 1.3 mum, enhanced after chemical reduction, enabled us to determine the main polaron parameters for the 1.3-mum band, and to make a possible assignment of the 3.2 mum band. We assign this to direct transitions between energy levels of polarons trapped at Nb-Nb sites and those of polarons trapped at Nb-Li antisites in a lattice. A number of peculiarities are revealed in the absorption behavior at the low-energy edge from 1000 to 3000 cm(-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. [References: 43]