Influence of structural defects on the optical properties of Mg:LiNbO3 single crystals

A comparative analysis of the dependences of the refractive index of Mg:LiNbO₃ single crystals on the Mg concentration is performed in the visible and IR transparency regions. The results of measurements at the wavelengths above 4 μm suggest the four-step mechanism of incorporation of the Mg impurity to the LiNbO₃ crystal in the range of molar concentrations of Mg from 0 to 7.1%. The resonances observed between 1 and 2 μm and between 2.8 and 3.5 μm in the transparency region are interpreted as contributions from polaron states. It is shown that the resonance in the region between 1 and 2 μm is strongly enhanced in crystals with the molar concentration of Mg above 5.1%.