Dispersion of the dielectric function real part for Mg:LiNbO3 crystals at terahertz frequencies

We study dispersion of the dielectric function real part epsilon' in the terahertz-frequency range for bulk and periodically poled congruent LiNbO3 and Mg:LiNbO3 crystals. The concentration of Mg in Mg:LiNbO3 samples was close to 5 mol%, which is the photorefractive threshold. Approximate expressions for extraordinary polariton dispersion dependence were obtained in the range 0.5-6.5 THz. The influence of Mg-dopant on the optical properties of crystals in the terahertz range is revealed. Changes of the defect structure of lithium niobate crystals are discussed.