Complex extraordinary dielectric function of Mg-doped lithium niobate crystals at terahertz frequencies
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We study the dispersion of the extraordinary dielectric function real and imaginary parts in the wide terahertz-frequency range of the lowest polariton branch for bulk LiNbO3 and Mg:LiNbO3 crystals. At frequencies 0.1–2.5 THz, both dispersion parts are measured by means of standard time-domain terahertz spectroscopy, and at higher frequencies up to 5.5 THz, the dielectric function real part is determined using a common scheme of spontaneous parametric down-conversion under near-forward Raman scattering by phonon polaritons. A special approach is applied for measuring of the dielectric function imaginary part at frequencies 1–3 THz, based on the analysis of visibility of three-wave second-order interference under spontaneous parametric down-conversion. The generalized approximate expressions are obtained for complex dielectric function dispersion within the lower polariton branches of LiNbO3 and Mg:LiNbO3. It is shown that the well-known decrease in terahertz-wave absorption of lithium niobate crystals under Mg-doping is caused by changes in the defect structure and reduction of coupling of the terahertz-frequency polaritons with Debye relaxational mode.
KeywordsLiNbO3 Dielectric Function Lithium Niobate Defect Mode Lithium Niobate Crystal
The authors are grateful to Dr. M.V. Chekhova, V.V. Kornienko, K.A. Grishunin, and Dr. D.V. Lopaev for the discussions and some helpful remarks. This study was supported by the Russian Foundation for Basic Research (Project Nos. 14-22-02091, 16-02-00258, 16-29-03294) and by Ministry of Education and Science of the Russian Federation (State Task No. 11.144.2014/K).
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