Felinskyi S. Anomalous dispersion and reflection of the electromagnetic waves in the terahertz band in dielectric crystals

The anomalous dispersion effects due to terahertz oscillations polar crystal on the distribution terms of electromagnetic radiation and material parameters of crystalline media are presented in this work. The analytical solution of the problem about physical conditions for the formation of regions with negative dielectric permittivity (DP) and the problem solution of minimum reflection of terahertz electromagnetic waves in a crystal with a single polar vibration are presented. The main tool for the quantitative determination of areas with negative DP was dispersion analysis of experimental spectra of far-infrared (terahertz) reflection, and Raman spectra in cases of noncentrosymmetric crystals in the band from 1 THz to10 THz (from 30 cm-1 to 300 cm-1). The generalized methods are applied for simulation and calculation of the terahertz anomalies study, particularly the phase portraits of complex functions DP and the refractive index in the negative DP are used. The validity of theoretical relations with respect to the criterion of the existence of areas with negative DP is experimentally proved. The analysis is performed using the spectroscopic data for terahertz reflection at 7 dipole-active vibrations in three crystalline modifications of boron nitride crystal, and the 4 crystal modes in MnF2 at temperature changes from 5K to 300K. The classifying method for the polar vibrational modes by the damping degree in the crystals is proposed, and it is used for the analysis of a total of 34 well-known fundamental modes have previously studied in crystals LiNbO3, LiTaO3 and LiIO3. The frequency region with an abnormally low reflection of terahertz radiation is determined. Based on the two models which are complementary for the resonant interaction of terahertz waves with polar vibrations in crystalline media it is shown that the reflection coefficients R?0,01% in full accordance with the experimental reflection spectra in several crystals.