ИСТИНА

We have measured and analyzed the THz-wave dispersion parameters of bulk ferroelectric crystals Sn2P2S6, LiNbO3, and Mg:LiNbO3. The extraordinary dielectric function real and imaginary parts are determined for bulk LiNbO3 and Mg:LiNbO3 crystals in the wide spectral range of the lowest polariton branch using different methods of passive linear THz spectroscopy. At frequencies 0.1– 2.5 THz measurements were made by the method of pulsed terahertz time-domain spectroscopy (TDS), using two femtosecond laser-based schemes for generation and detection of the THz radiation. Advantages of each scheme are compared and analyzed. At higher frequencies up to 5.5 THz, the dielectric function real part is determined using a scheme of near-forward Raman scattering by phonon polaritons. A special approach is applied for measuring absorption coefficients at frequencies 1–3 THz, based on analysis of visibility of three-wave second-order interference under spontaneous parametric down-conversion. Imposing all the experimental data, for each crystal we determined the generalized approximate expression for dispersion of the complex dielectric function within the whole lower polariton branch. It is shown that the 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. Using the TDS method we studied evolution of the refractive index and the dielectric function dispersion of Sn2P2S6 crystal in the frequency range 0.1 - 0.4 THz at temperatures before and after the ferroelectric phase transition at the Curie temperature TC = 640С. For the waves polarized orthogonally to the vibrations of the soft mode, a break in the dependence of the refractive index near the phase-transition temperature was observed. It is shown that the imaginary part of the dielectric constant in this frequency range is well approximated by means of three vibration resonances. Dependence of the parameters of these excitations on temperature in the vicinity of the phase transition is analyzed. The ultrafast dynamics of the phonon excitation at the frequency of ferroelectric mode in Sn2P2S6 has been investigated also using the two-color pump–probe active spectroscopy scheme. Results, obtained for different temperatures of the crystal, before and after TC, do not show effect of frequency softening.