Negative dielectric permittivity of nonmagnetic crystals in the terahertz waveband

Abstract

Physical conditions for occurrence of the spectral bands with the negative dielectric permittivity in nonmagnetic crystalline media in the terahertz waveband have been studied in this work. It has been shown that damping the polar vibrations has a primary effect on formation of the negative dielectric permittivity in real crystals at resonance (terahertz) frequencies, and phonon attenuation imposes significant restrictions both on the frequency range and the minimum achievable value for all dielectric tensor components. Within frameworks of the single-oscillator model, the authors have obtained: i) the criterion for the existence of the negative dielectric permittivity, which is based on physical and spectroscopic parameters of the crystal, ii) analytical expressions for calculation of the frequency band where the dielectric permittivity takes negative values. Frequency regions and the minimum value of negative dielectric permittivity are quantitatively defined in the crystal LiTaO₃. It is proved the applicability of the obtained relationships in cases of relatively complex phonon vibration spectra.