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Optical properties of LiInSe 2 in the THz frequency regime

We investigate the optical properties of LiInSe2 (LISe) by THz time-domain spectroscopy (THz-TDS). The experimental results indicate that LISe has excellent transparency in the THz frequency region. We determine quantitatively the birefringence of LISe. The refractive index and absorption coefficient along the three dielectric axes of LISe are reported in the THz frequency region. The lattice vibrations of LISe are analyzed on the basis of the THz-TDS measurement in combination with infrared and Raman spectra. The origin of the low-frequency vibrational modes below 3 THz as interatomic bond-bending modes is revealed.