Temperature-dependent optical properties of AgGaS2 in the terahertz range

The temperature-dependent optical properties of the AgGaS2 crystal were characterized in the 0.2–2 THz range by terahertz time-domain spectroscopy (TDS). The refractive indices and the absorption coefficients along and orthogonal to the optical axis (c axis) were measured, and the temperature-dependent Sellmeier equations were fitted for various applications of linear and nonlinear optical devices. An E mode vibrating orthogonal to the c axis centering at 1.03 THz (34 cm−1) and a B2 mode along the c axis centering at 1.86 THz (62 cm−1), were observed for the first time. The phonon softening during the temperature rise was discussed and analyzed with the help of Raman spectroscopy. According to the measured absorption spectrum of AgGaS2, we also fitted the parameters of the low-frequency phonon modes at room temperature with the Lorentz dispersion model, which is helpful in predicting the lattice dynamics and optical behaviors of the crystal. •The temperature-dependent optical properties of the AgGaS2 crystal were characterized and the low-frequency phonon modes were observed.•The phonon softening of AgGaS2 was studied based on absorption and Raman spectroscopy.•The dielectric properties of AgGaS2 were fitted by the Lorentz model.