Optical properties and potential applications of ɛ-GaSe at terahertz frequencies

Ordinary and extraordinary refractive indices of an ɛ-GaSe crystal at terahertz (THz) frequencies are experimentally determined in this study. Fitting experimental data by THz time-domain spectroscopy (THz-TDS) and a Fourier transform infrared spectrometer (FTIR), we proposed revised complex dielectric functions and Sellmeier equations of GaSe for both ordinary and extraordinary waves from 0.2 to 100 THz. Phonon vibrational modes and overtones in the THz frequency range are examined in detail. The high magnitude of the figure of merit (FOM~103 at 1 THz), the large birefringence (Δn~0.76 at 1 THz). and the low absorption coefficient (α~0.2 cm−1 at 1 THz) of GaSe are also identified. Potential applications to practical photonic devices such as phase shifters at THz frequencies are proposed.

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