Abstract:The optical constants of a nematic liquid crystal, 4′-n-pentyl-4-cyanobiphenyl (5CB), in the frequency range 0.3–1.4 THz were determined by terahertz (THz) time-domain spectroscopy. The real parts of the extraordinary refractive index ne and the ordinary refractive index no of 5CB varied from 1.74 to 2.04 and from 1.59 to 1.83, respectively. Liquid-crystal 5CB exhibits a relatively small absorption loss in this frequency range. The birefringence of 5CB was found to be as large as 0.21. The experimental results indicate that liquid-crystal 5CB is potentially useful for device applications in the THz frequency range.