Terahertz time-domain spectroscopy of nematic liquid crystals

Abstract

Liquid crystals are candidate materials for optical devices operating in the Terahertz (THz) frequency region of the electromagnetic spectrum. Proposed devices include THz phase shifters and THz quarter wave plates. To assist in designing for these applications, the fundamental properties of the materials should be determined. Fundamental optical properties to be determined over the frequency range of interest are the refractive index n and the absorption coefficient α. According to the orientation of the liquid crystals relative to the polarisation of the light field, ordinary and extraordinary values for the refractive index may be distinguished. In early work, employing time-domain spectroscopy, a rise in both no and ne with optical frequency in the THz region was reported. Later work, employing two-colour generation of THz radiation, indicated the values of no and ne were both relatively constant in the THz region. We have now made measurements of the two common nematic liquid crystals K15 and E7 using time-domain THz spectroscopy and confirm that no and ne show little change over the spectral region 0.15 to 1 THz.