The complex refractive indices of the liquid crystal mixture E7 in the terahertzfrequencyrange

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We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefrin-gence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases (26°C–70°C).The ex-tinction coefficient of E7 at room temperature is less　than　0.035　and　without　sharp　absorption　features　in　the　frequency　range　of　0.2–2.0　THz.　The　extraordinary　(ne)and ordinary (no)indices of refraction at 26°C are 1.690–1.704 and 1.557–1.581, respectively, giving rise to a birefringence of 0.130–0.148 in this frequency range. The temperature-dependent(26°C–70°C)order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared.