The optical constants of liquid crystals (LCs) are of fundamental interests and essential for liquid-crystal-based applications.[1] We have previously reported the optical constants of several LCs (5CB, PCH-5, and E7) in THz frequency range.[1,2] The ordinary and extraordinary indices (no and ne) of 5CB, for example, are 1.55 - 1.75 and 1.70 - 1.96, respectively, from 0.2 - 0.8 THz at room temperature In this work, we report the accurate complex optical constants of LC, 5CB, in the frequency of 0.2 - 1 THz as a function of temperature by using a photoconductive-antenna-based THz time-domain spectrometer. Two cells, LC cell and reference cell, are inserted in turn in the collimated THz beam path. The spectrometer is purged with dry N2 to ensure the relative humidity is < 5% during the experiment. The LC cell, which consists of two fused silica windows separated by Mylar (250 micron), is filled with 4'-n-pentyl-4-cyanobiphenyl (5CB, Merck). The polyimide is spin coated on the inner surfaces of two substrates, followed by baking and rubbing processes in order to obtain low pretilt (<1 degree) homogeneous LC alignment. The reference cell consists of two fused silica windows in contact with each other with the same total thickness to it of LC cell. The ordinary and extraordinary indices of 5CB were measured by linear polarized THz wave for which polarization is perpendicular and parallel to the aligned direction of LC molecular, respectively. A numerical method was used to extract the indices by comparing the signals for reference cell and LC cell. The frequency dependent and temperature dependent complex indices will be reported. The no and ne of 5CB are determined to be 1.577 - 1.597 and 1.762 - 1.786 in frequency range of 0.2-1 THz at room temperarure, respectively. The temperature dependent no and ne at 0.50 THz are shown in Fig. 1. The birefringence of 5CB at 0.50 THz can be obtained from ne-no which is from 0.205 to 0.143 from 25 - 33 degree C (Nematic phase). The temperature dependent birefringence also provides the information on order parameter of 5CB that agrees well with birefringence in visible range. The environmental humidity during the measurement affects significantly the accuracy of the optical constants. In summary, the temperature-dependent optical constants from 0.2 to 1 THz of 5CB are presented. The birefringence of 5CB is about 0.21 - 0.14 in nematic phase. The extinction coefficient of 5CB is small (<0.02). The order parameter of aligned 5CB from the temperature dependent birefringence agrees with that in visible range.

[1] R.P. Pan, Y.P. Lan, C.Y. Chen, and C.L. Pan, Liquid Crystal Element for Fine Tuning of Laser Wavelength, Mol. Cryst. Liq. Cryst., 413, 499, 2004. [2] C.Y. Chen, C.F. Hsieh, Y.F. Lin, R.P. Pan, and C.L. Pan, Magnetically Tunable Room-Temperature 2 Liquid Crystal Terahertz Phase Shifter, Optics Express, 12, 2630, 2004. Figure 1. The temperature dependent refractive indices of 5CB at 0.50 THz.