向列型液晶E7在不同溫度下的光學常數之時析兆赫光譜研究

Abstract

兆赫波時域光譜技術是一種研究物質在兆赫波段光學性質的 方法。本論文是利用兆赫波時域光譜技術來研究一種已經被廣 泛運用在可見光波段的混合性向列型液晶E7，其在向列相和均相態 時，在不同溫度(從攝氏26度至攝氏70度)以及頻率範圍(0.2 THz ~ 1.4 THz)的光學常數。實驗結果顯示此種液晶在兆赫波段下仍保有 正的複折射現象，而且複數折射率的虛數部分皆小於0.04，也沒有明顯的吸收譜線。在攝氏26度時，E7的長軸方向與短軸方向折射率分別約為1.71與1.57，即其雙折射約為0.14。另外，我們從雙折射數據中所得到的液晶有序參數資訊也與可見光波段者吻合，由此更確定了此次量測的準確性。另一方面，本研究也成功地驗證液晶折射率溫度相依的extended Cauchy equations。本研究成果也將會是液晶元件日後在遠紅外區波段各種應用的重要依據。

We have used terahertz (THz) time-domain spectroscopy to investigate the frequency dependence and temperature dependence of the optical constants of a widely used liquid crystal mixture E7 in both nematic and isotropic phases. The extinction coefficient of E7 at room temperature is less than 0.04 and without sharp absorption features in the frequency range of 0.2~1.4 THz. The extraordinary and ordinary indices of refraction at 26□C are around 1.71 and 1.57, respectively, giving rise to a birefringence of about 0.14 in this frequency range. The temperature-dependent order parameter extracted from birefringence data agrees with values in the visible region quite well. On the other hand, the extended Cauchy equations describing the temperature dependence of refractive indices of liquid crystal is also confirmed in this study.