液態晶體於兆赫波段之元件設計與測量

Abstract

目前兆赫波段之元件相當缺乏，大部分元件有著可調範圍過小及操作溫度過低的缺點。我們已成功的研究出一種利用磁場對液晶作用之可調變的相位延遲器，並進一步製作應用性較高的電控液晶相位延遲器，這些元件皆可在室溫下操作且其調變量皆大於2pi，並以此元件為基礎設計可調波長之二色性濾波器。 另一方面我們使用二維金屬孔洞陣列之異常穿透光特性，在兆赫波段下達到濾波的效果。其狹窄的穿透頻帶及其高的穿透率讓此類研究受到重視，可惜的是此穿透頻率是固定的。於是我們利用旋轉磁場控制液晶分子方向，搭配二維金屬孔洞陣列，成功的製作出兆赫波段下之磁控可調濾鏡。 除了上述與頻率相關之元件外，利用液態晶體具有雙折射特性與參考光學邦加球，我們設計並製作與頻率無關的四分之一波板。此四分之一波板在特定頻率範圍下皆可適用(0.20 到 0.50 THz)且可藉由控制液態晶體，使其可適用範圍改變到0.30到0.70 THz。 上述元件對於兆赫波之實用化及對材料之非破壞性檢測，有很大的助益。

Presently, optical components are short in the terahertz (THz) frequency range. Most THz tunable devices have the problems with small tunable range and cryogenic operating temperature. We have demonstrated a magnetically controlled liquid-crystal (LC) THz phase shifter. In addition, we study and design the electrically controlled LC-based THz phase shifter. Tunable phase shift up to 360° at 1 THz is achieved and can be operated over a broad range near room temperature. These components are used to solve the current lack of the THz birefringent filter. The filter is an important component in optics. It has been shown that a two-dimensional metallic-hole array (2-D MHA) can work as a filter for THz waves. The narrow central frequency and high efficiency of the MHA draws much attention. Previously, we have measured the temperature-dependent refractive indices of LCs (5CB and E7) in THz range. The measured results show that the LCs have large birefringence (~0.2) and small absorption. Here we use a rotary magnetic field to rotate the direction of the LC molecules. Then we combine the characteristics of the MHA to design a magnetically tunable THz filter. Nonetheless, traditional birefringent devices can be used only at a single corresponding frequency, because the phase retardation has strong wavelength dependence. An achromatic tunable THz birefringence quarter-wave plate is demonstrated. The phase retardation of this device is 90° from 0.20 to 0.50 THz and the operation range could be changed from 0.30 to 0.70 THz by using magnetically controlled birefringence in three nematic liquid crystal phase retarders. The above mentioned devices can promote the practicability of THz biological material nondestructive testing.