液態晶體於兆赫波段之光學性質與應用

Abstract

兆赫波時域光譜探測是一個研究物質在兆赫波段光學性質很有用的一種方法。它可以同時提供電磁波通過待測物質後之電場相位及振幅的資訊。我們利用這個方法量測兩種典型的向列型液晶（5CB和E7）在兆赫波段下之折射係數。實驗結果顯示這兩種液晶在兆赫波段下仍保有正的雙折射現象（∼0.2）並且吸收係數不大（∼0.02）。根據這樣的結果，我們成功地設計並製作了電控及磁控之液晶可調兆赫波相位延遲器。我們利用了三明治結構的液晶盒及磁控在1THz下達到超過360度的相位連續可調範圍。這對於可調兆赫波相位延遲器來說是一個里程碑。有了可調相移器的經驗，我們更進一步的將之用於可調Lyot濾波器。在Lyot濾波器中，我們利用了一個固定的相移器及一個可調的相移器組成一個相位延遲單位以確保所有的相位可調範圍都被用於調變可通過的頻率。其通過兆赫波之中心波長可以從0.388THz被連續調到0.564THz（~40%）。

The complex refractive indices of some typical nematic liquid crystal (LCs), 5CB and E7, in the THz frequency range have been determined by using the terahertz (THz) time-domain spectroscopy. For 5CB, no~1.58 and ne~1.78, and the birefringence is as high as 0.20 throughout this frequency range. Using magnetically controlled birefringence in 5CB and E7, the room temperature liquid crystal THz phase shifters/retarders have been demonstrated successfully. A phase shift over 2□ at 1 THz is achieved by employing a sandwiched cell structure. Using the tunable phase retarder, we also demonstrated the first THz tunable Lyot filter to our knowledge. The central passband frequency of the filter can be continuously tuned from 0.388 THz to 0.564 THz (a fractional tuning range of 40%).