高對比度且無偏振依賴性之液晶相位光柵元件

Abstract

利用雷射光激發染料摻雜液晶(DDLC)薄膜近期已被廣泛的研究。當染料分子受光激發後，產生光致異構化效應，伴隨偶氮染料的轉動引致液晶分子的旋轉，故DDLC薄膜可應用在液晶相位光柵之製作。本論文的主軸是使用負型液晶摻雜偶氮染料分子Methyl Red (MR) ，搭配垂直配向液晶盒製作高對比度且無偏振依賴性的液晶相位光柵元件。在本研究中使用兩種液晶配向方法:摩擦與光配向。我們探討了不同摩擦配向深度對光配向後液晶錨定方向角的影響，發現經光配向後液晶錨定方向角，取決於有摩擦配向及光配向的錨定力互相競爭而達到的平衡狀態，而其角度會因不同的摩擦配向深度而改變。我們製作的最佳液晶相位光柵配向方法為摩擦配向深度300 μm搭配適當的光配向時間，並量測該液晶相位光柵的對比度及一階繞射強度與電壓的對應關係，我們的實驗架構是偏振入射光方向和摩擦配向方向分別夾角為0°、45°和90°。最後我們將上述之垂直配向DDLC光柵元件和以傳統方式製造水平配向之DDLC光柵元件比較，證明本論文所提出之垂直配向膜所製作的液晶相位光柵元件具高對比度且類無偏振依賴性。

Liquid crystal (LC) alignments by using photo-induced dye-adsorption (PIDA) in dye-doped liquid crystal (DDLC) film have been studied extensively. The conformation of dye molecules such as Methyl Red in a LC mixture could be changed by absorption of an incident laser beam. The precipitation of conformation-changed dye molecules onto the LC-cell substrates would alter the LC-alignment. We call such LC-alignment as the PIDA method. In this thesis, we have used both rubbing alignment (as the first step) and the PIDA method (as a second step) to fabricate our DDLC samples. We have found that the resulted LC-alignment direction was the balance between the strength and direction of rubbing alignment and that of the PIDA method. We have fabricated the best DDLC phase grating by using rubbing alignment with rubbing depth of 300μm and the PIDA method within a range of photo-exposure. We have measured the first-order diffraction efficiency and contrast ratio of the fabricated DDLC phase grating by using an experimental scheme that the polarization of the incident light formed angles of 0°, 45° and 90° with respect to the rubbing direction for LC alignment. For comparison, we have also fabricated and measured DDLC phase gratings by using rubbed polyimide films for parallel alignments to replace vertical alignments. We conclude that DDLC phase gratings fabricated by using rubbed vertical-alignment film followed by the PIDA alignment can achieve polarization-independent and high contrast ratio in the first-order diffraction.