三穩態雙頻膽固醇液晶元件之光電特性及其應用

Abstract

本論文以雙頻液晶為材料摻雜手性分子（R5011），提出一個新型的電控三穩態雙頻膽固醇液晶元件和研究其驅動機制，使三種穩態紋理（平面態、焦錐態、穩定橫向螺旋態）中任意兩種穩態做直接、可逆切換。首先由介電頻譜術獲得兩個決定性頻率（f1、f2），實驗結果證實只要在特定電壓下施加一個頻率（f1）低於鬆弛頻率（fR）就能誘導電流體效應，讓液晶盒從平面態切換到焦錐態或穩定橫向螺旋態；同樣地，施加一個頻率（f2）高於雙頻液晶的反轉頻率（fc），讓介電各向異性從正值反轉到負值，液晶盒就能從焦錐態或穩定橫向螺旋態切換為平面態。本實驗結果展示兩種切換三穩態模式，第一種模式為固定低頻（f1）調控電壓達成三穩態，再升至頻率（f2）讓液晶盒回到平面態；第二種模式為固定特定電壓，僅調控頻率達成三穩態。吾人進一步以第二種模式切換焦錐態(散射態)和穩定橫向螺旋態(穿透態)之灰階調控光譜特性。 此外，吾人研究不同螺距之雙頻膽固醇液晶元件光電特性，證實從平面態以電流體效應誘導形成穩定橫向螺旋態之反應時間長，且長螺距之穩定橫向螺旋態容易因為自由能和配向關係切換為平面態。因此吾人進一步針對長螺距之穩定橫向螺旋態作光電特性改善，摻雜少量濃度高分子進行光固化穩定紋理，成功改善穩定性和反應時間。最後本研究提出平面態和穩定橫向螺旋態的直接、可逆切換應用，搭配正交偏振片，實現可調式波長開關和光開關。

This thesis demonstrate an electrically switchable tristable-dual-frequency cholesteric liquid crystal (DFCLC) devices based on dual-frequency materials doped with chiral additives. We report on a driving scheme, enabling the direct, and reversible switching between any two of the three stable states—planar (P), focal conic (FC), and uniformly lying helix (ULH) . In this study, two decisive frequencies designated f1 and f2 are obtained by means of dielectric spectroscopy. According to the mechanism of the voltage-induced ULH state and the material properties of the DFLC, it is suggested that the frequency f1 should be low enough to induce the electrohydrodynamic instability. Accordingly, we further demonstrate that the switching from the P state to either the FC or ULH state can readily be carried out by applying a voltage pulse at f1 to the DFCLC cell. Likewise, it is suggested that the frequency f2 should be higher than the crossover frequency of the DFLC to modulate the dielectric anisotropy from a positive to a negative value. The directly backward switching from the FC or ULH state to the P state can thus be achieved by regulating the frequency of the applied voltage pulse from f1 to f2. As the results of spectral characteristics, the DFCLC cell can be tuned into multiple gray levels as stable states. In addition, we examine the electro-optical properties and applications of tristable DFCLC devices based on different pitches. We found that the response time from P to ULH state is relatively long because of the electrohydrodynamic effect. On the other hand, the ULH state of long pitch DFCLC is instability due to the free energy and the alignment of cell. Therefore, we overcome these problems by incorporating a polymer network by means of photopolymerization of the small concentration of reactive monomers added to the DFCLC. Furthermore, applications of the proposed DFCLC as a light shutter and a switchable waveplate under the crossed polarizer are suggested in accordance with the feature of direct two-way switching between the P and ULH states.