光切換與光可調式三穩態手性斜垂向列液晶元件

Abstract

本篇論文提出一個新型的穩態手性斜垂向列液晶之切換機制，以摻雜光敏偶氮旋性物質作為手性分子，並利用其可逆的光致同素異構反應發展出新型的三穩態系統。過去在電控雙穩態手性斜垂向列液晶的研究中，皆必須使用少見的雙頻液晶作為液晶主體，且在製程上也須精確地控制配向劑混合的濃度、配向的參數、液晶盒厚與螺距比值參數和預傾角度等。相反地，本研究之三穩態切換機制不僅只需使用一般向列液晶（如E7）即可外，也大幅度降低了製程的難度，使穩態間能夠直接切換，並使光學穩態由二提高到三穩態，故增加了其未來應用和發展性。本研究也將此三穩態系統摻入二色性染料作為無偏振片之節能型光調制或開關元件，並利用此三穩態結構當作一維光子晶體之缺陷層，實現光強調控和波長調控之元件。且實驗結果展示了此三穩態結構在摻雜二色性染料之後，穩態切換灰階調控的光譜特性及實體樣品圖，並透過相位補償的概念提升其對比度至12:1。另一方面，在一維光子晶體中藉由改變入射光的偏光方向或以光調控形成多數穩態的方式，可展現作為光通道的缺陷峰之強度和波長可調控性。此外，利用指紋螺旋態可有效抑制混合模態，使其中心波長附近的缺陷峰穿透率降至1%以下，故相較於傳統的雙穩態斜垂向列液晶，本研究所採用的缺陷層系統，可拓寬20%的穿透強度調控範圍。

This thesis demonstrates a new photo-induced tristable chiral-tilted homeotropic nematic liquid cyrstals (T-CTHN) based on the trans–cis isomerization of the azo-chiral materials. The antecedent device, biased bistable chiral-tilted homeotropic nematic liquid crystals (B-CTHN), requires the atypical dual-frequency liquid crystal as LC host and a high degree of precision in fabrication including polyimide (PI) mixture concentration, rubbing parameters, cell gap over pitch ratio (d/p) and the pretilt angle. As advancements, the tristable switching mechanism can not only apply to some more readily available nematic materials (e.g. E7) but also reduces the complexity of the fabrication process in all mentioned aspects. Furthermore, this mechanism makes the stable states switch directly as well as widens states from bistable toward tristable, hence broadening the applications of the T-CTHN devices prospectively. Some other variations such as dye-doped T-CTHN as a polarizer-free energy-conservative light modulator or a shutter device, and T-CTHN photonic crystal (T-CTHN–PC)—in which T-CTHN as a defect layer in one-dimentional PC multilayers—as controllable attenuator or wavelength selector have been demonstrated. As the results of spectral characteristics and the prototype photos, the dichromatic-dye-doped T-CTHN can be tuned into multiple gray levels as stable states, and the contrast ratio (CR) was improved to 12:1 with the augmentation of a phase-compensating homogeneous cell. On the other hand, the defect modes in T-CTHN–PC can be as light channels—tunability of transmittance and wavelengths which depend on the incident polarization director or photo-manipulated multiple stable states. Besides, with the fingerprint state (FP) by which the mixed-mode (M-mode) is effectively suppressed, and the defect peak transmittance nearby the central wavelength recedes down to less than 1%, broadens the tunable range by 20% than B-CTHN–PC.