膽固醇液晶是僅次於向列型液晶在產業應用方面(例如膽固醇液晶電子紙)最廣泛的液晶種類，這是由於膽固醇液晶具備於多樣態(包含會產生選擇性布拉格反射之平面態、會產生散射之焦錐態與穿透式之垂直態)之間的可電控性，因此可電控膽固醇液晶一直是液晶領域的重要研究課題。然而傳統膽固醇液晶之平面態於外加電場作用下容易被破壞而無法進行反射波段有效連續調控，近年來傾斜螺旋膽固醇液晶的開發解決了此問題，此類膽固醇液晶在電場作用下展現可於超寬廣頻譜範圍內有效行布拉格反射波帶之連續調控能力，過去的研究文獻發現利用於膽固醇液晶裡加入液晶二聚體後，在外加特定電場作用下，形成傾斜螺旋結構，這是由於液晶二聚體具有特殊的彈性常數特性(K33 < K22)所致。傾斜螺旋膽固醇液晶之週期結構具備超廣波長範圍可連續電調控特性，因此在光學通信、量子光學和光電子學等領域具有相當之應用潛力。
本論文題目為『基於分子馬達之傾斜螺旋型膽固醇液晶製備可光電雙重調控與手性反轉之平面光學研究與應用』，研究動機乃為了追求傾斜螺旋膽固醇液晶具備更完善的外部可操控性，藉由結合傾斜螺旋膽固醇液晶之可電調控性與分子馬達之可光控旋性，製備可光電雙控性之傾斜螺旋膽固醇液晶。本研究除了研究傾斜螺旋膽固醇液晶之可光電雙控性機制外，進一步利用由λ/4波片與空間光調制器組成可產生任意線偏振分布之微圖案曝光系統，製備可超廣波長範圍之可光電調控反射波段與旋性反轉之幾何相位繞射元件。
本論文研究所使用的材料固定為摻雜液晶二聚體與手性分子馬達之膽固醇液晶，研究內容分成三部分，第一部分為透過改變交流電場大小，量測電致傾斜螺旋膽固醇樣品之反射頻譜變化與紀錄對應之偏光顯微影像。實驗結果發現，電致傾斜螺旋結構之螺距及傾角會隨著外加電場強度的降低而增加，這使得樣品反射峰在如此電控下可從藍光區位移至紅外光區(450 nm–1400 nm)。
實驗第二部分，在固定電場強度下，以紫外光照射電致傾斜螺旋膽固醇液晶之過程，量測反射頻譜與旋性之動態變化。實驗結果發現，在持續照射紫外光下，分子馬達啟動光致異構化機制而引致材料手性反轉。由於電致傾斜螺旋結構的產生需在材料具有足夠手性強度下才能發生，因此分子馬達的光致異構化效應造成材料的手性強度變化會促使樣品之反射峰先衰降消失，再於較長波長處出現增高與反旋；前者乃是因為光致異構化引起左手分子馬達濃度降低而右手分子馬達濃度升高，越發增強的外消旋效應使材料整體左手手性強度不足而引起左旋傾斜螺旋結構崩解；後者乃因為持續照光之下使右手分子馬達濃度持續上升且超過持續降低的左手分子馬達濃度，反旋後的外消旋效應持續衰弱而使右手手性強度足以形成右旋傾斜螺旋結構。上述傾斜螺旋膽固醇液晶在光致反旋後的反射峰位置與反旋前相比紅移了幾十奈米(約70–100 nm，取決於初始反射峰之波長位置)，本論文進一步透過幾個附加實驗結果與參考相關文獻和理論提出定性解釋。實驗結果也發現在持續照射紫外光下，反旋後的反射波段仍可維持穩定，因此結合第一部分電場調控的量測，使反旋後的反射頻譜亦能夠從藍光區位移至紅外光區(450 nm–1500 nm)。
第三部分乃利用前兩部分之研究成果應用於製備三種幾何相位繞射元件，包括兩種可產生渦流光之繞射光柵元件與可產生複雜圖案之全像繞射元件，實驗中利用藍(488 nm)、綠(532 nm)、紅(633 nm)三原色雷射光與柱面透鏡驗證帶有渦流特性之繞射元件產生之反射繞射光輸出特性與光電場的調控特性。實驗結果證實傾斜螺旋膽固醇液晶所製備之繞射光柵元件具備光電雙重調控特性與旋性可切換性。

This thesis is entitled “Optoelectronically tunable and chirality-invertible flat optics using molecular-motor-based heliconical cholesteric liquid crystals.” The research motivation is to pursue better external controllability of heliconical cholesteric liquid crystals (CLCs). By combining the electrical controllability of the heliconical CLC with optically controllable helical conversion of molecular motors, a heliconical CLC with optoelectro-controllability is prepared. In addition to studying the optoelectronically controllable mechanism of heliconical CLC, this study further utilizes a micropatterned photoalignment system composed of λ/4 waveplates and a spatial light modulator (SLM) to produce arbitrary linear polarization pattern for preparation of geometric phase diffraction elements (GPDEs). These GPDEs can be electrically tuned in position of PBG continuously in an ultrawide spectral range and be optically reversed in circular handedness and/or sign of topological charges.
The CLC materials used in this study include nematic liquid crystals (LCs), LC dimers, and chiral molecular motors. The first part of the experiment is to study the electrical tunability of the heliconical CLC sample. The reflection spectra and polarized optical microscope (POM) images of the heliconical CLC samples are measured at different magnitudes of the applied AC electric field (3 kHz). As the applied electric field strength decreases, both the pitch and the tilt angle of the helical structure increase. Therefore, under such electrical control, the PBG of the sample can be moved from the blue region to the near-infrared (NIR) region (450–1400 nm). Similar electrical tunability of PBG can also be achieved in the same sample after UV-irradiation-induced chirality inversion.
The second part of the experiment is to study the optical switchability of the heliconical CLC sample. The dynamic changes in reflection spectrum and circular handedness of the heliconical CLC are measured during the UV irradiation at a fixed electric field magnitude. The PBG position of the heliconical CLC can be optically switched between two different color regions (e.g., blue and green regions and green and red regions) and different circular handedness. Further experiments verify that the optical switching properties of the sample are attributed to the pitch change and chirality inversion of the sample, which is achieved through trans-cis photoisomerization induced by UV irradiation of the molecular motor molecules.
The third part focuses on the fabrication and investigation of three GPDEs, including two fork grating elements that can produce optical vortices and a holographic diffraction element that can produce complex image patterns. One blue (488 nm), green (532 nm), and red (633 nm) laser sources and a cylindrical lens are used to verify the output characteristics of far-field diffracted light with optical vortex characteristics and the control characteristics under electric and optical fields. Experimental results confirm that the diffraction grating element prepared by heliconical CLCs has photoelectric control characteristics and helical switchability.

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