以曲電與介電效應產生之均勻橫向螺紋態的配向機制與電光響應

Aligning Mechanism and Electro-Optical Responses of the Uniform Lying Helix State Based on the Flexoelectric and Dielectric Effects

林岳辰(Yueh-Chern Lin)

指導教授 ： 李偉

國立交通大學/光電學院/照明與能源光電研究所/碩士(2019年)

摘要

本研究於正型的棒狀膽固醇液晶中摻雜適量的彎曲型液晶分子CB7CB並成功的增加了樣品的曲電特性，筆者以藉此探討其在不同電場條件下的紋理形成與電光響應。在頻率f < fULH的範圍中，我們可以透過對樣品施加合適大小的電壓來誘導出介電效應與強烈的曲電效應，並藉此形成無缺陷的均勻橫向螺紋態。當外加交流電場頻率介於fULH與fFC之間時，曲電效應的大小隨著頻率的增加而逐漸降低，經降電壓處理後所得到的紋理為均勻橫向螺紋態與焦錐態的混合。而若是交流電壓的操作頻率大於fFC，則所形成的紋理皆為低散射的焦錐態，此時樣品中的曲電效應也幾乎完全消失。值得注意的是，若將此均勻橫向螺紋態擺放於正交偏振片間，且設置其光軸平行於任一偏振片的穿透軸時，我們的實驗結果證實固定電壓振幅大小並調整其頻率便可在不改變相位的情況下達到光強度的調控。這再次驗證了曲電效應的強度具有明顯的頻率依存性，而介電效應的大小卻幾乎不受操作頻率影響。總而言之，摻雜CB7CB的膽固醇液晶在均勻橫向螺紋態中呈現了可觀並頻率相依的曲電響應，而此特徵卻幾乎無法在典型的膽固醇液晶中被觀察到，因此本研究也為著次世代快速應答顯示器的開發提供了莫大的幫助。

關鍵字

均勻橫向螺紋態 ； 彎曲型液晶 ； 曲電效應 ； 介電效應

並列摘要

In this study, we attempt to promote the strength of the flexoelectric response by incorporating a proper amount of bent-core liquid crystal (LC) dimer CB7CB into rod-like cholesteric liquid crystal (CLC) with positive dielectric anisotropy, and investigate the voltage-dependence of electro-optical responses and the formation of various textures. With suitable voltages, we evidence that defect-free uniform lying helix (ULH) can be obtained, based on the dielectric and strong flexoelectric effects, under the frequency f conditions of f < fULH. For frequencies in the regime between fULH and fFC, the strength of the flexoelectric effect get decreased with rising frequency and the coexistence of both focal conic (FC) and ULH is generated, whereas for the cell treated with the voltage at f > fFC, the flexoelectric effect almost disappears completely and the resulting texture is FC. Attractively, when placing the ULH cell between crossed polarizers, with the helical axis being parallel to the transmission axis of either linear polarizer, our study demonstrates that the light intensity can be controlled without phase change by adjusting the frequency of AC voltage at a fix amplitude. This indicates that the strength of the flexoelectric effect can be modified by the frequency, but in the meantime the dielectric effect is nearly invariant. In consequence, CB7CB-doped CLC in the ULH state exhibits considerable and frequency-dependent flexoelectro-optical responses which can scarcely be acquired in typical rod-like CLCs, thus this study promotes a giant potentiality for practical application of ULH for fast-response LC devices.

並列關鍵字

Uniform lying helix ； Bent-core liquid crystal ； Flexoelectric effect ； Dielectric effect

參考文獻

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