利用電控膽固醇液晶傾斜螺旋態結構調制偏振體積光柵之光學特性。;Electrically controllable optical properties of polarization volume gratings based on the oblique helicoidal state of chiral nematics

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/92383

Title:	利用電控膽固醇液晶傾斜螺旋態結構調制偏振體積光柵之光學特性。;Electrically controllable optical properties of polarization volume gratings based on the oblique helicoidal state of chiral nematics
Authors:	曾禹翔;Tseng, Yu-Shiang
Contributors:	光電科學與工程學系
Keywords:	液晶;liquid crystal
Date:	2023-08-11
Issue Date:	2024-09-19 15:48:39 (UTC+8)
Publisher:	國立中央大學
Abstract:	本論文研究分為三大主題，第一主題是水平配向之傾斜螺旋態膽固醇液晶於不同溫度下的反射頻譜變化，以及其結構變化與操作電壓變化。在此部分研究中，固定材料比例、液晶盒厚度和配向膜種類，僅改變溫度一參數，溫度對於傾斜螺旋態膽固醇液晶的特性影響顯著。產生傾斜螺旋態結構需添加液晶二聚體，此可降低整體液晶混合物的彎曲彈性係數(K33)，而改變溫度(升溫)會使其彎曲彈性係數(K33)上升，使傾斜螺旋態的特性越不明顯。最後透過4×4貝里曼矩陣計算反射頻譜之理論值並與實驗量測到的反射頻譜進行擬合，最後透過擬合得到其螺旋週期以及錐角。第二主題為偏振體積光柵之傾斜角、繞射角、繞射效率及史托克參數(S3)量測，探討相同x方向週期，並改變膽固醇液晶的螺距使其傾斜角改變，進而改變繞射角；最後探討Q參數對於布拉格光柵偏振選擇性影響。第三主題為前兩主題的結合，製作傾斜態膽固醇液晶之偏振體積光柵(Oblique Helicoidal State of Polarization Volume Grating, OHPVGs)，預計透過傾斜螺旋態膽固醇液晶可電控調制螺距長度的特性製作出可電控調制繞射角之光柵，但實驗上卻無此現象，本論文對此現象的推測為Q值太小，故繞射角與螺距長度及傾斜角無關，只與波長及光柵週期及液晶折射率相依。此外，本論文研究仍有發現OHPVGs可解決施加電場下之水平配向之傾斜螺旋態膽固醇液晶反射頻譜中短波長上升之問題，本論文第六章提出解決辦法以及未來可繼續研究之方向。;The research topic in this thesis is divided into three main parts. The first topic focuses on the reflection spectra and structural changes of homogeneous alignment of oblique helicoidal state of chiral nematics at different temperatures, as well as the variations in the operating voltage. In this study, the concentration of the adopted materials, liquid crystal (LC) cell gap, and alignment films are kept invariant, while only the temperature parameter is varied. The temperature has a significant impact on the characteristics of the oblique helicoidal state of chiral nematics. The formation of an oblique helicoidal state requires the addition of LC dimers, which reduces the overall bend elastic constant (K33) of the LC mixture. The bend elastic constant (K33) increases with the temperature, making the characteristics of the oblique helicoidal state less prominent. The Berreman 4×4 matrix is used to fit the measured reflection spectra, and the pitch length and cone angle are obtained through the fitting process. The second topic involves the measurements of the slanted angle, diffraction angle, diffraction efficiency, and Stokes parameter (S3) of the obtained polarization volume gratings (PVGs). It explores the variations of the slanted angle by adjusting the pitch length of chiral nematics with the same period along the x-directional, which in turn affects the diffraction angle. Finally, the Q parameter′s impact on Bragg volume gratings′ polarization selectivity is examined. The third topic involves the combination of the first two topics, aiming to fabricate Oblique Helicoidal State of Polarization Volume Gratings (OHPVGs). The goal is to achieve electrically controllable modulation of the grating′s diffraction angle by manipulating the pitch length through the use of oblique helicoidal state of chiral nematics. However, the experimental results did not show this phenomenon. The thesis speculates that the small Q value prevents the diffraction angle from being related to the pitch length and slanted angle, and instead, it depends on the wavelength and the grating′s periodicity and refractive index of LCs. In addition, this study also found that OHPVGs can address the issue of a short-wavelength increase in the reflection spectrum of homogeneous alignment of the oblique helicoidal state of chiral nematics under an applied electric field. Chapter 6 of this thesis proposes some solutions and outlines potential research directions in the future.
Appears in Collections:	[Graduate Institute of Optics and Photonics] Electronic Thesis & Dissertation

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