二色性染料摻雜向列型液晶光柵之非對稱繞射特性研究;Studies of Asymmetric Diffraction Characteristics of Liquid Crystal Gratings based on Dichroic Dye-Doped Nematics

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題名:	二色性染料摻雜向列型液晶光柵之非對稱繞射特性研究;Studies of Asymmetric Diffraction Characteristics of Liquid Crystal Gratings based on Dichroic Dye-Doped Nematics
作者:	劉翰儒;Liu, Han-Ju
貢獻者:	光電科學與工程學系
關鍵詞:	液晶光柵;光配向;LC grating
日期:	2026-01-14
上傳時間:	2026-03-06 18:29:04 (UTC+8)
出版者:	國立中央大學
摘要:	本論文主要利用摻雜二色性染料(Dichroic Dyes)之向列型液晶為材料，搭配表面配向方式使液晶排列的不同，探討其液晶盒因材料與結構關係所造成的非對稱繞射光學特性。實驗中液晶盒採用光配向製成，結構為一週期性交錯排列的扭轉向列型液晶(Twisted Nematic Liquid Crystals, TNLCs)與水平配向液晶(Homogeneous Alignment LCs, HALCs)所構成之週期性液晶光柵，與另一為均勻水平配向(HA)所組成的液晶盒。液晶光柵具有不同分子排列結構之區域所構成，結構上同時具備折射率週期性差異所形成之相位光柵，以及因摻雜二色性染料所造成之吸收差異所對應之振幅光柵。在未施加電壓的情況下，先以週期結構面探討，並分析此面同時具有相位光柵與振幅光柵兩種光柵效應，藉由量測並比較其第零階與第一階繞射光之強度分佈與偏振特性。另經計算得到當入射光線偏振方向平行y軸時，其繞射光之第零階與第一階繞射之線偏振度約0.83；當入射光線偏振方向平行x軸時則皆約0.61。若從水平配向面入射，經過分析在此面皆未具有相位光柵與振幅光柵兩種效應，且在入射光偏振方向與水平配向面垂直時，所量測到之第零階繞射之出射光偏振角度為32º，與理論值45º相差13º，後續探討其可能由亮暗週期寬度比非1：1所導致，經計算分析後寬度比約為2 (TNLC)：3 (HALC)。進一步地對液晶盒施加一電壓，使液晶分子趨於垂直基板排列時，經計算得到由週期結構面在不同入射偏振方向下之第一階繞射效率分別從垂直x偏振方向入射的1.8%與平行x軸偏振方向入射的1.4%下降至趨近於0%；反之，第零階繞射效率分別由垂直x偏振方向入射的26%與平行x軸偏振方向入射的23%提升至約49%，此情形下TNLC與HALC區域之折射率差異與吸收差異均消失，整體光學行為趨近於單一折射率n_o，結果顯示，該向列型液晶光柵具偏振選擇性與入射面方向非對稱繞射特性，且其繞射光學行為可透過外加電場進行調控。;This study investigates the asymmetric diffraction behavior of nematics cell doped with dichroic dyes by combining material absorption effects with structural modulation induced by surface alignment. The devices were fabricated using a photoalignment technique. One substrate consists of a periodic liquid crystal (LC) grating composed of alternating twisted nematic LC (TNLC) and homogeneous alignment LC (HALC) regions, while the other is a uniformly HA alignment. Both of the alignment layers are fabricated by photoalignment technique using brilliant yellow, azo-material. Due to the coexistence of periodic refractive-index modulation and dichroic-dye-induced absorption contrast, the LC grating simultaneously exhibits phase-grating and amplitude-grating effects. Under the voltage-off condition, diffraction measurements from the periodic-structure side show that the degrees of linear polarization of both the 0th and 1st diffracted beams are approximately 0.83 for y-polarized incidence and 0.61 for x-polarized incidence. In contrast, when light is incident from the homogeneous-alignment side, neither phase grating nor amplitude grating effects are observed. For polarization perpendicular to the homogeneous alignment direction, the polarization angle of the 0th-order transmitted beam is approximately 32°, deviating from the theoretical value of 45°, which is attributed to a non-ideal grating duty cycle of approximately 2 (TNLC):3 (HALC). When an external electric field is applied, the 1st-order diffraction efficiencies from the periodic-structure side decrease to nearly zero, while the 0th-order diffraction efficiency increases to approximately 49% for both polarization states. Under this condition, the refractive-index and absorption contrasts between the TNLC and HALC regions vanish, and the optical response approaches that of a uniform medium with a single refractive index n_o. These results demonstrate that the proposed nematic LC grating exhibits polarization-dependent and incident-side-dependent asymmetric diffraction, which can be effectively controlled by an external field.
顯示於類別:	[光電科學研究所] 博碩士論文

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