| 摘要: | 本論文的研究可分為三大部分，且三部分之研究皆著重於探討液晶在空間中產生之排列變化所對應的光學調制。在第一部份之研究中，顯微鏡影像色彩中所隱含之液晶結構資訊即為此部分之討論重點，在該研究中，我們成功利用影像色彩擷取出膽固醇液晶之傾斜螺旋結構中的錐角及其螺距隨外加電場強度變化之趨勢，而在施加電場由0.6 V/μm連續下降至0.45 V/μm之過程中，藉由紀錄的影像色彩所辨別出的膽固醇液晶錐角從40°逐漸升至90°，後續將此趨勢與理論預測之趨勢變化相比較，可發現藉由影像色彩所辨別之液晶結構為暫態傾斜螺旋液晶結構，其在電場調制下所呈現之結構變化趨勢與理論預測之穩態傾斜螺旋結構有顯著區別，藉此也凸顯本研究提出之液晶結構分析方法可有效地藉由影像色彩及時解析液晶結構變化，進而補足一般以自由能變化推算之結構變化趨勢所無法克服之缺點。在第二部分之研究中，滿足特定條件下之90°扭轉向列型液晶在不同施加電壓下所表現之光學特性為此部分之研究重點，此液晶光學系統可在不同施加電壓下將偏振方向平行或垂直於入射面液晶排列方向之線偏振態由0°連續旋轉至90°，且在過程中，出射光之線偏振度皆高於0.95，而在該部份研究中，我們以邦加球分析線偏振入射光在此液晶結構中傳遞時所對應之偏振變化過程，而藉由該視覺化分析可發現當扭轉液晶結構滿足此特定條件時，位於中間液晶層之相位延遲主導偏振態演化之最終結果，而後續再搭配矩陣方法分析，發現兩片皆符合特定條件之串疊扭轉液晶結構所對應之瓊斯矩陣即為旋轉矩陣，由此推導結果便可得知，該結構在特定電壓區間下可提供0°至180°之旋轉角調制變化，此外，該調制效果也不僅限於特定偏振方向之線偏振入射光，針對任意線偏振方向之入射光，該串疊液晶結構皆可表現出相同之旋轉角調制變化。而在最後一部份的研究中，我們主要探討向列型液晶光柵之彎曲彈性係數變化所對應之結構形變影響，在研究中我們藉由摻雜液晶二聚體之方式大幅降低整體液晶之彎曲彈性係數，此外，在實驗中我們也將光配向週期作為實驗變因，進而探討配向週期變化所引致之液晶排列形變，綜觀整體實驗數據，可發現當配向週期接近液晶層厚度時，彎曲彈性係數之大小變化對於液晶光柵之排列有顯著影響，對於維持原始彎曲彈性係數之向列型液晶光柵，當基板表面配向週期接近液晶層厚度時，其表現出的光學週期為表面配向週期的兩倍，而在此部分之實驗基礎上，也可發現若將向列型液晶光柵之彎曲彈性係數降低，則即使在配向週期接近液晶層厚度之情況下，該光柵元件中的液晶導軸方向仍可依造基板表面所施加之配向週期排列，而該液晶光柵便可將入射光大角度繞射且同時維持高繞射效率。;This thesis is divided into three parts, with discussions all focused on the optical modulations induced by the changes in the spatial orientation of liquid crystals (LCs). In the first study, we primarily focused on the LC structural details underlying the image colors recorded using a polarized optical microscope (POM). Using the POM image colors, we have successfully extracted the variation trends of the cone angle and pitch of heliconical structures under different electric stimuli. The deduced cone angles range approximately from 40° to 90° when the applied electric field is continuously decreased from 0.6 V/μm to 0.45 V/μm. By comparing these variation trends with those ranging approximately within 30° obtained through theoretical predictions, we found that the trends deduced from the image colors correspond to the structural variations of the heliconical structures in the transient state, exhibiting distinct variation patterns compared to those of the heliconical structures in the stable state. This significant difference also highlights the fact that the proposed method can effectively analyze the real-time variations of helical LC structures, thereby overcoming the limitation that free energy calculation can only predict the variation trend corresponding to the heliconical structures in the stable state. In the second study, we aimed to elucidate the optical properties of 90°-twisted nematic LCs (TNLCs) under a specified condition and subjected to different voltages. This twisted nemtatic LC optical system can rotate linearly polarized light by angles ranging from 0° to 90° when the incident polarization direction is parallel or perpendicular to the LC alignment on the input surface. Under this condition, the output polarization state maintains a degree of linear polarization above 0.95 throughout the entire operating range. During research, we initially employed the Poincaré sphere to analyze the polarization evolution of linearly polarized incident light propagating through the specified TNLCs. This visualized method clearly revealed that the phase retardation of the LC layers near the center of the TNLCs governs the final state of the polarization evolution. In addition, by integrating the insights obtained from the Poincaré sphere with the matrix method, we also discovered that the Jones matrix of an optical system composed of two identical TNLCs under the specified condition is a rotation matrix. According to this deduction, the optical system is capable of rotating incident light with arbitrary linear polarization states to an angle within the range of 0° to 180° under a specific range of applied voltages. In the last study, we mainly focused on the influences of structural deformation within nematic LC (NLC) gratings, induced by the variations in the bend elastic constant. During this study, the bend elastic constant of the NLC gratings was decreased using LC dimer dopants. Additionally, the alignment period was introduced as a variable for further investigation in this study. According to the overall experimental results, it is evident that the LC arrangement within the NLC gratings strongly depends on the value of the bend elastic constant when the alignment period approaches the thickness of the NLC gratings. For NLC gratings with the original K33 value of the employed LCs, their optical periods become twice the alignment periods imposed on the substrates when the grating thicknesses approach the alignment periods. Based on this result, we conclude that if the bend elastic constant is significantly decreased, the LC directions within the NLC gratings can effectively follow the periodic alignment directions imposed on the substrates when the alignment period is close to the thickness of the gratings. Consequently, such NLC gratings can effectively deflect incident light at a large angle while maintaining high diffraction efficiency. |
