介電鬆弛與介電熱效應於複雜中間相材料之研究與光電應用

Abstract

介電加熱物理在複雜中間相材料特別是液晶一直是一個非常複雜的問題。此外，介電鬆弛行為在中間相材料一直是非常混亂和無序。從統計力學得出的德拜鬆弛定律，Cole–Cole方程，Cole–Davidson方程，Havriliak–Negami鬆弛和Kohlrausch–Williams–Watts函數在過去一百年來陸續被提出。然而，本論文中提出了廣義介電鬆弛定律，把上述看似不同的方程式統一，所有的介電鬆弛都只是廣義介電鬆弛定律在某些情況下的變體。我們還使用廣義介電鬆弛定律來推導廣義介電加熱定律。特別適合描述雙頻液晶中容易引起介電加熱效應現象。在論文中，雙頻液晶或雙頻膽固醇型液晶的介電加熱效應能有許多應用。我們提出了一種基於介電加熱效應的低操作電壓方法。加熱效應引發交換頻率的增加，隨後改變液晶的粘滯力允許低操作電壓。此外，我們還展示了通過介電加熱效應實現膽固醇型液晶螺旋結構的局部變形。變形誘發的缺陷模態可以用介電加熱大小進行調節。此外，我們還展示了由二元手性摻雜混合物製成的光子膽固醇薄膜，其選擇性反射在可見光區域擁有電控的特性。最後，本文還展示了基於介電加熱的電熱光效應。與常規液晶材料中眾所周知的電光效應相比，電熱光效應顯示出不尋常的光電行為。基於介電加熱效應，本論文不只提出了完整理論，還展示出了眾多光學上的實際應用。

Physics of dielectric heating in complex mesogenic materials especially liquid crystal materials (LCs) has been a complicated topic. The dielectric relaxation process for mesogenic materials is usually chaotic without obvious order. In the last century, the Debye relaxation law, the Cole–Cole equation, the Cole–Davidson equation, the Havriliak–Negami relaxation, and the Kohlrausch–Williams–Watts function have successively been derived from statistical mechanics. In this thesis, we propose a general dielectric relaxation law that unifies the above-mentioned formulas, and we point out that these formulas are only variations of the general formula in different conditions. We applied the general dielectric relaxation formula for derivation of the law of general dielectric heating. This formula is especially suited for describing the easily induced dielectric heating effect in dual-frequency liquid crystals (DFLCs). In this thesis, we further exploit this heating effect in various applications for both nematic DFLCs and cholesteric DFLCs (DFCLCs). For nematic DFLCs, we propose to apply dielectric heating for reducing driving voltage. The dielectric heating results in increased cross-over frequency and also reduces viscosity of DFLCs to allow for lower driving voltage. For DFCLCs, we demonstrated that with dielectric heating, local deformation in cholesteric structure can be achieved. The defect mode in cholesteric structure can be controlled through the degree of dielectric heating. Furthermore, we have shown that cholesteric film with binary chiral agents exhibits electrically tunable selective reflection in visible wavelength. Lastly, we demonstrated the electrothermo-optical effect based on dielectric heating. Comparing to the commonly-known electro-optical effects, the electrothermo-optical effect exhibits anomalous behaviors. Based on dielectric heating, this thesis proposed various practical applications in the field of optics.