含膽固醇液晶缺陷層之一維光子晶體的光控與電控穿透光譜

Abstract

本研究將一維光子晶體中摻入含偶氮旋性分子的向列型液晶缺陷層，提出了一種缺陷峰可照光切換及電場調控的複合型光電元件。根據偶氮旋性分子光敏的同分異構化反應，我們可以利用照射不同強度的紫外光及綠光的方式在膽固醇液晶平面態及向列型液晶扭轉態與水平態等液晶紋理間切換，進而得到三組穩定的缺陷峰。此外當我們將液晶分子穩定於膽固醇液晶平面態及向列型液晶水平態時，以交流電場的施加可以得到兩種獨特的可調性：其一是利用電場可調控膽固醇液晶光穿透強度的特性來控制缺陷峰強度；另外是以向列態液晶分子電控雙折射率的效應來操縱缺陷峰的波長。其他缺陷峰光譜在不同偏振方向的觀察結果也會一一闡述。更進一步地，我們在液晶盒與紫外光源間引入一偏振片，探討紫外光之偏振態對液晶分子排列之影響。當紫外光的偏振方向垂直於基板配向時，我們定義此光為增益光，增益光的施加能夠促進偶氮旋性分子同分異構化的進行。反之，正交於增益光偏振方向的光定義為減益光，減益光的照射不僅會抑制同分異構化反應，更能夠破壞解旋狀態時的順式態穩定結構，使其加速回到具有螺旋結構的反式態。有別於現今含偶氮旋性分子向列型液晶的相關研究，我們提出了一種嶄新的方式；即，使用單一光源（在本研究中為紫外光）達到觸發偶氮旋性分子同分異構化反應的機制。

This thesis demonstrates a photoswitchable and electrically tunable hybrid photonic structure consisting of a one-dimensional photonic crystal (1D-PC) infiltrated with an azo-chiral-doped liquid crystal (ACDLC) as the defect layer. Based on the photoswitching among the planar-nematic (N), twisted-nematic (T), and planar-cholesteric (CLC) states, attributable to the photoinduced trans-cis isomerization of the azo-chiral dopant, three stable and switchable sets of defect-mode peaks can be realized by controlling the intensity of the green and ultraviolet lights irradiated to the PC/ACDLC cell. Moreover, when respectively stabilizing the cell in the nematic and planar-cholesteric states, upon the application of external voltages, two unique tunabilities in wavelength and intensity can be obtained in our proposed PC/ACDLC hybrid in accordance with the electrically controlled birefringence and voltage-controlled light-scattering strength in the nematic and cholesteric states, respectively. Other spectral features of the proposed hybrids are clarified accordingly. Furthermore, we also investigated the effect of polarized UV light on the isomerization process of azo chiral dopant as well as the textural transition in the ACDLC cell. When the polarization state of UV light is perpendicular to the rubbing direction, the photo-induced isomerization will be fastened which facilitates the unwinding of molecular helices. In the contrary, setting the polarization state of UV light in parallel to the rubbing direction can rewind the ACDLC cell to the helical state, presumably due to the photo-induced instability of the azo chiral molecules in the cis state. Consequently, we propose a new pathway for an ACDLC, enabling the isomerization process of azo chiral molecules and textural transition of LC states to be triggered by a single light source.