以封於介電反射鏡間的三穩態膽固醇液晶製作多功能光電元件

Abstract

本論文依一維光子晶體/液晶混合結構，以三穩態膽固醇液晶作為缺陷層並封於兩介電反射鏡間，藉由缺陷峰頻譜的電壓可調控性與光學記憶性，提出一個多功能光電元件的設計概念。我們所使用的膽固醇液晶缺陷層具有三個光學穩定態，分別為平面態、散射態與均勻橫向螺紋態。同時，我們巧妙的將膽固醇液晶的反射波段設計在光子晶體的光子能隙波段內。 利用外加電場控制元件在三個穩定態間切換，根據研究結果，在平面態與橫向螺旋態之間切換可控制在特定波長範圍內之缺陷峰的穿透度達到波長選擇的效果。此外，在散射態與橫向螺旋態間切換可以透過改變電場的振幅和頻率調制在光子能隙內之缺陷峰的穿透度，達到光開關與光強度調變的效果。若將元件固定在橫向螺旋態，可藉由增加頻率，提高液晶分子排列的秩序性進而調整缺陷峰的穿透率。最後在1D PC/CLC的光譜量測系統中我們使用了一片偏振片，可以在膽固醇液晶橫向螺旋態時藉由轉動偏振片，達到選擇模態的效果。綜上所述，本論文所提出之含三穩態膽固醇液晶之一維光子晶體結構可視為一個低耗能且具有光開關、波長選擇器及強度調變器之多功能光學元件。

In this study, a one-dimensional-photonic-crystal (1D PC)/liquid-crystal hybrid structure consisting of a tristable cholesteric liquid crystal (CLC) sandwiched between two dielectric mirrors is fabricated and purposed as a new design of an energy-efficient multifunctional photonic device based on its electrically tunable and optically memorable spectral features of defect modes. Here, the CLC exhibits three optically stable state—planar (P), scattering (S) and uniform lying helix (ULH). Specifically, the reflection band of the CLC is masterly set within the photonic bandgap (PBG) of the 1D PC. By electrically modulating the device between stable CLC states, experimental results indicate that switching the device between the ULH and the P states enables the suppression in the transmission of partial defect modes within the PBG, revealing the feature of wavelength selection. In addition, switching between the S and ULH states can be regarded as the light-intensity on and off states for defect modes in the visible spectrum, respectively. When fixing the CLC in the ULH state, increasing the frequency of the applied voltage results in the enhancement of light transmission owing to the promoted molecular ordering. Finally, two sets of defect modes with distinct optical profiles can be realized by rotating the transmission axis of the polarizer inserted between the light source and the device. This device possesses many alluring features such as tristable switching and As a result, our proposed PC/CLC provides a new pathway for the design of multifunctional and energy-efficient optical switches, light shutters, wavelength selectors, and electrically controllable intensity modulators.