以液晶元件作為光學調制器並實現其增強光電性能上的應用

Liquid Crystal Devices as Modulators for Applications of Optoelectronic Performance Enhancement

10.6342/NTU.2015.02223

黃久菖

液晶 ； 白光發光二極體 ； 可變色溫 ； 照明設計 ； 膽固醇型液晶 ； 硒化鎘奈米棒 ； 光致螢光 ； 缺陷線 ； liquid crystal ； white light-emitting diode ； tunable color temperature ； illumination design ； cholesteric liquid crystal ； CdSe nanorod ； photoluminescence ； disclination

國立臺灣大學物理學系學位論文

2015年

博士

趙治宇

英文

液晶材料以被廣泛地運用於顯示技術上而為大眾所熟知，但由於其分子獨特、以及多樣化秩序性的排列，使其擁有優越、多元的光學性質及廣泛的應用領域，於是非僅侷限於顯示上。在本論文中，我們將單一液晶元件運用於兩個不同的領域：固態照明及光學量測，分別使用向列型以及膽固醇型兩種不同型態的液晶來進行。結果不僅展現出液晶高度的適用性，同時保留了液晶元件易於操作的優點，其效能更勝過一般研究或商業上所運用的方法。 在固態照明的應用中，我們使用了90°扭旋配相之向列型液晶，製成一可藉由操控電壓，調節透射光量之元件，並以此元件搭配藍光發光二極體和圖樣化螢光粉塗層陣列，構成一可調節色溫之白光發光二極體；由操作電壓控制透射藍光的光量，此白光發光二極體之色溫變化可跨越ANSI C78.377所規範之白光區域，甚至可精確地落於黑體輻射曲線上。色溫變化之趨勢、大小、位置亦可隨需求調整；藉由選擇適當色溫之藍光發光二極體與螢光粉，所構成之可變色溫白光發光二極體，其色溫可自3700 K變化至6100 K，且變化軌跡接近並與黑體輻射曲線相仿。有別於一般製作可調變白光發光二極體的方法，此模式不僅提供了一個易於設計製作、直覺式調控之可變色溫白光發光二極體，亦可應用目前高度發展之相關液晶技術，來修飾、或增進諸多白光發光二極體的表現能力，且其抑制藍光亮度的概念，更可以減少為大眾所擔心的，現行發光二極體所帶來的高強度藍光危害。 在光學量測方面，我們使用了膽固醇型液晶，並將硒化鎘奈米棒粒子摻雜於其中，製成一液晶元件。以膽固醇型液晶為介質，散佈於其中之硒化鎘奈米棒粒子展現出光致螢光增強的效果，此效果於三種不同的成分之膽固醇型液晶均有不同程度的表現，顯示膽固醇型液晶對於奈米粒子之光致螢光的增強效果具一般性。螢光光譜的量測數據亦顯示了增強的程度與膽固醇型液晶的光學螺距有關，光學螺距越短，增強的程度越高，於實驗中我們所獲得最高的增強倍數為4.27倍，較一般的表面電漿共振方法要來得高出許多；探討造成增強現象的原因，我們歸納出是由膽固醇型液晶中所存在的缺陷線所造成。應用膽固醇型液晶之平面螺旋態來增強螢光強度，可以在不破壞材料、干擾訊號的狀況下，以簡單的步驟達到極佳的強化效果，若再運用膽固醇型液晶本身獨特的性質，更可以提供多元的應用層面。

Liquid crystals (LCs) are well known materials which are widely utilized in the region of displays. Their various unique optical properties of LC molecules, which are resulted from different orientations, make them potentially applicable to diverse applications other than displays. In this dissertation, we have successfully applied LCs to two different regions: illumination and optical enhancement with two different kinds of LCs, nematic and cholesteric LCs, being used respectively. The results have shown that the LCs not only possess high capability of application in these regions but also exhibit comparable efficiencies to the methods generally used. Additionally, it brings benefits of simple integration and operation which do save more efforts and costs. In the application of illumination, 90° twist nematic LCs are used to fabricate a transmittance controllable device. Combining with blue light-emitting diodes (LEDs) and a patterned phosphor layer, we construct a correlated color temperature (CCT) tunable white light LED. The variation of CCT of this white LED passes through ANSI white quadrangles on chromaticity diagram, which can even be finely adjusted to the black body locus. Moreover, the manner of the variation of CCT can be regulated. With proper replacement of blue LED and phosphor, we fabricated a white LED with variable CCT ranged from 3700 K to 6100 K for track close to black body locus. Different from commercially CCT-tunable white LEDs, this approach not only provides a simple way to design, construct and manipulate a CCT variable LED, but with applying well developed LC technologies, the performance of white LEDs can also be modulated and improved. Additionally, with the utilization of transmittance adjustment of the blue light, the problem of blue hazard of white LED which many users worry about can be solved as well. In the application of optical enhancement, we have doped cadmium selenide (CdSe) nanorods into cholesteric LC (CLC) cells. The constructed cells filled with three different kinds of CLCs generally show an amplified photoluminescence (PL) phenomenon. The spectra also reveal that the efficiency of amplification is related to the optical pitch of CLCs. It displays an inversely proportional property, that is, a greater enhancement of the PL signal is achieved in the samples with shorter pitch length of CLCs. The highest PL amplification acquired in this study is 4.27-fold which is much higher than the traditional method of surface plasma resonance. The analyses have shown that the enhancement phenomenon is attributed to the presence of oily streaks in CLCs which possess optical properties benefiting the excitation of CdSe nanorods. Different from the traditional method, this simple method can enhance the PL signal efficiently without damaging the materials or interfering in the signals. Moreover, with versatile properties that CLC possessed, this study suggests that the method could provide a potential way for PL signal manipulation in many optical fields.

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