利用九十度扭轉式液晶聚合物網絡之偏振無關液晶相位調制器

Abstract

本文提出一種偏振無關液晶相位調制器，主要是利用九十度扭轉式液晶聚合物網絡(簡稱T-PNLC)。其機制結合兩種偏振無關液晶相位調制器，第一種為剩餘相位型液晶相位調制器，其液晶分子有一個相同的傾角，且指向矢隨機分佈在T-PNLC液晶盒的中間層。另一種為雙層型液晶相位調製器，靠近T-PNLC的上下基板，液晶指向矢方向為相互垂直。這種混合型的T-PNLC，其電控相位移大小介於剩餘相位型與雙層型之間，且操作之前不需額外偏壓。藉由SEM觀察，我們得知T-PNLC的domain夠大(~10μm)，一顆聚合物又夠小(~0.2μm)，所以為低散射的相位調制器。另外，T-PNLC的偏振相關性與光聚合時外加電壓、光聚合時溫度、液晶盒厚度、及光入射角相關。實驗中，當液晶盒厚度為7μm，其相位調制器在施加30 Vrms電壓下可達到0.28π之相位變化，反應時間約1.5ms。最後我們將T-PNLC做光學分析，估計出T-PNLC中雙層型液晶分子的平均預傾角(50~70 degree)、剩餘相位型液晶分子的平均預傾角(73~83 degree)及其他參數。此偏振無關之相位調制器未來可應用於電控焦距之微液晶透鏡陣列或雷射指向。

A polarization independent liquid crystal (LC) phase modulation using polymer-network liquid crystal in a 90 degree twisted cell (T-PNLC) is demonstrated. Two polarization-independent mechanisms are adopted. The first one is the phase modulation from the middle layer LC directors with random but same tilt orientations and can be classified as the residual phase type. The other one is the phase modulation from the directors near the two substrates with nearly orthogonal orientations and can be classified as the double-layered type. T-PNLC is a scattering-free phase modulator because of large domain size (~10μm) and small polymer grain (~0.2μm) observed by SEM. No bias voltage is needed when we operate T-PNLC. The phase modulation range is between the residual phase type and the double-layered type. In addition, the polarization dependency of T-PNLC depends on the effects of the curing voltage, the curing temperature, the cell gap, and off-angle of an incident light. In the experiments, the maximum phase shift of T-PNLC is around 0.28π at 30 Vrms with the cell gap as 7 μm. The response time is around 1.5 msec. The average pretilt angle of liquid crystal in double-layered region(50~70 degree), residual phase region (73~83 degree), and other parameters can be estimated base on our optical analysis of T-PNLC. The potential applications of the polarization independent T-PNLC are electrically tunable-focus LC micro-lens arrays and laser beam steering.