超音波操控向列型液晶排列之研究(II)

Abstract

剪應變與壓力梯度都具有改變長鏈奈米材料的排列秩序，後者通常需要許多週期才 能完成整個程序，例如：超音波誘發排列。本計畫以超音波原理及液晶的光學性質為基 礎，探討超音波改變向列型液晶配向的機制及其可能的應用。在過去一年的研究中，已 獲得的沒水式超音波實驗數據顯示，穿透液晶的光強度極值發生於洩漏聲導波的模態頻 率。向列型液晶主要受到液晶盒內聲導波生成的影響，才發生分子重排。數值模擬結果 亦發現，為了維持液晶層的聲壓梯度，大部分的聲導波能量消耗於玻璃層。僅有Scholte 波與高頻的A0 模態具有界面聲波特性，能夠將聲壓侷限於液晶層，卻不耗損能量於玻 璃層。基礎於第一年的初步研究成果，提出後續的兩年期計畫。 第二年研究期間將以液晶盒內的界面聲波，操控向列型液晶的配向，建立適當的理 論模型，決定超音波重排直立及水平配向之向列型液晶的最佳操作條件、反應時間及恢 復時間。實驗採用指插換能器激發界面聲波，實驗數據將驗證理論預測的結果。第三年 的研究將應用超音波操控液晶排列的技術，開發兩種液晶透鏡。分別以界面聲波之駐波 及圓形平板的軸對稱共振模態取代電場，調制液晶盒光學元件，成為偏光菲涅耳透鏡及 可調式點聚焦透鏡。本計畫的研究成果將有助於發展操控奈米材料排列的實用技術，提 供創新的產品技術給國內以液晶光學為基礎的平面顯示器產業。

Shear strain and pressure gradient can alter the orientational order of long chained nanoscale materials. The latter method, such as ultrasound induced alignment, usually needs a large number of cycles to complete the process. A research program based on the principles of ultrasound and optical properties of liquid crystals is proposed to explore the mechanism of acoustic re-alignment of nematic liquid crystals and its feasible applications. The first year research has been granted and performed. Experimental evidence indicates that re-alignment of nematic liquid crystals is mainly caused by guided acoustic waves propagating over the liquid crystal cell immersed in water tank. The extreme value of optical transmission through the liquid crystal cell occurs at the modal frequencies of leaky guided waves. Numerical simulation also shows that most energy carried by guided acoustic waves in consumed in glass plates to sustain sound pressure gradient in the liquid crystal layer. Only the Scholte waves and high-frequency A0 mode are able to confine sound pressure in liquid crystal layer without energy dissipation into glass plates. Based on the preliminary study in the first year, a further two-year research program is proposed. In the second year, research interest will focus on manipulation of nematic liquid crystals using interface waves transmitted into the cell. A theoretical model will be developed to determine the optimal operating conditions, response time, and relaxation time for acoustic re-alignment of both homeotropically and homogeneously aligned nematic liquid crystals. Theoretical prediction will be verified with experiments using interdigital transducers as transmitters of interface waves. In the third year, two kinds of liquid crystal lenses will be developed. The polarized Fresnel lens and a point-focus lens made of nematic liquid crystals will be developed using standing acoustic interface waves and axisymmetric resonant vibration modes of circular plates, instead of electric field. This research can benefit the progress in manipulation of long chained nanoscale materials and provide innovative technology to domestic industry of liquid crystal based flat panel display.