聲導波對於向列型液晶排列的影響

Abstract

向列型液晶是單光軸雙折射晶體，受到電場、磁場、聲場的影響，液晶分子的排列會發生改變。液晶受聲場作用致使分子排列扭轉，改變折射率，影響光穿透強度，稱為液晶的聲光效應。斜束超音波照射液晶試片時，不直接對液晶分子作用，液晶分子受到折射進入液晶試片的超音波生成聲導波作用，本文根據超音波理論及實驗探討聲導波對於垂直配向液晶層聲光特性的影響。 實驗以沒水式超音波換能器激發0.2~7 MHz的縱波，斜束入射沒於水槽的液晶試片，調整液晶層間隙、超音波強度、入射角等參數，以光接收器量測穿透液晶試片之光強度，間接觀察液晶分子排列之變化。在各入射角所生成的聲導波共振頻率處，穿透光強度明顯發生變化。光強度變化除了會趨於飽和穩態外，有時候還會呈現崩潰之後再度飽和的現象，彷彿液晶分子扭轉超過極限後，分子再度重排。 實驗結果與施文斌同學模擬液晶試片聲導波的頻散曲線結果比對，發現聲導波S0、S1模態明顯改變液晶分子之排列，A0模態也會改變分子排列，但是不明顯。液晶分子排列受到高頻聲導波影響的反應時間較低頻者縮短甚多，雖然尚未達商業利用價值，但是值得進一步研究。

Nematic liquid crystals (NLC) are optically uniaxial materials. Their alignments are easily disturbed by applied external fields such as electric, magnetic, acoustic fields. A change in the refractive index of NLC results from the realignment of liquid crystal molecules. The influence of acoustic waves on the optical transmission through the liquid crystal cell is called the acousto-optical effect of NLC. The liquid crystal molecules do not be directly perturbed by obliquely incident acoustic beam. Instead, they are influenced by the acoustic guided waves which are induced by the refracted ultrasound into the sandwiched liquid crystal cell. This thesis investigates the acousto-optical effect on vertically aligned NLC due to acoustic guided waves in liquid crystal cells based on experiments and theories of ultrasound. In experiments, the liquid crystal cells are placed in a holder within a water tank and are obliquely insonified by an immersion ultrasonic transducer in a broad frequency range from 0.2 to 7 MHz. The acousto-optical effect was observed by a variation of optical transmission through the cell. The optical transmission was measured as a function of the gap of liquid crystal cell, acoustic intensity, incident angle, and so on. Experimental evidence indicates that optical transmission becomes significant at the occurrence of acoustic guided waves in the cell. The optical transmission not only reaches saturation with the increase of acoustic intensity and duration but also collapses and followed by becoming saturated again. It seems that realignment occurs once the molecules rotate over an extreme tilt angle. The comparison of experimental results and the simulated dispersion curves of acoustic guided waves in liquid crystal cells done by Shih shows that S0, S1 modes have strong effect on the realignment of liquid crystal molecules. The A0 mode has only sight influence. In addition, liquid crystal apparently has faster response in high frequency range than at low frequencies. Even though the response time does not reach the required commercial standard, it is worth further investigation on acoustic realignment of liquid crystals using guided waves.