聲導波操控向列型液晶排列的研究

Abstract

向列型液晶具單光軸雙折射性質，受到聲場擾動，液晶分子的排列會偏離平衡狀態，使穿透液晶的偏極光強度產生變化，稱為液晶的聲光效應。本研究以鈮酸鋰晶片上的指差換能器激發表面聲波，經耦合液體傳遞至液晶層與玻璃構成之三明治結構中，生成高頻聲導波，轉動液晶分子的光軸。液晶分子微小，不具可視性，故採用雷射光束的穿透率作為定量指標，並以偏光顯微鏡輔助觀察，評估聲導波對於液晶分子排列的影響。 實驗證實聲導波可影響液晶分子排列，穿透光強訊號的上升時間隨超音波驅動電壓的增加而縮短，下降時間則隨液晶層厚度增加而延長。平行於液晶盒傳遞的聲波相速度隨驅動頻率提高而略為遞減，推測其具有頻散特性。本研究以碘酒為染料，滴入液體耦合層，發現折射進入液體耦合層的聲場輻射會引起聲場流。液晶層厚度大於25 μm的試片受到聲導波的輻射，會產生垂直於波前的聲場流，聲場輻射與聲場流都會影響液晶分子排列，使穿透光強均勻性發生區域性變化。液晶層厚度增加時，穿透光強的頻譜極值數目也會增加。

Nematic liquid crystals (NLC) are optically unaxial, birefringent materials. Molecular orientation of NLC could be altered from their equilibrium states by acoustic field. Variation of the polarized light intensity transmitted through a liquid crystal cell disturbed by acoustic waves is called the acousto-optical effect of NLC. This thesis presents an investigation on this effect induced by acoustic guided wave propagating in the liquid crystal layer sandwiched between two glass plates. The interdigital transducers fabricated on lithium niobate substrates were used to generate surface acoustic waves, which refracted into the liquid crystal layer through a coupling fluid. The transmitted laser intensity was measured as a quantitative index to interpret the acousto-optical effect because the nanometer-scale liquid crystal molecules are invisible. Besides that, a polarized microscope was also used to observe the phenomena accompanied with realignment of liquid crystal molecules due to acoustic guided waves. Experimental results indicate that liquid crystal molecules could be reoriented by acoustic guided waves. The transmitted laser intensity response extends to longer rise time with the increase of driving voltage for ultrasound. On the other hand, the fall time grows as the thickness of liquid crystal layer increases. The phase velocities of acoustic waves traveling in the liquid crystal cell slightly reduce with the increase of driving frequency. It reveals that the acoustic waves are dispersive. Iodine dye was used to observe the acoustic streaming induced by radiation of acoustic guided waves which were refracted through the coupling liquid layer in the present experimental setup. For the liquid crystal specimens having thickness greater than 25 μm, acoustic streaming field could be induced by radiation of acoustic guided waves. The acoustic streaming has an orientation perpendicular to the wavefront. Both acoustic radiation and acoustic streaming can change molecular orientation and induce local fluctuations of transmitted light intensity. It is found that the increase in thickness of liquid crystal layer also induces more extreme spikes in transmitted light intensity.