錨定能對液晶盒動態響應的影響

Abstract

液晶的動態行為在液晶元件光學反應的研究中，是一個非常基本也非常重要的課題。液晶的動態特性不僅與液晶的材料性質有關，更與液晶排列方式有著密不可分的關係。液晶的排列方式會被邊界條件所左右，其邊界條件可用錨定能密度來表示，錨定能密度則可由easy axis的位置與錨定能係數來決定。此外，在向列型液晶中，其指向矢的轉動與流動是互相耦合在一起的。例如：液晶指向矢的非均勻轉動會造成流動的發生，而流動亦會干擾指向矢的轉動，此效應可以在扭轉型液晶盒的光學跳躍現象與雙穩態驅動機制中發現。 本論文中，我們先研究錨定能密度並用雙穩態液晶盒為例，來探討其對流動效應的影響。在錨定能密度中，預傾角是easy axis在極角方向的傾角。我們推導出液晶盒在外加電壓下，預傾角與相位延遲的關係，解決了測量垂直排列反射式液晶盒預傾角的問題。在測量錨定能係數時，我們發現若外加電壓使得邊界上的指向矢偏離easy axis變大，只考慮一項的錨定能密度函數是不合適的，此時必須加進高次項來修正。我們用包含高次項的錨定能密度模型，測量並分析二個水平排列與三個垂直排列的液晶盒所具有的錨定能係數。 此外，我們以水平配向，一邊界為強錨定，另一邊界為弱錨定的雙穩態向列型液晶盒（BiNem）為例，研究在不同脈衝波的作用下，液晶盒光學表現不同的原因與機制。我們發現其弱邊界上的指向矢，在脈衝波作用結束時的位置，會影響後來的液晶動態行為。最後，我們設計並做出一種新的雙穩態液晶顯示模態（BHN）的液晶盒，藉由液晶的流動效應與雙頻液晶的特性，構成轉換機制。在實驗上，其切換電壓僅需為5 V即可達成穩態間的轉換。

Consider as an important subject for the study of the electro-optical response in the liquid crystal (LC) device, the dynamic behaviors of liquid crystals are not only related to the intrinsic properties of the LC material itself but also to the spatial orientation of the LC directors. The arrangement of the directors in a cell is controlled by the boundary conditions, which can be described by the anchoring energy density determined by the position of the easy axis and the coefficients of the anchoring energy density. Moreover, there is a coupling between the translational motion and the rotational motion of the nematic LC directors. For example, the non-uniform rotations of LC directors induce the shear flow of the LC director; conversely, the shear flow disturbs the orientation of the LC directors. The shear flow effect can be observed in the optical bounce of twisted nematic cells and some of the switching mechanisms of bistable devices. In this research, we first studied the anchoring energy density and than used two bistable cells as examples to investigate the impact of the shear flow effect. Within the anchoring energy density, the pretilt angle is the tilt angle of the easy axis in the polar direction. We derived the relationship between the pretilt angle and the phase retardation as a function of applied electric field and solved the problem of obtaining the pretilt angle of a non-twisted, vertical aligned reflective cell. When measuring the anchoring energy coefficients, we found the higher order terms of the anchoring energy density cannot be negligible when the applied voltage becomes large. By using the anchoring energy density with one higher order term, we measured the anchoring energy coefficients of two homogeneous cells and three homeotropic cells. Furthermore, we investigated the optical response of Bistable nematic (BiNem, a homogeneous aligned cell with one weak anchoring substrate and one hard anchoring substrate) cell by applying voltage pulses with various durations. We found that the position of the weak-boundary director at the end of the pulse has a crucial impact on the relaxation behaviors. Finally, we designed and demonstrated a novel bistable mode, namely, the bistable chiral-tilted homeotropic nematic (BHN) LC device. The switching mechanism is achieved by the shear flow effect and the anisotropic properties of a dual frequency liquid crystal. The experimental switching voltage of the device is only 5 V.