纳米粒网络对于手性液晶反射光谱的展宽效应

Bragg Reflective Spectrum Broadened by Nano-particle-network in Chiral Liquid Crystals

依据分子呈螺旋型排列的介质的电磁场解计算了给定厚度的手性液晶的光谱反射率与其双折射的关系,依据布拉格反射的干涉条件计算了斜入射光的布拉格反射光谱的蓝移。从中发现:(1)若液晶的双折射大于0.2,液晶层的厚度达到3μm就可使反射率高于90%;(2)若要以液晶的平面态实现整个光谱范围的布拉格反射,液晶的双折射应当达到0.6以上;(3)入射角达到60°时手性液晶的反射光谱蓝移可达到100nm。然而,商品手性液晶的双折射只有0.2左右,为了将这样的手性液晶用于在整个可见光谱反射的显示器件,就需要在液晶中引入聚合物网络,并在玻璃基片表面实现聚合物的凹凸结构。网络的作用是要在液晶层中建立随机分布的分子锚定机制,使手性液晶的螺距随位置而变化,从而展宽其布拉格反射谱。基片表面的凹凸结构用于诱导液晶螺旋轴的斜向随机分布,以利用倾斜螺旋结构布拉格反射的蓝移现象,进一步展宽布拉格反射谱。实验证明,在液晶中引入聚合物网络和表面的凹凸结构以后,手性液晶的布拉格反射区间由原来的80nm增加到120nm以上,对比度达到4∶1左右。

Based on the analytic expression of electromagnetic field solution of helical symmetric dielectric material, the paper presents therelationship between spectral reflectivity and birefringence of chiral liquid crystal, and the blue shift of Bragg reflection in the condition of oblique incidence. The theoretic results indicated that：（1）If birefringence of liquid crystal is greater than 0.2 and thickness of liquid crystal layer reaches to 3 μm, the liquid crystal layer will reflect more than 90° of the incident light; （2）To reflect the whole visible spectrum by Bragg reflection, An of the liquid crystals in plane alignment state should exceed 0.6; （3）When the incident beam inclines to 60° from normal direction, blue shift of the reflective spectrum will reach to 100 nm. On the other hand, as An of the commercial chiral liquid crystals is not larger than 0.2, to get the entire visible reflective spectrum, it needs to introduce polymer network into liquid crystals and make a sagging structure on the surface of substrates. The contribution of the network is to establish random anchorage that makes the pitch variety, hence broadening the Bragg reflection spectra. Random distribution of sagging structure on the surface of substrate is used to induce random screw axes of the chiral liquid crystal, which not only causes blue shift of Bragg reflection but also further stretches the reflection spectra. Experiments demonstrated that Bragg reflection spectrum could be broadened from 80 nm to more than 120 nm, and the contrast reaches to 4 ： 1 by introducing both polymer network and sagging structure on the substrate surface of a cell.