Bragg diffraction of higher orders on oblique helicoidal liquid crystal structures

For the oblique helicoidal (ChOH) structure of the chiral twist-bend nematic-forming mixture of CB7CB/CB6OCB/5CB doped by light-sensitive chiral compound based on azo-fragment, at oblique incidence of light two consequent states of Bragg reflection in the visible spectral range from 400 to 750 nm were experimentally observed in the course of decreasing the applied electric field. These states were assigned to different orders of Bragg’s diffraction occurring at ChOH layers with full pitch periodicity P. While the second and third orders of Bragg’s diffraction can be easily observed in conditions of standard electrooptic cell, the first order could be revealed only at very high electric field near the ‘red’ edge of the visible spectral range. For the second and third orders of diffraction there is a narrow range of electric field where both diffraction orders are observed (so-called hybrid state), with transmittance spectra of the ChOH structure showing two peaks at the short-wavelength and the long-wavelength edges of the visible spectral range. Model calculations of Bragg’s reflection of light were carried out using literature values of elasticity constants and dielectric permittivity, and the calculated plots of the selective reflection maximum vs. voltage were in good agreement with experimental data.

针对由CB7CB/CB6OCB/5CB组成的手性扭曲弯曲向列相混合体系，经基于偶氮片段的光敏手性化合物掺杂后所形成的倾斜螺旋（oblique helicoidal, ChOH）结构，当光线斜入射时，在400 nm至750 nm的可见光光谱范围内，实验观测到随外加电场降低过程中先后出现两种布拉格反射态。这两种反射态分别对应于具有完整螺距周期P的ChOH层所产生的不同级次布拉格衍射。尽管在标准电光池条件下可轻易观测到第二级与第三级布拉格衍射，但第一级衍射仅能在接近可见光光谱“红边”的极高外加电场下被观测到。对于第二级与第三级衍射而言，存在一段狭窄的电场范围可同时观测到这两种衍射级次（即所谓的杂化态），此时ChOH结构的透射光谱在可见光光谱的短波端与长波端各呈现一个峰值。本研究采用已发表文献中的弹性常数与介电常数数值开展了光布拉格反射的模型计算，所得选择性反射峰值随电压变化的计算曲线与实验数据吻合良好。