An experimental investigation of discrete changes in pitch in a thin, planar chiral nematic device

液晶 材料科学 锚固 平面的 背景(考古学) 折射率 反射(计算机编程) 光学 跳跃 谱线 光电子学 物理 生物 结构工程 计算机图形学(图像) 工程类 量子力学 古生物学 程序设计语言 计算机科学 天文
作者
H. G. Yoon,Nicholas W. Roberts,Helen F. Gleeson
出处
期刊:Liquid Crystals [Taylor & Francis]
卷期号:33 (4): 503-510 被引量:30
标识
DOI:10.1080/02678290600633501
摘要

Abstract The phenomenon whereby sudden changes in helicoidal pitch occur in thin chiral nematic devices with strong surface anchoring has been studied experimentally. This has been done with the aim of examining the underlying process in the context of existing theory. Thin devices (∼2.5 µm thick) containing two commercially available chiral nematic liquid crystals were employed in the study. The samples were chosen for their dependence of pitch on temperature; one exhibits a very slow increase in pitch with increasing temperature (95.6% of the Merck material BL131 in BL130), while the second shows the more common rapid reduction in pitch with increasing temperature (the Merck mixture TM1001). High resolution reflection spectra were obtained for the devices and a numerical fitting algorithm, based on the Berreman 4×4 matrix technique, provided accurate information on changes in the pitch, refractive indices, device thickness and changes in the surface director across the pitch jump region. We demonstrate that changes in refractive index and device thickness are negligible in analysis of the reflection spectra across the pitch jumps. We further show that the pitch changes discontinuously at the pitch jump, with no variation in its value as the process occurs. We find evidence that the surface directors also change by less than 10° during this process in a manner analogous to coiling or uncoiling a constrained spring. This mechanism differs somewhat from others proposed in the literature.
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