Flat optics with cholesteric and blue phase liquid crystals

ABSTRACT

The cholesteric (Ch) phase is an ancient liquid crystal (LC) phase, with its roots dating back to the days of Friedrich Reinitzer. It is most well known for its ability to selectively reflect circularly polarised light with the same rotation handedness as the helical structure, and have found applications in polarising and display devices. Most studies to date utilising the reflective properties of ChLCs, however, have treated ChLCs as simple dielectric mirrors in which light follows the law of reflection. We have recently shown that through controlling the phase of the Ch helix, it is possible to control the phase of reflected light. Because the phase can be controlled over 0–2π, the reflected wavefront from a planar device can be designed to possess non-specular properties, such as deflection and lensing. In this paper, we aim to provide a comprehensive understanding of the phenomenon by presenting theoretical and experimental results on the dependence of the reflected light phase on the helix phase, and the effect of chiral handedness on the phenomenon. Also, we show that wavefront manipulation based on the same concept can be achieved in Ch blue phases (BPs), which are chiral LCs with three-dimensional periodicity.     

Flow of cholesteric liquid crystals—I: Flow along the helical axis

It is shown that the essential features of Helfrich’s permeation model for flow along the helical axis of a cholesteric liquid crystal can be derived approximately on the basis of the Ericksen-Leslie theory.     

聚甲基丙烯酸胆甾醇酯共聚物的合成、结构与性能研究 Ⅳ.PMACE的相态转变及光学性质

Finkelmann和等人对侧链胆甾型高分子液晶的研究表明,将具有液晶功能的低分子基团,经过一个软段连接到柔性高分子主链上的梳型高分子在一定的温度下可以形成液晶态,调节侧链高分子液晶的分子结构、软段长度,可以改变其相态转变温度及微区形态。前已报导具有不同侧链结构的聚甲基丙烯酸胆甾醇酯共聚物的合成、相态转变及光学性质,本文通过对聚甲基丙烯酸胆甾醇乙烯酯共聚物(PMACE)的液晶态及结晶态的微细结构及相态转变与胆甾侧链含量关系的研究,给出了液晶态的形成条件及结构特征。     

THE STRUCTURE AND OPTICAL PROPERTIES OF COPOLYMER OF CHOLESTERIC ESTER

The structure and switching properties of liquid crystalline side chain copolymers of cholesteric ester of 1,2-hydroxypropyl 2,4-di-isocyanatoluene methylmethacrylate (PHCPM) have been studied in detail. The cholesteric mesophase of PHCPM is shown by polarizing microscopy, X-ray diffraction and selective light reflection. Solution of PHCPM in CHCl_3 is optically anisotropic; its optical properties were determined by specific rotation [α], circular dichroism (CD) and wide-angle hght scattering (WALS) methods.     

黄原胶溶致液晶行为的研究

以小角激光光散射法、偏光显微镜法及旋转粘度计法研究了黄原胶水溶液在不同浓度和温度下的光学性质。结构区域及粘性行为。发现当溶液浓度达到3％(临界浓度值)时,呈现胆甾型液晶特征,液晶的形成对温度有显著的依赖性。     

乙基纤维素液晶的螺旋性质及影响因素的研究

以乙基纤维素／丙烯酸体系为例，对乙基纤维素液晶的胆甾相性质及其影响因素进行了研究，测定了体系胆甾相结构的螺距。结果表明，乙基纤维素／丙烯酸液晶体的螺旋方向为左旋，随溶液液浓度的增大，胆甾相结构的螺距减小。胆甾相结合的螺距还明显地受到温度，压力、溶剂组成，高分子掺杂等因素的影响。     

Unexpected photoinduced phenomena in chiral–photochromic cholesteric copolymers with a triplet sensitizer.

For the first time the possibility of energy transfer from a triplet photosensitizer to chiral–photochromic fragments in photoactive cholesteric systems was demonstrated. For this purpose we prepared mixtures containing chiral–photochromic cholesteric copolymers with a triplet sensitizer—acrydine orange (AO). Chiral–photochromic groups in copolymers contain a C=C bond capable for undergoing E–Z isomerization during UV irradiation. All polymer mixtures form a chiral nematic phase displaying selective light reflection with _max~650–1,000 nm depending on the structure and concentration of the chiral groups. Irradiation of mixtures by visible light (>450 nm) leads to a shift of the selective light reflection peak to a long-wavelength spectral region. This effect is associated with a decrease of anisometry of chiral–photochromic fragments in copolymers during their E–Z isomerization. It is important to emphasize, that chiral–photochromic side groups of copolymers do not absorb visible light themselves; therefore, the previously mentioned changes can be explained by the energy transfer from photoexcited AO molecules in the triplet state to isomerizable fragments. The study of the kinetics of this process revealed a rather unexpected phenomenon: after the first 60–80-min irradiation, the helix pitch of the supramolecular structure of the mixtures increases, but after successive irradiation the helix pitch decreases. The possible explanations of this phenomenon were suggested. It was demonstrated that these mixtures may be used for irreversible recording of optical information.     

Electric field-driven structural changes in cholesteric shells for optical applications

Cholesteric liquid crystal (CLC) shells with planar anchoring condition were prepared by using a capillary microfluidic technology. Under the influence of an electric field, shells were studied by filling them in glass cells coated with transparent ITO working as an electrode. CLC shells with a planar texture having dark brushes at 0 V under crossed polarizers transformed into a focal conic domain, a fingerprint texture, and finally to an isotropic phase with an increase of electric field that was confirmed by inserting the Bertrand lens in the polarized light path. In addition, our studies also show that the electric-field-driven isotropic phase was due to dielectric heating and local heating effects. The calculated magnitude of the local heating result from the model was in agreement with the experimental observation together with the chiral nematic to isotropic transition due to conventional heating.     

Enhancement of the optical absorption in cholesteric liquid crystals due to photonic effects: an experimental study

An anomalous strong optical absorption was measured in a cholesteric liquid crystal (CLC) at both edges of its photonic band gap. The experiment was carried out by studying the luminescence generated by the CLC sample doped with a small amount of fluorescent dye. The material was excited with monochromatic light at different angles of incidence and polarisations. Clear peaks were found in the luminescence response at angles for which the pumping wavelength coincides with the positions of the gap edges. The effect is especially noticeable for excitation under circularly polarised light of the same handedness as that of the CLC helix, and it is the highest at the long-wavelength edge. The modification of the absorption is originated by the helicoidal (photonic) structure of the material, which drastically influences the propagation of electromagnetic waves at certain frequencies and polarisations. The results were analysed numerically using an extension of the Berreman method that incorporates absorption effects. Good agreement with the experiment was found.     

Lasing Spectra of Dye Doped Cholesteric Liquid Crystals Under Reversible Phototuning of the Frequency

We investigate the lasing frequency phototuning in a distributed feedback (DFB) laser based on cholesteric liquid crystals (CLC). To improve the CLC planar texture quality, we use new materials such as nematic ZhK-654 and an analog of cholesteryl oleate as a twisting additive. The selective excitation of the cis-form molecules of the azoxy-nematic is carried out with the use of a combination of an interference filter with maximum transmission at 436 nm and a cut-off filter. Using the orienting substrates with SnO₂ allows us to reduce the threshold pump power by more than an order of magnitude. The proposed combination of filters for the selective excitation enables us to align the ranges of lasing wavelengths in both directions.