Cholesteric liquid crystals, optical properties

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Several unique optical properties arise from helical structures of cholesteric liquid crystals. 

These unique optical properties of cholesteric liquid crystals have been investigated... 

Electronic displays make use of the fact that the orientation of the molecules in liquid crystals changes in the presence of an electric field. This reorientation causes a change in their optical properties, making them opaque or transparent, and hence forming a pattern on a screen. Cholesteric liquid crystals are also of interest because the helical structure unwinds slightly as the temperature is changed. Because the twist of the helical structure affects the optical properties of the liquid crystal, these... 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Other equally remarkable optical properties are associated with the selective reflection. At normal incidence, the reflected light is circularly polarized one circular component is totally reflected, while the other passes through unchanged. Also, quite contrary to what is found in normal substances, the reflected wave has the same sense of circular polarization as that of the incident wave. This is an important difference between the nature of the optical rotation of normal substances and of cholesteric liquid crystals. While the more familiar cases of optical rotation have their origin in the selective absorption of one circularly polarized component of the light, the form optical rotation of the twisted structure in cholesteric liquid crystals originates in the selective reflection of one circularly polarized component of the light. 

Fig. 7. Optical properties of right-handed cholesteric liquid crystals...

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

As mentioned in Chapter 1, cholesteric liquid crystals have a helical structure. The helical structure results in unique optical properties, such as selective reflection, circular dichroism, drastic optical activity, and electro-optic effects. 

The pitch P is the most important parameter of cholesteric liquid crystals. The physical properties of cholesteric liquid crystals are associated with P, such as selective reflection, optical rotation dispersion, circular dichroism, etc. The helical pitch is sensitive to the temperature and external field, for example, electric and magnetic field, chemical environment, pressure or radiation, etc. 

Penzien K. and Schmidt G.M. (1969) Reactions in chiral crystals an absolute asymmetric synthesis, Angew. Chem. Int. Ed. Engl. 8, 608-609. Saeva F.D., Sharpe P.E. and Olin G.R. (1975) Asymmetric synthesis in a cholesteric liquid crystal solvent, J. Amer. Chem. Soc. 97, 204-205. Thiemann W. and Teutsch H. (1990) Possible amplification of enantiomeric excess through structural properties of liquid crystals-model for origin of optical activity in the biosphere. Origin ofLife Evolution of Biosphere 20, 121-126 1986,16,420. 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

Cholesteric liquid crystals are periodic optical media. When doped with fluorescent dyes, they can be used to make cavity-free lasers [ 14,15]. In lasers, one of the important properties is spontaneous emission rate W, which is proportional to the density of states p, as pointed out by Purcell [16]. The density of states function is given by... 

The optical properties of cholesteric liquid crystals are very specific and are determined by the pitch and arrangement of the axis of the helix and the polarization of the incident light. In an external field changes occur both in the direction of the axis of the helix (texture transitions) and in its pitch (untwisting of the helix). Before considering these field variations let us give a brief account of the optical properties of cholesteric liquid crystals in the absence of a field. Comprehensive reviews of the topics have been given recently in [1, 2]. 

The optical properties of cholesteric liquid crystals have been studied extensively [23, 24]. Light propagation in helical cholesterics is governed by the spatially periodic relative dielectric tensor, whose eigenvalues are C and i for the electric field parallel and perpendicular to the director n. It can be written as... 

Optical Properties of Cholesteric Liquid-Crystal Films... 

The encapsulation process for ChLC is mainly attributed to its transport and optical properties [15]. Firstly, since viscosity of pure ChLC is close to that of water, its fluidity prevents ChLC from being coated on flexible substrates. Secondly, when a cholesteric liquid crystal is pressed, the flow generated inside makes the displayed image erase. Therefore, droplet dispersions by encapsulation act as a protector for its bi-stability and optical properties. The additional advantage is that encapsulated cholesteric liquid crystals are self-sealing the materials confined to the droplets cannot flow through an interface of the droplets. 

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