Scattering in Cholesterics

In terms of their optical properties, a prominent feature of cholesterics is the helical stmcture of their director axes. Such helicity gives rise to selective reflection and transmission of circularly polarized fight. These processes may be quantitatively analyzed, using the electromagnetic approach given in the next section and in Chapter 7. We begin here with some general observations. 

On the other hand, an incident left circularly polarized fight will be totally transmitted (Fig. 4.3b). In the case of oblique incidence a similar analysis shows that higher-order diffractions are possible. The Bragg diffraction becomes 

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R. M. Hornreich and S. Shtrikman, Theory of light scattering in cholesteric blue phases, Phys. Rev. A, 28,... 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

FIGURE 6.17. Thermodynamic bistability in cholesteric liquid crystals with homeotropically oriented director at opposite boimdaries (a) scattering state (b) transparent homeotropic state and (c) intensity curve of the cell, placed between crossed polarizers, Ae > 0. 

Despite all the drama at the cellular interface when e < 3.5 (Figure 15.2), q is linear in e q/qo = 0.19fi -I- 0.29, independent of the dynamics. On the other hand, Vx has two responses. When <2, Vx/Vei = 0.32e + 0.3 the cholesteric texture left by the traveling phase boundary is unstructured and there are no director relaxation processes observed when v 0. When e> 2, xhei = 0.60e + 0.03 Ex increases twice as fast as when < 2. The texture left behind the traveling interface is a uniform array of line defects (one line per cell) that slowly disappears by defect coarsening when y 0 [1]. We note that taking into account the scatter in the data, the intercept of Vx s e > 2 line is zero and that it intersects its < 2 line at 1 where the breathing mode starts. 

The texture change or memory effect is observed in cholesteric materials with negative dielectric anisotropy [71]. The liquid crystal layer is homogeneously oriented by boundary forces to form the planar texture which is completely transparent if the band of selective light reflection is outside the visible spectrum. The substrates are covered with conducting films that are in contact with the liquid crystal. When a d.c. or low frequency field is applied, the sample is transformed to the so-called focal conic texture. In this texture, the liquid crystal is broken up into small domains which are randomly oriented and have diameters of a few microns. Since these domains are optically anisotropic, they act as scattering centers for visible light. Therefore the focal conic texture exhibits a milky white appearance. 

J. D. Parsons, C. Hayes, Fluctuations and light scattering in a compressible cholesteric liquid crystal. Mol. Cryst. Liq. Cryst. 1975, 29, 295-309. 

B. Kellenevich and A. Coche, Relaxation of Light Scattering in Nematic-Cholesteric Mixtures, Mol. Cryst. andLiq. Cryst, 24, p. 113 (1973). 

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