Cholesterics light scattering

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 3.5.3 Chiral liquid crystals without a chromophore may reflect colors because of light-scattering effects in helices of 400-700 nm pitches. Models of the cholesteric phase and the Schadt-Helfrich cell for liquid crystal displays are given. Two perpendicular polarization filters let light pass only if its direction of polarization has been rotated by the liquid crystals. If the hquid crystals are destroyed by an electric field, no light is transmitted, because the crossed polarizers quench it.

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

When supertwisted, i.e., twisted up to an angle exceeding 90, liquid crystal cells were known [31], but the importance of these structures for display applications was not clear. It was shown that the electrooptical response of supertwisted cells almost always revealed a hysteresis behavior due to the appearance of transient light scattering domain structures [119, 120]. When studying the early works devoted to the electrooptics of cholesteric structures we could imagine that domain appearance is their intrinsic feature and cannot be avoided [121]. However, this is not true and might have been discovered before 1980 if the results of [119, 120] could have been analyzed more carefully. 

Another type of bistability, called in [57] thermodynamical bistability, is observed, if one of the bistable states is disordered, resulting in strong light scattering [60-64]. Consider, for example, unwinding the cholesteric structure with a homeotropically oriented director at the boundaries. Fig. 

It is clear that the transition from a nematic (or cholesteric) to a smectic phase will result in the divergence of certain of the elastic constants. In particular, k22 and 1 33 will diverge at the N-SmA phase transition. The type of divergence observed will depend on the nature of the phase transition, which can be either first or second order [143, 144]. The transition is second order if the nematic phase is sufficiently wide, such that the nematic order parameter is saturated at the transition. Both de Gennes [145] and McMillan [146] developed theories of the SmA-N phase transition that have implications for light scattering. The form of the divergence of the twist and bend elastic constants can be written as ... 

Figure 13. Three states of a cholesteric phase change device operated in light scattering mode. The grandjean texture (a) and homeotropic field-on state (b) are optically clear the focal-conic state (c) represents an alternative field-off state, which is optically scattering.

T. Harada, P. P. Crooker, Light scattering through the Isotropic-Cholesteric phase transition of a Cholesteric liquid crystal, Phys. Rev. Lett. 1975, 34, 1259-1262. 

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

C. C. Yang, Light Scattering Study of the Dynamical Behavior of Ordering Just Above the Phase Transition to a Cholesteric Liquid Crystal, Phys. Rev. Lett., 28, p. 955 (1972). 

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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