Electro-Optical Effects in Cholesteric Phase

Cholesteric liquid crystals show many electro-optic effects. Among them the cholesteric-nematic phase transition effect is the most interesting one which was addressed in the early part of this chapter. Others are the square grid effect, storage effects and color effects, etc. 

In this chapter we shall only be concerned with electro-optic and thermo-optic switching effects in thermotropic side-chain polymer liquid crystals. We will consider briefly the synthesis and structure of such compounds and show how the nematic, cholesteric and smectic phases arise. Since the optical properties of each of these phases are different, and may be altered depending on alignment within the phase, each gives rise to different electro-optic effects. If these are coupled to the use of dye additives or substituents, then it will be realized that a wide range of electro-optic devices based on dichroism or fluorescence as well as birefringence or scattering power may be fabricated. These will be considered and discussed in terms of their performance and potential applications. Finally, possible uses of polymer-low molar mass liquid crystal solutions will be considered in terms of electro-optic device applications. 

Display devices can also be constructed using the field effect, the cholesteric memory effect and the cholesteric-nematic phase change effect [259, 262]. The recognition of the useful electro-optical properties of liquid crystals has stimulated efforts in synthesis of new mesomorphic materials. Today, more than 6000 compounds are available but an ideal liquid crystal is still elusive. 

Fig. 6.24 The electro-thenno-optic effect of nematic-cholesteric LC mixture MBBAiEBBA CN = 45 45 10. The material here is same with the thermo-optic effect [90]. The transmission variations are enhanced by the electric field application. The scattering centers in the nematic phase are turned to a clear state with the electric field application and then the irradiated portion becomes clear. Thus, the dynamic figures can be displayed...

Fig. 6.25 The electro-thermo-optic effect of smectic-cholesteric mixture COB CN = 90 10. The light transmission is greatly decreased by the electric field application [91]. The light scattering centers are created in the smectic phase by the irradiation on to a part of the LC cell, and they are erased by the electric field application and become clear. This characteristic can be applied to a dynamic display. M mesophase, I isotropic phase...

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