Nematic liquid crystals device applications

Liquid crystals have interesting electro-optical properties. When subjected to small electric fields, reorientation and alignment of the liquid crystal molecules takes place, which produces striking optical effects because light travels more slowly along the axes of the molecules than across them. This has led to their use in optical display devices for electronic instruments such as digital voltmeters, desk calculators, clocks, and watches. Nematic liquid crystals are most commonly used in these applications. Cholesteric materials are added to provide memory effects. 

Room temperature nematic liquid crystals have been developed for electro-optical applications [13-15]. In particular, twisted nematic (TN) liquid crystal displays have been widely used for practical display devices [ 13-15,38). In the TN cells, nematic liquid crystals form twisted alignment due to the influence of rubbed aligmnent polymer layers coated on the substrates (Fig. 7a). The TN cells are placed between two crossed polarizers. Without electric fields, the twisted LC aligmnent induces optical rotation of incident polarized... 

The initial research on electro-optic phenomena in side-chain polymer liquid crystals concentrated on systems that exhibited nematic phases so that a ready comparison could be made with low molar mass mesogens. Such measurements have established that electro-optic devices are feasible and have allowed elastic constants to be deduced from applications of the continuum theory. This theory, originally derived for low molar mass nematic liquid crystals, defines a relationship for the free energy density F in terms of the elastic constants (/ ) and the director n such that ... 

Planar-aligned nematic liquid crystals also display a nonlinear birefringence which has been used to produce switches in both transmitted power and polarization state. This, allied with their dielectric anisotropy and thus sensitivity to applied electric fields, permits their application to a wide range of optical/optoelectronic devices. 

This effect has been observed experimentally in comparatively thick cells (d 50 /xm) [113]. In cells with d 20 /xm, the final twisted state (in the field) proves to be insufficiently stable and the nematic liquid crystal layer is gradually transformed into a planar structure. The addition of small quantities of cholesteric liquid crystals to the initial nematic mixture enables a stable twisted structure to be achieved with the application of a field and improves the electrooptical characteristics of the device. The electrooptical response of electrically induced twist nematic cells includes intensity oscillations observed both in the switching on and switching off regimes [114]. These oscillations take place due to the variation of birefringence, which are not important in the usual twist effect. 

The FLCs or AFLCs mentioned above are liquid crystalline materials showing a chiral smectic C phase or related phases. Although nematic liquid crystal possesses only directional order, smectic liquid crystals shows layer structures or periodic order of the liquid crystalline molecular centres. In this respect, smectic liquid crystalline materials have more in common with crystalline materials than with nematic liquid crystalline materials. As a result, the alignment of smectic liquid crystal is quite different from that of nematic liquid crystals. Smectic liquid crystals show a large variety of defects because they possesses highly ordered structiu-es. Moreover, the layer structures are irreversibly destroyed by applying stress. This phenomenon poses a major problem for display applications. Therefore, several techniques to prevent the application of force to the liquid crystalline materials of FLC or AFLC devices have been proposed. 

One of the intrinsic problems for the practical application of FLC displays is the low mechanical stability of the molecular orientations. The initial molecular orientations in an FLC material are easily destroyed by the application of mechanical pressure and/or mechanical shock. The materials do not return to their initial states, unlike molecular orientations in nematic liquid crystals, which are also disturbed by mechanical pressure and/or shock but usually return to their initial states. The low stability of FLC devices under exposure to shock is attributed to the presence of the smectic layer structure. The molecular orientation of the smectic phases is highly ordered in comparison with that of the nematic phase. 

Display devices based on nematic liquid crystals, notably small liquid crystal televisions, are used as SLMs due to their increased availability [1]. However, in most of the above applications the speed of operation is important, and nematic liquid crystals are too slow, so the emphasis here is on SLMs that use the faster electrooptic effects to be found in chiral smectic liquid crystals. We will start by looking at how these interact with light. 

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