Twisted nematic electrooptic effect

The majority of current commercial liquid crystal displays are based on the twisted nematic electrooptic effect using active matrices to give, for example, complex computer lap-top screens [13]. It is often overlooked that these displays are based on long pitch chiral nematic materials to remove so-called reverse twist, and the original papers [254, 255] referred to the effect as a positive planar cholesteric to nematic phase... 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

For a nematic LC, the preferred orientation is one in which the director is parallel everywhere. Other orientations have a free-energy distribution that depends on the elastic constants, K /. The orientational elastic constants K, K22 and K33 determine respectively splay, twist and bend deformations. Values of elastic constants in LCs are around 10 N so that free-energy difference between different orientations is of the order of 5 x 10 J m the same order of magnitude as surface energy. A thin layer of LC sandwiched between two aligned surfaces therefore adopts an orientation determined by the surfaces. This fact forms the basis of most electrooptical effects in LCs. Display devices based on LCs are discussed in Chapter 7. 

Certain aryl amines are endowed with unusual electronic properties. Schiff s bases such as 4-methoxybenzylidene-4-n-butylaniline were the original experimental chemicals used in the early development of electrical and electrooptical displays. Liquid crystal display (LCD) technology relies on twisted nematic or supertwisted nematic effects. Later nematogen development was based on cyanobiphenyls103. 

Most of the electrooptic effects are based on light scattering or on light absorption by polarizers or by dissolved dyes. Electrooptic effects which belong to the second group are tunable birefringence, the twisted nematic effect and the guest-host effect. 

The surface flexoelectric energy, which is found from (4.2) and (4.3). Attaining the minimum of the nematic free energy, (4.5) or (4.6), it is possible to derive the equilibrium director distribution in a static case. To find the response times, we have to solve the equations of nematodynamics in the electric field. The corresponding analysis shows that the director reorientation is always accompanied by the macroscopic flow, the so-called backflow [5]. (The only exclusion is the pure twist rotation of the director [1].) Backflow considerably affects the characteristic times of the electrooptical effects in uniform structures, especially in the case of strong deformations of the initial director orientation [3, 5]. 

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. 

In Fig. 62 this linear electrooptic effect is compared with the quadratic effect controlling the state of a twisted nematic device. 

The electrooptical (or magnetooptical) effect corresponding to the geometry in Fig. 4.4(c) is called the T-effect. It is caused by a pure twist deformation and is most amenable to theoretical analysis. Unfortunately, it is less suitable for experimental investigation, since the field induces optical biaxiality in the nematic liquid crystal. The effect, for which the relaxation kinetics were considered in Section 4.1.6, must not be confused with the twist effect which includes combined T, S, and B deformations (to be discussed in Section 4.2). 

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