Twisted nematic device electric field effects

It should be mentioned that Fraser [92] has carried out a theoretical investigation of the twisted nematic device that incorporates surface pretilt of the director and electric field effects. Much of the analysis is naturally extended from the ideas presented on pretilt in Section 3.4.2 and electric field effects in Section 3.5. 

The kickback effect can have serious implications in twisted nematic devices (cf. Section 3.7 above). Examples of such complications in connection with experiments and displays have been reported and can be found in the brief article by Gerritsma, van Doom and van Zanten [111]. Numerical studies showing the influence of back-flow in the twisted nematic cell under the application of a strong electric field being switched on or off have been carried out by van Doom [64]. 

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. 

With the help of Fig. 3.18, we are now in a position to illustrate briefly how a twisted nematic display operates. For more details on this and other effects related to liquid crystal displays the reader is referred to the review articles by Schadt [242] and Scheuble [243]. This type of device, based on the experiments involving electric fields carried out by Schadt and Helfrich [241], was patented for technological applications by these authors [125] in 1970 and by Fergason [84] in 1971. Fergason also went on to develop other patents on twisted nematic displays, most notably one issued in 1973 [85]. 

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