Structures supertwist

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

SEE Supertwist birefringence effect device effect in a 180° to 270° twisted nematic structure that uses a combination of interference and polarization guiding to achieve an optical effect with sharp threshold behavior, making it particularly suitable for multiplexing. 

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. 

The geometry of a SBE display is typical for the supertwisted displays (Fig. 4.20). A 270 supertwisted nematic layer is oriented with a 28 director tilt at the boundaries to prevent the appearance of light-scattering domain structures. In an SBE cell the ratio d/P = 0.75, i.e., three-quarters of the helix pitch, is fitted within the layer thickness d. When the field is switched on the director reorients to nearly homeotropic configuration (dielectric anisotropy Ae > 0). Two polars used in the SBE display are located at angles (3 and 7 with respect to the director projection on the input (Li) and output substrates. 

High contrast and uniformity of transmission characteristics at oblique incidence are also beneficial features of the SBE mode which considerable improve the legibility of supertwist displays. Better viewing angles than that for the 90° twist structure seem to be a peculiar feature of highly twisted chiral nematics. Figure 4.23 demonstrates this for a 200° supertwist cell in comparison with the usual 90° twist cell [127]. 

FIGURE 4.23. Transmission-voltage characteristics, (a) 90° twist nematic structure and (b) a 200° supertwisted chiral nematic. Angles of light incidence i are shown at the curves, = 180° (Fig. 4.15). 

TABLE 4.6. Different types of electrooptical effects in supertwist structures. 

In Table 4.10 we do not consider the guest-host effect in supertwist structures [163], as these modes are less attractive than those for pure supertwist cells without dyes. One of the reasons for this is longer response times and a lower brightness in the off state. 

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