Supertwist Effects

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 effects of bistability and hysteresis in supertwisted nematic layers were first investigated in [122]. To obtain twist angles larger than 90 , nematics were doped with a small amount of an optically active material. Thus a cholesteric (or chiral nematic) with a large pitch P was created, so that the pitch value had to adjust the boundary conditions for the directors on the substrates. The corresponding texture was first discovered by Grandjean and is discussed in Chapter 6. In 1984 the display based on the Supertwist Birefringent Effect (SBE) was proposed [123]. 

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. 

Transmission spectra [98, 124] for the blue (B) and yellow (Y) modes in the on and off states are shown in Fig. 4.21. The considerable wavelength dependence of the transmission of the SBE display imposes certain limitations to their applications. 

FIGURE 4.20. The geometry of the SEE display, (a) An SEE cell with the directors twisted at an angle of 270 L and L2 show the projection of the directors on the two substrates) is placed between (P) polarizer and (A) analyzer oriented at angles and 7 with respect to Li and L2, respectively, (b) Distribution of the director angles inside the supertwist cell in the off and on states, 0 is the director pretilt angle and ( m is the maximum rotation angle. 

---

This book was conceived as a renewed version of the earlier published original book, Electro-Optical and Magneto-Optical Properties of Liquid Crystals (Wiley, Chichester, 1983) written by one of us (L.M. Blinov). That book was first published in Russian (Nauka, Moscow, 1978) and then was modified slightly for the English translation. Since then new information on electrooptical effects in liquid crystals has been published. Novel effects have been discovered in nematics and cholesterics (such as the supertwist effect), and new classes of liquid crystalline materials, such as ferroelectric liquid crystals, appear. Recently, polymer liquid crystals attracted much attention and new electrooptical effects, both in pure polymer mesophases and polymer dispersed liquid crystals, were studied. An important contribution was also made in the understanding of surface properties and related phenomena (surface anchoring and bistability, flexoelectricity, etc.). 

The current (third) period, which may be called a colonization, involves wide electrooptical investigations of novel effects in ferroelectric liquid crystals [9, 10] and a study of exotic materials like polymeric and lyotropic mesophases, blue phases in cholesterics, well-ordered smectics, and so on. For conventional (nematic and cholesteric) phases the accent was shifted to the optimization of the material properties for electrooptical devices, though novel phenomena like the supertwist effect [11] and a gamma of linear electrooptical effects [12-14] have also been discovered. 

Figure 6. Mid-cell tilt angle versus voltage for a range of cells having different twist angles, showing the increase in slope and onset of bistability resulting from the supertwist effect. The liquid crystal is E7 in each case, with the pitch length matched to the twist angle.

The key to the origin of the supertwist effect, which leads to increased threshold steepness, is that at large twist angles where the cell becomes bistable, the energy of the... 

Scheffer T J, Nehring J, Kaufmann M, Amstutz H, Heimgartner D and Eglin P 1985 24 80 character LCD panel using the supertwisted birefringence effect Dig. Tech. Papers Int. Symp. Soc. Information Display 16 120-3... 

This supertwisted nematic display utilizes a twist of the Hquid crystal director of 270° within the ceU rather than 90° (45). The basic operation of the ceU is unchanged ia that the effect of the analyzer on light which has been rotated by 270° is the same as for 90° rotation. 

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... 

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. 

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. 

Thus the supertwist birefringent effect was a step forward to the development of a new generation of matrix addressed displays with very high information contents and excellent viewing characteristics. However, several characteristics of the SBE mode need further improvement. They are as follows ... 

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

TABLE 4.8. Effect of various chiral nematic parameters and cell geometry on the electrooptical characteristics of supertwist cells. 

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. 

As described above (Chapter 4), due to the birefringence effect, the first versions of supertwisted displays operated in two-color modes, i.e., yellow and blue. It was not very impressive and strongly limited their applications. Since 1987 several black-white variants of supertwist displays were proposed. Three of them are shown in Table 8.2. 

---