Nematic operating modes

In LCDs various LC modes of operation are applied like twisted nematic (TN) [5], super twisted nematic (STN)[6], vertically aligned nematic (VAN) [7,8], optically compensated birefringence (OCB) [9] and in plane switching (IPS) [10-12]. The LC mode used depends on the demands of a specific application, like the viewing angle, power consumption and manufacturing cost. 

Liquid crystal displays (LCDs) have many advantages over other display types. They are flat and compact, possess extremely low-power consumption (microwatts per square centimeter in the case of the twisted nematic effect), their color and contrast does not fade with an increase in the illumination intensity, they work both in transmissive and reflective modes in a wide operating temperature range and with a long lifetime. Besides that, LCDs are the most economically produced flat display systems. LCDs have... 

Table 8.7 shows that the parameters of the prototype light valve (CdS-nematic) are much worse than that of the a Si-FLC device. The operation speed of the latter comes closer to the solid electrooptical crystal modulator (PROM), but with a considerably higher resolution. Liquid crystal light valves on a Si-FLC operate using the Clark-Lagerwall mode [21], the electroclinic eflFect [22], or the deformed helix ferroelectric effect [24]. The operation speed in the two mentioned cases could be 10-100 times faster than mentioned in Table 8.7. 

The turn-on time of presently available liquid crystals operating in the dynamic scattering mode is relatively long. According to Kochlman and Felici [101] the turn-on time of a nematic layer is given by the expression... 

As the rate of twist in a liquid crystal cell increases, the ability of the layer to rotate the plane of polarization of light is diminished. The light becomes elliptically rather than linearly polarized, and the major axis of the ellipse is rotated. This is the situation in the liquid crystal cells used for the super-twisted nematic display. Such displays are therefore normally operated in a variable birefringence mode [25]. In order to allow for the optical rotatory power of the liquid crystal layer, special choices of the optical thickness of the layer and the angles at which the polarizing filters are set allow efficient... 

Figure 1. Configuration and off- and on-state of a normally white mode operated twisted nematic (TN) cell.

Since the LCD was first developed, there have been many variations on the simple display mode shown. Different combinations of molecular orientations, surface treatments, and electric fields have yielded new, faster displays with higher contrast ratios. The liquid crystal material itself has also been highly optimized by the synthesis of a host of different liquid crystal molecules and the preparation of finely tuned liquid crystal mixtures. In this book, we do not discuss the many iterations of LCD improvement. Instead, we discuss one additional mode for the LCD, the twisted nematic display, as this is one of the most commercially successful and long-lasting modes of operation. 

For chiral nematic liquid crystals, the method outlined previously for a planar nematic cell has been shown to be quite effective. For smectic-A the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for a ferroelectric liquid crystal (FLC), smectic-C for surface stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabi-lization. f On the other hand, smectic-A (Sm-A ) cells for soft-mode FLC (SMFLC) operation are easier to prepare using the methods described above. ... 

---