Problems of Bistability Realization

The existence of two or more thermodynamically stable states with different optical transmission is a very important feature of FLC cells. As mentioned above, bistable switching or bistability is realized in the Clark-Lagerwall effect. In this section, we will consider the influence of physical parameters and cell configuration on the appearance of the phenomenon. Reliable reproduction of bistability conditions is very crucial for technological applications. 

FIGURE 7.26. Electrooptical switching of FLC cells for different polar anchoring energies Wp. Short pulses of the electrical field E with different signs are applied 

A new operation mode was proposed [74] with asymmetric surface conditions (polar anchoring is zero to one substrate and assumes a finite value to the other). It was shown by computer simulation that the splayed and uniform bistable states appeared. Polarization charges strongly affect the director distribution in the splayed state and define optimum FLC cell thickness which, in this case, could be more than 4 /x. In [74] a possible theoretical interpretation is given of the earlier experimental results [156], where bistability was observed in rather thick (4-14 /i) cells with asymmetrically treated substrates (see also [157]). 

Methods of computer simulation show that the electric field threshold of bistability (7.74) depends both on polar and dispersion anchoring strength coefficients [75, 158]. With increasing anchoring we have to increase the switching amplitude of the electric field (Fig. 7.27). For Wj = 0 it is possible to write the following relationship for the bistability threshold [75]  

FIGURE 7.27. Bistability diagram in the anchoring coordinates wp — WpdfK, Wd = WddfK) for different values of the reduced electric field e = jK [158]. 

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