Deformed helix mode , ferroelectrics

Beresnev, L., Chigrinov, V. G., Dergachev, D. I., Poshidaev, E. P., Funfschilling, J., and Schadt, M., Deformed helix ferroelectric liquid crystal display a new electrooptic mode in ferroelectric chiral. smectic C liquid crystals, Liq. Cry.st., 5, 1171-1177 (1989). 

It should be noted that phenomenologically this effect is analogous to the deformed helix electrooptical mode observed in ferroelectric liquid crystals where coupling of an electric field with the spontaneous (instead of flexoelectric) polarization is used [83, 84]. 

The scattering effects observed during the deformation of the ferroelectric helix have not yet been satisfactorily investigated [115]. For instance, one should explain the correlation between temperature dependence of the helix pitch and intensity of the scattered light [113], as well as the effect of FLC physical parameters on the response times and hysteresis behavior of transmission-voltage characteristics. Moreover, these effects have not been studied in commercial FLC mixtures operating at room temperature. Nevertheless, these electrooptical modes might be useful for applications in nonpolaroid FLC displays for realization of the optical memory, etc. 

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 orientations of the molecules of the FLC materials are classified by the presence or absence of a helical structure. The most famous FLC device is the SSFLC (Surface Stabilized FLC) [1], in which the helical structure of the FLC material is unwound. While a variety of molecular orientations have been applied in SSFLC devices, three molecular orientations appear to be the most useful in practical FLC displays.These are the bookshelf-layered structure and the Cl-uniform (CIU) and the C2-uniform (C2U) orientations [2]. Each of these structures shows monostability or bistability, depending on the material and its alignment properties. The monostable orientations are applicable to active matrix FLC displays while the bistable orientations are applicable to passive matrix FLC displays. FLC displays with a helical orientation have also been investigated. One useful FLC mode with the helical orientation is the DHF (deformed-helix ferroelectric) mode [3]. This mode is monostable and is thus suitable for an active matrix drive method. 

In the case of a helical pitch shorter than the wavelength of visible light, colouration due to selective reflection from the helical structure disappears and the rotation of the optical axis by deformation of the helical structure can be used for optical switching. This mode is called the Deformed-Helix Ferroelectric (DHF) liquid crystal mode. The DHF mode can realize stable continuous grey scale. Moreover, the viewing angle dependence of the contrast is small even in... 

Fig. 6.1.2 The DHF (Deformed Helix Ferroelectric) liquid crystal mode.

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

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