Deformed helix ferroelectric

Fig. 13.11 Deformed Helix Ferroelectric effect. Scheme of observation of the effect (a) and the picture of distortion of the hehcal stmcture (b) in the zero, positive and negative field. P and A are polarizers and analyser, ITO means indium-tin oxide electrodes, Iq and I are intensities of incoming and outgoing beams. Note that at E = 0 the hehx is harmonic, for 0 < l l < anharmonic and for l l > l J unwound...

Helix Distortion and Deformed Helix Ferroelectric effect... 

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). 

The Deformed Helix Ferroelectric (DHF) effect was observed in the very first investigations of FLC structures [1, 92], but the first adequate explanation was given in [93-96]. The geometry of the FLC cell with a DHF effect is presented in Fig. 7.18. The polarizer (P) on the first substrate makes an angle / with the helix axis and the analyzer (A) is crossed with... 

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.

This effect is observed in a geometry where the cholesteric axis h is homogeneously oriented in the plane of the cell (along x) and an electric field is applied to the electrodes of a sandwich cell along the z axis [137,138]. In this case, the helical structure, even the ideal one, is incompatible with the planar boundary conditions, and splayed and bended regions form near the boundaries. Thus, according to Eq. (38), the flexoelectric polarization arises in those regions which can interact with the electric field. The distortion is very similar to that observed in the ferroelectric smectic C phase (see Fig. 24) for a so-called deformed helix ferroelectric effect [139]. 

Besides the ambitious efforu to arrive at well-aligned SSFLC structures, some work has been dedicated to other electrooptic effects as are well known with LMM FLCs, such u the electroclinic (SMFLC). the antifenoclectric (AFLC), and the deformed helix ferroelectric effect (DHF). 

Further research work has been devoted to creating new FLC polymers with multifunctional properties. Recently, Scherowsky reported on fluorescent FLC polymers [136]. The endeavor to produce FLC polymers with strong SHG activity will be dealt with in Section VI. So tar we have discussed variouB applications with an eleclrooptic effect aiudogous to the SSFLC Qark-Lagerwall effect observed on LMM FLCs. htoy of these applications are conceivable with other electrooptic effects such as the antifer-roelectric. electrodink. or deformed helix ferroelectric effect, in particular if gray scale is required. 

J. Ffln chilling and M. Schadt, High speed deformed helix ferroelectric (DHFVUquid cryilal displays sirith video potentiaL Proc. I3ih Int. Disp Res. Conf., Strasbourg. France, 1993, pp. 63—66. 

Until the mid-1990s and after 20 years of intense research on nematic field-effect LCDs it was still uncertain whether LCDs and LC materials could indeed meet the short response time requirements and the optical quality required for LCD television. Therefore, parallel to nematic LCD research, strong efforts were made to find effects based on the inherently faster responding ferroelectric liquid crystals (FLCs). Unfortunately, FLCs proved to be difficult to surface-align, rendering them up to now commercially applicable only for niche products such as electronic eye shutters or time sequential LCD projection. FLC examples are the surface-stabilized ferroelectric (SSF)-LCD of Clark and Lagerwall [40] which initiated FLC-LCD development and the deformed helix ferroelectric (DHF)-LCD of Beresnev et al. [41], In 1995 a TFT-addressed black-white DHF-LCD television prototype with 20 ps response time and broad field of view was developed by the author and coworkers in collaboration with Philips [42] (Fig. 6.5a). 

Figure 6.10 Light transmission spectra of 5 p.m DHF-FLC cell placed between two crossed polarizers versus applied voltage at a frequency of color switch of 100 Hz the horizontal size of the micrographs is 500 p.m [19]. Reproduced from E. P. Pozhidaev, G. Hegde, P. Xu, and V. G. Chigrinov, Electrically controlled birefringent colors of smectic C deformed helix ferroelectric liquid crystal cells. FLC 07 Program, FLC International Conference on Ferroelectric Liquid Crystals, 0-36 (2007)...

E. P. Pozhidaev, G. Hegde, P. Xu, and V. G. Chigrinov, Electrically controlled birefringent colors of smectic C deformed helix ferroelectric liquid crystal cells. FLC 07... 

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