Liquid crystal display ferroelectric

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Polymerization of reactive monomeric liquid crystals is one method for stabilizing the liquid-crystalline thin films. Another approach is to form chemical gels of liquid crystal molecules with low molecular weight by construction of a polymer network. This method has been investigated for the stabilization of ferroelectric liquid crystal displays. Guymon et al. reported that a polymer network produced by photochemical cross-linking accumu-... 

By appropriately installing two polarizers on two surfaces of the cell the bright/dark states can be obtained by changing the polarity of the applied voltage. The response of the liquid crystal cells is much faster than other liquid crystal displays. The response time is inversely proportional to the spontaneous polarization Ps and applied electric held E, and is linear in proportion to viscous coefficient 77. It is typically tens of microseconds. In comparison, the relaxation time is generally tens of milliseconds for other liquid crystal displays. The ferroelectric liquid crystal display exhibits the... 

Figure 6.38. The chiral smectic C phase (a and b) and the ferroelectric liquid crystal display (c and d).

Microphase Stabilized Ferroelectric Liquid Crystal Displays... 

The spontaneous molecular polarization of ferroelectric liquid crystals, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. A low electric field of only a few volts can switch the ferroelectric liquid crystal between two equally stable states with opposing polarization directions. This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the ON state. The power required to run ferroelectric liquid crystal displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, ferroelectric liquid crystals can switch hundreds of times faster than a... 

Ferroelectric liquid crystals also show high resolution due to the very thin gaps allowed by their mode of action. In the transmission mode, they are typically less than 2 xm thick, compared to 4-8 xm in nematic LCDs. In reflective ferroelectric liquid crystal displays, this thickness is reduced by half, resulting in a thickness of less than 1 jxm. Thus, pixels as small as 5 xm have been demonstrated. 

The switching of the director in the surface stabilised ferroelectric liquid crystal cells (SSFLC) [8] has briefly been discussed in Section 13.1.2. Due to its importance for ferroelectric liquid crystal displays we shall discuss this effect in more detail. The geometry of a planar cell of thickness d is shown in Fig. 13.1.2. Now, the helical structure is considered to be unwound. We are interested in the field and time behaviour of the director or c-director given by angle cp(r), and this process is considered to be independent of z and y- coordinates. The smectic C equilibrium tilt angle 9 is assumed constant. 

Figure 4.9 Schematic diagram of the bookshelf cell structure of the surface-stabilized ferroelectric liquid crystal display, (a) The director is along the direction Ai when the applied field is up. (b) The director is along the direction Ai when the applied field is down, (c) Directions of the polarizer and analyzer.

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). 

L.A. Beresnev, R. Buchecker, N.I. Chernova, V.G. Chigrinov, J. Funf-schilling, M.V. Loseva, Yu.P. Panarin, E.P. Pozhidaev, and M. Schadt, Bistable Ferroelectric Liquid Crystal Display Cell, USA Patent No. 5,327,273. Date of patent July 5, 1994. 

In typical FLC (Ferroelectric Liquid Crystal) displays, the FLC material is sandwiched between two substrates separated by around 2 pm. A number of molecular orientations have been found to be suitable for use in FLC displays and investigated. Some of these are summarized in Fig. 6.1.1. 

Ferroelectric liquid crystal displays have potential as very fast displays and also do not require active matrix addressing technology. Due to their fast response times, they also have potential applications as high-speed electro-optical shutters or spatial light modulators. However, due to fabrication... 

The ferroelectric smectic C liquid crystal display has not, at the time of writing, achieved extensive commercial use. It nevertheless stands as an important device, both because of its potential application in complex displays, which will not require an active matrix, and because of its intrinsic scientific interest. In addition, ferroelectric liquid crystal displays show faster switching rates (of the order of microseconds) than conventional nematic-based displays. 

On a macroscopic scale, the spontaneous polarization vector in the optically active phase spirals about an axis perpendicular to the smectic layers (Fig. 20), and sums to zero. This macroscopic cancellation of the polarization vectors can be avoided if the helical structure is unwound by surface forces, by an applied field, or by pitch compensation with an oppositely handed dopant. The surface stabilized ferroelectric liquid crystal display utilizes this structure and uses coupling between the electric field and the spontaneous polarization of the smectic C phase. The device uses a smectic C liquid crystal material in the so-called bookshelf structure shown in Fig. 21a. This device structure was fabricated by shearing thin (about 2 i,m) layers of liquid crystal in the... 

B. Bahadur, Supertwisted Dye Effect, Super-twisted Birefringence and Ferroelectric Liquid Crystal Displays, Litton Data Images Report, March 1986. 

Illustration of the Deformed HeKx Ferroelectric Liquid Crystal Display (DHFLCD) mode. 

N. Yamamoto, Y. Yamada. E Mori, R Orihara, and Y. Ishfeashi. Ferroelectric liquid crystal display with high contrast ratio. Jpa. J. AppL Pkys. 28324 (1989). 

Y. Sato, T. Tknaka. H. Kobayaahi. K. Aoki. R Wsianabe, R Ihkeshiia. Y. Ouchi. R Thk ezoc. and A. Pukuda. High quality ferroelectric liquid crystal display with quasi-bookshelf layer structure, Jpa. J. AppL Pkys. 28.L483 (1989). 

M. Isogai, K. Kondo, T. Kitanum, A. Mukoh. Y. Nagae, and R Kawakami. Bistable ferroelectric liquid crystal display device using conventional ducknesa cell, Proc. 6th InL Disp Rex Conf. "Japan Display S6," Ttkyo, 1986, pp. 472>474. 

Y. Yatnada, N. Yamamoto, K. Mori K. Nakamura. T Hagtwara, Y. Suzuki 1. Kawamura. R Orihara, and Y. IshibBslii. Ferroelectric liquid crystal display usiiig tristable swiiching. Jpn AppL Phys. 29 1757 (1990). 

The realization of this device geometry was first applied in 1980 in the surface-stabilized ferroelectric liquid crystal display and provided much faster switching times than the nematic devices of the time (<0.1 ms) however, the main drawback of the smectic device has been the stability of liquid crystal alignment within the pixels. Nematics are very fluid-like, and after a deformation, they rapidly revert to their previous uniform state of alignment (think about what happens when you press on your laptop screen). Smectics are much more viscous and unfortunately do not self-repair when deformed. 

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