Ferroelectric liquid crystal cell

In a chiral smectic (Sc ) phase, the tilt angle is the same within a layer, but the tilt direction processes and traces a helical path through a stack of layers (Figure 43). It has been demonstrated that when such a helix is completely unwound, as in a surface stabilized ferroelectric liquid crystal cell, then changing the tilt of the molecules fi om +0 to —0 by alternating the direction of an applied field results in a substantial electro-optic effect, which has the features of veiy fast switching (%1 - lOps), high contrast and bistability [87]. The smectic A phase of chiral molecules may also exhibit an electro-optic effect, this arises due to molecular tilt fluctuations which transition is approached, which are combined with a... 

The backbone affects the dynamic behavior of the ferroelectric liquid crystalline polymer. Sandwiching the two kinds of ferroelectric liquid crystals between two ITO-coated glass plates of 1.5 microns gap respectively, one constructs a SSFLC (surface stabilized ferroelectric liquid crystal) cell. The switch time between two optical states r is determined by... 

We can answer the last question if consider a constraction of the so-called surface stabilised ferroelectric liquid crystal cell or simply SSFLC ceU [9]. Such SSFLC cell is only few micrometers thin and, due to anchoring of the director at the surfaces, the intrinsic helical stmcture of the SmC is unwound by boundaries but a high value of the spontaneous polarisation is conserved. The cell is con-stracted in a way to realise two stable states of the smectic C liquid crystal using its interaction with the surfaces of electrodes, see Fig. 13.6a. First of all, in the SSFLC cell, the so-called bookshelf geometry is assumed the smectic layers are vertical (like books) with their normal h parallel the z-axis. Then the director is free to rotate along the conical surface about the h axis as shown in Fig. 13.6b (Goldstone mode). It is important that, to have a bistability, the director should be properly... 

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. 

Johno, M., Chandani, A.D.L., Ouchi, Y., Takezoe, H., Fukuda, A., Ichihashi, M., Furukawa, K. Smectic layer switching by an electric field in ferroelectric liquid crystal cell. Jpn. J. AppL Phys. 28, L119-L120 (1989)... 

Effect of material constants on the orientation structure of ferroelectric liquid crystal cells, Figures 8 and 9, T.C. Chieu, Journal of Applied Physics, 64, p. 6234 (1988). Reproduced by permission of the American Institute of Physics. 

In surface stabilized ferroelectric liquid crystal cells the liquid crystal is sandwiched... 

Figure 19. Director alignment in a ferroelectric liquid crystal cell.

Finally, doping surface-stabilized ferroelectric liquid crystals with charge-transfer complexes, for example, tetramethyltetra-thiafulvalene/octadecyltetracyano-1,4-qui-nodimethane, has been utilized to improve their bistability [37] Ions from the CT complex form an internal electric field in reverse to the applied pulse. By applying this phenomenon, a ferroelectric liquid crystal cell with perfect bistability and inverted memory characteristics was designed [37]. Ono and Nakanowatari developed a method for the determination of the internal electric field and studied a TCNQ doped ferroelectric liquid crystal [38]. 

A. Ptiknda, Y. Ouchi, H. And. R Ihkano, K. Ishikawa, and H. Ihkezoc. Cxwnpicxitics in the structure of ferroelectric liquid crystal cells. The chevron structure and twisted stales. Uq Cryst 5 1055 (1989). 

K. Iloh, M. Johoo, Y. Thkantshi, Y. Ouchi R Tkkezoe, and A. Fukuda, Self-recovery from alignment damage under AC fields in antifcrroelectric and ferroelectric liquid crystal cells, Jpn. J. AppL Phys. 30 735 (1991). 

F. GicBelmann, L Dierking. and P. Zagenmaiet, Strong electroacoustic effect in ferroelectric liquid crystal cells, MoL Cryu. Liq. CrysL Lett 8 103 (1992). 

Sato H, Fujikake H, Lino Y, Kawakita M, Kikuchi H (2002) Flexible grayscale ferroelectric liquid crystal device containing polymer walls and networks. Jpn J Appl Phys 41 5302-5306 Sato H, Fujikake H, Kikuchi H, Kurita T (2003) Rollable polymer stabilized ferroelectric liquid crystal device using thin plastic substrates. Opt Rev 10(5) 352-356 Schrader DM, Jean YC (1988) Positron and positronium chemistry. Elsevier, Amsterdam Shinkawa K, Takahashi H, Fume H (2008) Ferroelectric liquid crystal cell with phase separated composite organic film. Ferroelectrics 364 107-112 Simha R, Somcynsky T (1969) On the statistical thermodynamics of spherical and chain molecule fluids. Macromolecules 2 342-350... 

Takahashi T, Umeda T, Furue H, Kobayashi S (1999) Modelling and computer simulation of the electrooptic response of polymer-stabilized ferroelectric liquid crystal cells. Jpn J Appl Phys 38 5991-5995... 

I. Dahl and S.T. Lagerwall, Elastic and flexoelectric properties of chiral smectic-C phase and symmetry considerations on ferroelectric liquid-crystal cells, Ferroelectrics, 58, 215-243 (1984). 

Xue Jiu-Zhi, M.A. Handschy and N.A. Clark, Electrooptic response during switching of a ferroelectric liquid crystal cell with imiform director orientation, Ferroelectrics, 73, 305-314 (1987). 

W.-S. Kang, H.-W. Kim, and J.-D. Kim, Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light. Liquid Crystals 28, 1715 (2001). 

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... 

Surface Stabilized Ferroelectric Liquid Crystals (SSFLC)116 Here all three vectors of spontaneous polarization (Fs) are initially aligned by surface effects in thin cells (ca 2 pm). The switchability is due to 180° rotation of the Fs vectors on a cone. 

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

Calamitic metallomesogens forming a chiral smectic C phase (SmC ) are ferroelectric materials. Due to the low symmetry of this phase when the helix is unwound (C2) the molecular dipoles are aUgned within the layers of the SmC phase, giving rise to ferroelectric order in the layers. Because the SmC phase has a helical structure, there is no net macroscopic dipole moment for the bulk phase. However, it is possible to unwind the helix by application of an external electric field or by surface anchoring in thin cells. Under such conditions, a well-aligned film of the ferroelectric liquid crystal can exhibit a net polarisation, called the spontaneous polarisation (Ps). Ferroelectric liquid crystals are of interest for display applications because the macroscopic polarisation can be switched very fast by an... 

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... 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

It has been known for a long time that the surface ordering of a nematic (or other non-polar) liquid crystal is influenced by the ferroelectric domains of the anchoring substrate. In a work by M. Glogarova at al. [69], it is shown how the properties of a liquid crystal cell can be modulated and stabilized using a ferroelectric material as an anchoring substrate. These results motivated us to consider that the EFM technique could be efficiently used to create surfaces with variable anchoring conditions on a micrometric scale. 

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 Cells with Non-ferroelectric Liquid Crystal... 

Fig. 13.14 A model showing distribution of the electric field strength over an ahgning insulating layer ( //,) and a ferroelectric liquid crystal layer (A c) SSELC cell.

One of the main reasons, if not the only reason, that liquid crystals are of great importance in display applications is their ready response to externally applied electric fields [1,2]. Their direction can be easily changed by electric fields produced by the application of a few volts across the liquid crystal cells. They are either dielectric or ferroelectric materials with high resistivities and thus consume little energy. When the liquid crystals reorient, their optical properties change dramatically because of their large birefringences. In this chapter, we will first discuss how liquid crystals interact with externally applied electric fields, and then consider their applications. 

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.

This problem is overcome by Clark and Lagcrwall in their invention of the surface-stabilized ferroelectric liquid crystal (SSFLC) device [16], shown in Figure 4.9. The liquid crystal is sandwiched between two parallel substrates with the cell gap, h, thinner than the helical pitch, P, of the liquid crystal. The inner surface of the substrates is coated with alignment layers which promote parallel (to the substrate) anchoring of the liquid crystal on the surface of the substrate. The smectic layers arc perpendicular to the substrate of the cell, while the helical axis is parallel to the substrate. Now the helical twist is suppressed and unwound by the anchoring. 

The spontaneous electric polarization Ps of a thermotropic ferroelectric liquid crystal is usually measured with the so-called triangular wave method [16]. For this method, a triangular voltage U is applied to the liquid crystal and the so induced current in the liquid crystal is measured. This current can be divided into several contributions. If measuring a thermotropic SmC liquid crystal, the main contributors to the total current /ei. are the ohmic current, caused by the resistance / ei. of the liquid crystal, the capacitive current, due to the capacity C of the cell, and the polarization reversal current, which originates from the spontaneous electric polarization Ps of the polar liquid crystal. 

There are more possibilities for modulating light propagation using ferroelectric liquid crystals, particularly ferroelectric LC polymers, besides the ferroelectric switching in an SSFLC cell as shown in Fig. 32b. 

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. 

The book is subdivided into three parts. The first three introductory chapters include consideration of the nature of the liquid crystalline state of matter, the physical properties of mesophases related to their electroop-tical behavior, and the surface phenomena determining the quality of liquid crystal cells giving birth to many new effects. The second part (Chapters 5-7) is devoted to various electrooptical effects in nematic, cholesteric, and smectic mesophases including ferroelectric compounds. Here major emphasis is given to explaining the physical nature of the phenomena. The last part (Chapter 8) is a rather technical one. Here recent applications of liquid crystalline materials in electrooptical devices are discussed. 

The author would like to propose another method of determining the alignment layer polarity. In this method, the cell structure of the twisted FLC mode described in Ref. 35 is used. Consider the cell structure for which the rubbing directions on the top and bottom plates are set rectangularly, as shown in Fig. 5.2.9. A ferroelectric liquid crystal of 45° tilt angle, showing no smectic... 

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