Surface Stabilized FLC

Recall that in the one-elastic-constant approximation K] = K22== K), the dynamical Equation (4.92) for a ferroelectric liquid crystal under an applied field, see 

For a typical FLC material and device . f 10 N, 0—20°, Cj —lO F/m, E 10 V/m, P 10 C/m, and e = cos 0+(8 /8j )sin 0 - 1. If e is appreciable so that the magnitude of the dielectric term [second term on left-hand-side of Eq. (6.34)] tn be comparable to that of the spontaneous polarization (third) term, then the elastic term becomes the smallest among the three. Under this assmnption. Equation (6.34) becomes 

The optimal transmission occurs at 6=22.5° and 5=ti. In order to achieve a uniform rotation, the SSFLC layer thickness is often limited to about 2pm. As mentioned before, FLC cells, in general, switch faster than nematic cells the response time ranges from Ips to several tens of microseconds, owed largely to the lower rotational viscosity, larger spontaneous polarization, and higher switching fields. 

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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... 

Fig. 2. Schematic drawing of the bistable switching of a ferroelectric liquid crystal in the surface stabilized FLC configuration.

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. 

The Clark-Lagerwall Effect. This effect is observed in thin surface-stabilized FLC (SSFLC) cells where the smectic layers are perpendicular to the substrates, the thickness is less than the helical pitch (delectric field of opposite polarity switches the direction of the spontaneous polarization between the UP and... 

For chiral nematic liquid crystals, the method outlined previously for a planar nematic cell has been shown to be quite effective. For smectic-A the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for a ferroelectric liquid crystal (FLC), smectic-C for surface stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabi-lization. f On the other hand, smectic-A (Sm-A ) cells for soft-mode FLC (SMFLC) operation are easier to prepare using the methods described above. ... 

FLC phases in the surface stabilized geometry possess a single C2 axis of symmetry, and therefore polar order and non-zero x<2) in the simple electronic dipolar model. Thus, it is not surprising that experiments aimed at measuring this property were first reported shortly after the Clark-Lagerwall invention. Early studies (14-15) described second harmonic generation in (S)-2-Methylbutyl 4-(4-decyloxybenzylideneamino)cin-namate, the first ferroelectric liquid crystal, also known as DOBAMBC (1). 

To obtain fast LC photoresponse, a new guest/host system was developed, in which ferroelectric LCs (FLCs) were used as a host LC. FLCs exhibit spontaneous polarization (Ps) and show microsecond responses to change in applied electric field (flip of polarization) in a surface-stabilized state.1261 If a flip of polarization of FLC molecules in the surface-stabilized state can be induced by light in the presence of an applied electric field, photoresponse in the microsecond time region might be achievable. 

A mixture of an azobenzene derivative and an FLC (Figure 2), in which the concentration of the azobenzene guest was 3 mol%, was prepared in the surface-stabilized state in a very thin LC cell. Then the mixture was irradiated with light at 366 nm to cause trans-cis photoisomerization of the azobenzene guest molecule. It... 

One very important advantage of FLCPs over amorphous poled polymers is the possibility of phase matching, which is a precondition for efficient SHG. The mismatch of the refractive indices of the fundamental wave and the second harmonic due to the dispersion may be overcome using the birefringence of the FLC material in the geometry shown in Fig. 37 [15]. Due to the C2 symmetry of the unwound smectic C phase (achieved by surface stabilization or by application of a DC electric field along the y direction), the tensor has only four independent elements, which may be denoted in the contracted d tensor notation as... 

In the sections on smectic liquid crystals, first the alignment and molecular orientation of surface stabilized ferroelectric liquid crystals (SSFLCs) are treated in detail. Next, the alignment technologies needed for the occurrence of bistability are detailed. Furthermore, liquid crystalline devices made of AFLC materials and the applications of FLC and AFLC materials to active matrix devices are discussed. 

At first in this chapter, ferroelectric liquid crystals (FLCs) and their most interesting application as surface stabilized ferroelectric liquid crystals (SSFLCs) are briefly explained. 

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. 

Figure 5.29 (a) Bookshelf geometry in a surface-stabilized ferroelectric liquid crystal (SSFLC) display showing two states of polarization. Here the surface acts to unwind the helix, (b) Chevron geometry in an FLC. This disturbs the switchable polarization in the bookshelf geometry... 

An interesting development beyond the FLC OASLM is the a-Si novelty filter [78]. This device uses an OASLM like structure to perform a simple motion detection to measure the difference between image frames. Rather than use the FLC material as a direct modulation medium, it uses the surface-stabilized characteristics of the FLC as a form of optic memory. The optical modulation of the FLC material is also converted from a polarization effect to an amplitude effect by the inclusion of a pleochroic dye with the FLC material. 

In a ferroelectric liquid crystal by reducing the cell gap of a cell to a critical value, the helix of the LC medium is unwound and the FLC medium takes a planar conformation due to the surface anchoring effect [35]. This device is called surface-stabilized (SS)-FLCD [35]. 

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 16 Different orientations of mesogens and polar axis for FLCEs in different assemblies (a) surface-stabilized LC cells, (b) freestanding films, (c) films oriented by deformation of weakly precrosslinked FLCs. From Ohm, C. Brehmer, M. Zentel, R. Adv. Mater. 2010,22 (31), 3366-3387. ...

Recently, studies of stabilized LC alignment obtained by using polymer networks have been reported. A photocurable monomer is added to the LC, and it is polymerized by UV exposure after the LC has been aligned either by the surface method or by usii electric or magnetic forces. An application for an PTiC was reported based on this approach, and good alignment of the FLC was reported [31]. 

The polarity of the alignment layer surface does not have much influence on alignment phenomena for nematic liquid crj talline materials. However, in the case of FLC materials, the polarity of the alignment layer surface shows an important effect. This is because the interaction between the spontaneous polarization and the polarity of the surface becomes important. This matter has been approached theoretically [27]. The stable director orientation in the SSFLC device was determined by minimizing the total free energy of the surfaces and the bulk elastic distortion as functions of cell thickness, cone angle, helical pitch, elastic constant and surface interaction coefficient. Because of the tendency of the direction of the spontaneous polarization to point either into or out of the substrate surface due to polar surface interaction, the director of the molecules twists from the top to the bottom surface. Therefore, the uniform state can only be stabilized in the case of a small surface interaction coefficient. 

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