Surface stabilized ferroelectric liquid crystal SSFLC

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. 

Fig. 5.2 Helical structure of SmC phase (a) and surface stabilized ferroelectric liquid crystals (SSFLC) (b and c)...

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. 

As discussed in Chapter 4, surface stabilized ferroelectric liquid crystals (SSFLCs) have two stable states at zero field. The two states have different planar orientational angles as shown in... 

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. 

Surface-Stabilized Ferroelectric Liquid Crystals (SSFLCs)... 

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... 

This surface bistability is at the basis of chiral smectic C surface stabilized ferroelectric liquid crystal (SSFLC) devices [92]. As their name indicates, these devices are made of thin cells in which the walls, imposing the orientation of the molecules at the surfaces, unwind the spontaneous smectic C helix and stabilize two uniform configurations of the director in the cell. Switching between these two states can be done by applying an electric field. 

Figure 4. Layering in a surface stabilized ferroelectric liquid crystal (SSFLC) cell showing the degenerate cone for the tilt of the molecules.

In 1980 a much more brilliant approach was described by dark and Lagerwall in which the ferroelectric chancier of tte Sc phase is optimally exploited [2. They proposed the so-called surface-stabilized ferroelectric liquid crystal (SSFLC) structure, which has the unique combined properties of fast response speed (tens of microseconds) and bistabilily. The originally suggested SSFLC structure is based on the following ideas ... 

In helically modulated SmC liquid crystals, the bulk polarization is vanishingly small. The helicity can be unwound by an external field applied parallel to the smectic layers. It can also be unwound by surface effects if the samples are sufficiently thin (thickness pitch), leading to the so-called surface-stabilized ferroelectric liquid crystal (SSFLC) with a nonvanishing macroscopic polarization. 

The chiral smectic C phase has the unique property of a dipole perpendicular to the tilt direction of the mesogens. This results from the lack of a mirror plane due to the chirality of the mesogens. However, a macroscopic polarization is not observed, as the lilt direction changes from layer to layer to form a helical superstructure. The twist can be unwound by surface alignment and electrical fields in a so-called surface-stabilized ferroelectric liquid crystal (SSFLC) cell. ... 

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