Surface-stabilized ferroelectric liquid alignment

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. 

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. 

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. 

Effect of alignment layer conductivity on the instability of surface stabilized ferroelectric liquid crystal devices. Figures 1 and 2, TC. Chieu and K.H. Yang, Applied Physics Letters, 56 (14), p. 1326 (1990). Reproduced by permission of the American Institute of Physics. 

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. 

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. ... 

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). 

Aliphatic polyimides as alignment layers for surface stabilized ferroelectric smectic C liquid crystal cells, Figure 1, B.O. Myrvold, Liquid Crystals, 7 (2), p. 262 (1990). Reproduced by permission of Taylor Francis. 

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). 

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. ... 

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