Surface SSFLC cells

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... 

Fig. 1.4 Sketch of the surfaee-stabilized feiroelectric liquid crystal (SSFLC) cell structure. Due to the surface-stabilization, the helical structure of the SmC phase is unwound as only two director orientations on the tilt eone can be realized. These two director states correspond to either UP or DOWN polarization (redrawn after [15])...

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... 

DOWN positions which correspond to the LEFT and RIGHT positions for the director that moves along the surface of a cone, the axis of which is normal to the layers and parallel to the cell substrates. In the LEFT and RIGHT positions the director remains parallel to the substrates and the SSFLC cell behaves as a uniaxial phase plate. The total angle of switching equals the double tilt angle 6. 

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

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. ... 

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. 

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... 

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. 

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. 

Thus it passes through the top polariser and the cell appears bright. Because the switching in both directions is driven by the interaction of the spontaneous polarisation with the electric field (rather than the interaction between an induced polarisation and the electric field or a relaxation process when the electric field is removed), these surface stabilised ferroelectric hquid crystal (SSFLC) displays are much faster than twisted nematic and birefringent displays. 

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. 

In the case of SSFLCs, asymmetry of the polarities of the top and bottom surfaces determines whether the cell shows bistability, a twisted state or monostability. Because the 7a value does not change greatly for the surfaces of different polymers, the asymmetry of the cell can be evaluated by the difference, A7p... 

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. 

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. ... 

---