SSFLC device

The advantages of SSFLC devices derive to a large extent from the spontaneous macroscopic polarization P of the phase. For example the electrooptic rise time of a prototypical SSFLC light valve is inversely proportional to the magnitude of the polarization. In order to design new FLC materials with large P in a directed way, we... 

Another phenomenon that has potential applications is the field-induced tilt or the electroclinic effect. Unlike the SSFLC device, this effect does not possess bistability but it has a faster (submicrosecond) response. By using the same bookshelf geometry and a suitable polarizer and retarder arrangement, the electroclinic effect can be used for modulating a light signal with a transmitted intensity linearly proportional to the applied voltage or as a tunable colour filter. 

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 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. 

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. 

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. 

In its ideal stale fulfilling the cooditioas given in equation (3) with k 0. such a SSFLC device is a switchable kfl wave plate as it rotates the optical axis by 45 and the poUrization of the incident light by. It can be used as a display, sptttial light mothilalor. shutter, real-time mask, or in a linear array such as a printhead... 

This simplified model equation was used by Xue, Handschy and Clark [281] for modelling the electro-optic response in an SSFLC device. It has the solution [281]... 

The rotation around the (small) cone is another reason for the fast switching speed. For Ps = 10 nC/cm = 1 V/pm, y = 0.1 poise and = 22.5, r 10 ps The fast switching speed is a merit of ferroelectric liquid crystal devices. Regarding the bistability, on the one hand, it is good because it enables multiplexed displays of the ferroelectric liquid crystal on passive matrices on the other hand, the bistability is a problem because it makes it difficult to produce gray scales. Another issue with SSFLC is that it is more challenging to achieve uniform orientation in SSFLC than in nematic liquid crystals. 

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. 

SSFLCs have a very short switching time (in the order of microseconds) because of the direct coupling force of the spontaneous polarization and an electric field. This response is about a thousand times faster than that of conventional nematic devices, which utilize the dielectric anisotropy of the molecule. 

Both the dark and bright states of the SSFLCs shown in Fig. 5.1.4 are stable after an electric field is removed. This memory effect is a result of the surface anchoring effect and the fact that both states are elastically equivalent. The memory effect is very important for high multiplexing of displays in order to avoid the drawback of crosstalk, and to obtain an optical storage device. 

---