Bistable switching, ferroelectric liquid crystals

In a chiral smectic (Sc ) phase, the tilt angle is the same within a layer, but the tilt direction processes and traces a helical path through a stack of layers (Figure 43). It has been demonstrated that when such a helix is completely unwound, as in a surface stabilized ferroelectric liquid crystal cell, then changing the tilt of the molecules fi om +0 to —0 by alternating the direction of an applied field results in a substantial electro-optic effect, which has the features of veiy fast switching (%1 - lOps), high contrast and bistability [87]. The smectic A phase of chiral molecules may also exhibit an electro-optic effect, this arises due to molecular tilt fluctuations which transition is approached, which are combined with a... 

The spontaneous molecular polarization of ferroelectric liquid crystals, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. A low electric field of only a few volts can switch the ferroelectric liquid crystal between two equally stable states with opposing polarization directions. This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the ON state. The power required to run ferroelectric liquid crystal displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, ferroelectric liquid crystals can switch hundreds of times faster than a... 

Palto, S.P., Geivandov, A.R., Bamik, M.I., Blinov, L.M. The new role of alignment layers in bistable switching of ferroelectric liquid crystals. Numerical simulations and experimental results. Ferroelectrics 310, 95-109 (2004)... 

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. 

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

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. 

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. 

Today s ferroelectric liquid crystals are made on small sizes on silicon chips (liquid crystals on silicon) and presently are serving only a relatively small market. This is partially because of the problems with field-induced mechanical stresses related to their piezoelectricity. However, we believe that sooner or later the ferroelectric displays should be good enough to replace the nematic displays. They are inherently bistable, i.e., only those pixels should be readdressed that are showing change, and they offer switching with 1-100 microsecond ranges. Their main drawback is that they have two-dimensional... 

Fume H, Takahashi T, Kobayashi S (1999) Monostabilization of surface-stabilized ferroelectric liquid crystal using polymer stabilization. Jpn J Appl Phys 38(1) 5660 Fume H, Yokoyama H, Kobayashi S (2001) Newly developed polymer-stabilized ferroelectric liquid crystals microsized bistable domains and monostable V-shaped switching. Jpn J Appl Phys 40 5790-5794... 

There are two types of matrix addressing schemes— passive and active. The passive matrix (PM) addressing scheme requires the row and coluiim electrodes to address each individual pixel. This scheme still promises well in the area of bistable device such as ferroelectric liquid crystal (FLC) display and bistable twisted nematic (BTN) display because they do not need a control unit for gray-scale capability. The active matrix (AM) addressing scheme is the most developed and widely adopted one in cmrent LC displays. In this scheme, each pixel is cormected to a small electronic switch or TFT made with o-Si, poly-Si, or CdSe. This switch not only enables the pixel to hold the video information until it can be refreshed, but also prevents cross talk among neighboring addressed pixels. 

In solid state all the 10 pyroelectric crystal groups allow in principle for bistable switching behaviour. This is the proper ferroelectricity. Under certain conditions ferroelectricity (improper) can be realized in liquid crystals. This was shown by Meyer and coworkers115 in 1975. Since that time intense activities have been initiated, applying this property for flat-panel devices, switches, light modulators etc. In principle, three effects can be observed and used ... 

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