Focal-conic mode

For the liquid crystal in the homeotropic state, when the applied field is turned down, there are two relaxation modes. One is the H-F mode in which the liquid crystal relaxes into the fingerprint state (and then to the focal conic state) as discussed in the previous section. The other is the H-P mode in which the liquid crystal relaxes into the planar state [76,77]. The rotation of the liquid crystal in the H-P mode is shown in Figure 10.28. The liquid crystal forms a conic helical... 

In summary, if the hquid crystal is in the homeotropic state and the applied field is reduced, there are two possible relaxation modes. If the applied field is reduced to the region liquid crystal relaxes slowly into the fingerprint state and then to the focal conic state when the apphed field is reduced further. If the applied field is reduced below Ehpy the hquid crystal relaxes quickly into the transient planar state and then to the stable planar state. In bistable Ch reflective displays, the way to switch the liquid crystal from the focal conic state to the planar state is by first applying a high field to switch it to the homeotropic state, and then turning off the field quickly to allow it to relax to the planar state. 

Figure 13. Three states of a cholesteric phase change device operated in light scattering mode. The grandjean texture (a) and homeotropic field-on state (b) are optically clear the focal-conic state (c) represents an alternative field-off state, which is optically scattering.

On removal of the low frequency field, in thick cells, the texture relaxes back to focal conic with residual light-scattering properties. This has been described as the storage mode or memory effect in chiral nematic systems [162]. If a high frequency ac field is now applied (o)> co ), the dielectric torque restores the nonscattering planar texture and, just as in smectic A materials, this effect can be used in electrooptic storage mode devices [165], since we have a low frequency write and high frequency erase mechanism. 

Figure 2. Schematic drawing of a normal mode laser-addressed SmA device showing the initial clear state (homeotropic alignment) and the laser-addressed and storage (focal conic) states. Methods 1 - 3 are alternative processes for erasure.

In addition to the TN and IPS modes in nematic displays, himdreds of other displays methods were invented and tested over the last 30 years. Out of them the bistable nematic development based on surface flexoelectric interactions, the bistable cholesterics displays based on switching between planar and focal conic textures and the polymer dispersed liquid crystal displays found some applications. None of them, however, offer better than a few milliseconds switching time. 

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