Focal conic state

Fig. 5.8 A photoaddressed and multiswitchable cholesteirc LC display of 6.0 wt% chiral dopant 8 in LC host E7 in a 5 pm thick homeotropic aligned cell (1.5 in. x 1.5 in.). Top schematic cholesteric textures Middle demonstration of an image Bottom crossed polarized textures. Reproduced with permission from [76]. a Planar state, b Focal conic state, c Planar state. Copyright 2011 John Wiley Sons...

The ChLC has two stable states of planar and focal conic at zero field condition. In the planar state in Fig. 4a, the helical axis is arranged perpendicular to the cell substrate, and incident light is reflected backward due to the Bragg reflection. The reflection band is a narrow range of wavelengths and its bandwidth is defined as Xq = ( e no)P, where tig and no are the ordinary and extraordinary refractive index, respectively. In the focal conic state in Fig. 4b, the direction of the helical axis is randomly distributed within the cell in a way that Bragg reflection does not... 

Bistable Ch reflective displays are operated between the reflecting planar state and the nonreflecting focal conic state. When a Ch hquid crystal is in the planar texture, the refractive index varies periodically in the cell normal direction. The refractive index oscillates between the ordinary refractive index and the extraordinary refractive index rig. The period is PJ2 because n and - n are equivalent. The liquid crystal exhibits Bragg reflection at the wavelength 2o = 2h Po/2)=hPo for normally incident hght [28], where h= ne + no)/2 is the average refractive index. The reflection bandwidth is given by AmPq, where An = is the birefrin-... 

Transition between planar state and focal conic state... 

In bistable Ch reflective display applications, it is desirable that the threshold of the transition from the planar state to the focal conic state be high, so that the cholesteric liquid crystal can remain in the planar state and the display does not exhibit flicker under colimin voltage in addressing. 

When the liquid crystal is in the focal conic state and the externally applied electric field is increased, more liquid crystal molecules are ahgned parallel to the field. The liquid crystal is gradually switched to the fingerprint state. There is no sharp boundary between the focal conic state and the fingerprint state. When the appUed field is increased further, the pitch of the hquid crystal becomes longer, as shown in Figure 10.27. When the applied field is above a threshold Ec, the helical structure is unwound [29,48], the pitch becomes infinitely long, and the liquid crystal is switched to the homeotropic state. 

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. 

As discussed in previous sections, cholesteric liquid crystals exhibit two bistable states at zero field the reflecting planar state and the non-reflective focal conic state. They can be used to make multiplexed displays on passive matrices. In this section, we consider the drive schemes for the bistable Ch displays. 

Figure 10.30 The response of the histahle Ch hquid crystal to 40 ms wide voltage pulses, a initially in the planar state, h initially in the focal conic state.

Electric fields have been used to switch a cholesteric texture between the planar, the focal conic, and the homeotropic state. The planar and focal conic states persist after the field is turned off, the choice determined by the amplitude and frequency of the applied field and the rate at which it is turned off. These textural transitions were discovered by Heilmeier and Goldmacher [32] and are used today for color display panels that exhibit gray-scale memory [33,34]. 

Electronic Paper, Figure 4 Different textures of the cholesteric liquid crystal (a) planar state, (b) focal conic state and (c) homeotropic state. Rgure from [15]... 

Figure 12.34. Schematic illustration of a polymer-stabilized cholesteric texture (PSCT) in (a) the planar texture, (b) the focal conic texture, and (c) the homeotropic texture. Transforming the homeotropic ahgned state (c) back to the planar of focal conic state depends on the history of the voltage.

Such devices can be used in displays, when careful switching between the focal conic state and the homeotropic nematic state can provide a high information format [18, 19], albeit with slow writing speeds. 

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.

Figure 14. The read/write process in a smectic A thermoelectric scattering cell. Writing is accomplished by local heating of the cell to the isotropic state (b), which on cooling can be induced to form either a clear homeo-tropic state (a) if a field is simultaneously applied, or to a scattering focal-conic state (c) in the absence of a field. The field has no effect in the absence of simultaneous heating.

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.

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