Focal chiral nematics

FIGURE 8.46 Structure of the reflective bistable cholesteric LCD fabricated with a black PPy film as the electrode and the light absorbing layer. Relative reflectance as a function of applied AC voltage for the (a) chiral nematic mixture (CNM) and (b) CPD cells initially in either the planar (open symbols) or focal conic (filled symbols) state. Reflectance at 510 and 560 nm for the CNM and CPD cells, respectively, was measured after 200 Hz-AC field was applied for 20 ms and then removed. (From Kim, Y.C., et al.. Mol. Cryst. Liq. Cryst, 327,157,1999. With permission.)... 

Figure 2. Chiral nematic arrangements in thin cells (a) planar or Grandjean, (b) fingerprint, and (c) focal conic.

Figure 6. Photomicrographs of (a) planar, (b) focal conic, and (c) fingerprint textures in chiral nematics, and the schlieren texture demonstrating brushes in an apolar nematic (d) observations through crossed polarizers.

As a result of the layered nature of the chiral nematic structure, like the smectic A, it can also exhibit focal-conic textures [79] and both phases exhibit screw and edge dislocations. A dislocation corresponds to a displacement of the layered structure in a plane orthogonal to the layer and may be formed by the pairing of two disclinations of opposite sign. A screw dislocation has a singular line along the screw axis and is equivalent to a f-screw disclination in a chiral nematic. An edge dislocation corre-... 

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. 

In the White-Taylor device the chiral nematic (p A) is doped with an anisotropic dichroic dye. With homeotropic boundary conditions and low voltages, the focal-conic texture becomes axially aligned in the plane of the device. The dye spirals with the director and the random directions of the helix axis in the plane of the device ensure that unpolarized light is absorbed uniformly in this state. Application of a high field (see... 

Polariser - Glass Substrate. .Transparent Electrode Homeotropic Alignment Layei Chiral Nematic (Focal Conic)... 

Figure 38. Schematic operation of the White-Taylor dye guest-host chiral nematic electrooptic cell. In (a) for zero applied field the axis of each focal-conic domain is random in the x, y plane, as therefore is the dye, using homeotropic surface alignment. In (b) the texture is planar for the zero field state and therefore the dye spirals around the z direction. In (c) the focal conic (a) or planar (b) transition to homeotropic nematic has taken place above the threshold voltage V,], (WT). The black ellipses represent the dyes in the chiral nematic matrix.

A further interesting use of the focal-conic to homeotropic texture transition is in infrared modulation [272]. Here it was found possible to modulate infrared light at A=8-12 pm with a maximum transmission of 87%, a contrast of 93%, and turn on and off times of 1 ms and 125 ms, respectively. A further window examined was 3-5 pm, and this work suggests that other chiral nematic electrooptic effects could be exploited in the near infrared. In communications technology a 2x2 optical switch for fiber-optics has been developed [273] using a chiral nematic film and two switchable nematic waveplates. It has been demonstrated that this is suitable for LED or laser sources. The device worked at 1.318 pm and had switching times of 40 ms with -26 dB crosstalk between unselected fibers. There will clearly be further advances in this use of the unique optical properites of chiral nematics. 

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