Homeotropic chiral nematics

Figure 9.2. The homeotropic aligmnent of the chiral nematic phase.

The moduli were calculated from the threshold of the Frederiks transition ((4.9) induced by a magnetic (Ax > 0) and electric (Ae < 0)) field in homeotropically oriented liquid crystal layers. The same order of magnitude (10 -10 dyn), which is typical of conventional nematics, has been found for elastic moduli Kn and for other nematic polymers [233, 234]. Unwinding of the helical structure of chiral nematic polymers allowed the elastic constant K22 to be calculated K22 10" dyn for an arylic comb-like copolymer with cholesterol and cyanobiphenyl side-chair mesogens [229]). 

The concept of mechanical field induced orientation can easily be transferred to nematic elastomers with oblate chain conformation, i.e., side chain end-on elastomers with an even number of spacer atoms. In order to achieve a monodomain structure, a globally oblate chain conformation has to be established. This can be achieved by uniaxial compression or biaxial stretching of the polydomain elastomer which induces a uniform homeotropic alignment of the nematic director perpendicular to the film plane. Up to now, this orientaticMi technique has only been realized experimentally for chiral nematic elastomers [72]. 

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. 

For chiral nematic liquid crystals, the method outlined previously for a planar nematic cell has been shown to be quite effective. For smectic-A the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for a ferroelectric liquid crystal (FLC), smectic-C for surface stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabi-lization. f On the other hand, smectic-A (Sm-A ) cells for soft-mode FLC (SMFLC) operation are easier to prepare using the methods described above. ... 

A suitable surface treatment results in a homeotropic orientation for a nematic mixture incorporating a dichroic dye of positive contrast and an amount of a chiral dopant insufficient to overcome the surface forces and generate a twisted structure in the nematic phase. 

The static homeotropically aligned guest-host nematic mixture (plus guest dye and chiral dopant) is optically transparent and, therefore, the display appears colourless in the non-activated state. The lower limit of the pitch for a given cell gap, before a twisted nematic structure becomes energetically more favoured than the homeotropic nematic structure, is determined by the djp ratio ... 

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