Host-guest device

FIGURE 5 The upper panel shows homogeneously aligned, negative contrast, and the lower panel shows homeotropically aligned, positive-contrast, single-polarizer guest-host device effects. 

The quality criteria of guest-host devices in display systems are based on the following principal characteristics ... 

References [181, 182] demonstrated fluorescent displays with the possibility of switching between two colors [181] and optical shutters, switched by short electrical pulses [182] (less than 1 ms). The color of fluorescent displays is readable in the dark at a low level of ambient light, the displays possess excellent viewing characteristics at oblique viewing and are insensitive to any irregularity in cell thickness. Fluorescent displays can compete with the usual guest-host devices in some indoor applications. 

These materials may also be included in guest-host or side-chain polymer systems, similar to those exploited in electro-optic polymer studies. This would improve processability for waveguide devices. The coefficients quoted above show that such a doped polymer could function at reasonable power levels and waveguide dimensions with an active region 1-2 mm long. 

The stability of conjugated polymer-fullerene devices embedded in conventional polymers (guest-host approach) are higher due to encapsulation against environmental influences. 

Guest-host (GH) LCDs are coloured displays, whereby the colour and changes in colour are attributable to the absorption of incident light by a dichroic guest dye dissolved in a liquid crystalline host material, usually nematic, and the co-operative reorientation of these dichroic dyes in an electric field, i.e. electro-optical devices. 

The first nematic guest-host prototype nematic guest-host display device contained a nematic liquid crystal (4-butoxybenzoic acid) and a pleochroic dye (methyl red or indophenol blue) sandwiched between two (Nesa) electrodes dTn 12/im) rubbed uniaxially, but with no additional orientation layer, see Figure 3.14. One polariser was fixed to the front substrate surface with its direction of maximum absorption parallel to the rubbing direction and, therefore, the nematic director. 

The first nematic guest-host prototype nematic GH-LCD reported by Heilme-ier and Zanoni " contained methyl red (157) as the dichroic dye dissolved in 4-butoxybenzoic acid as the nematic liquid crystal host. Other hosts investigated later included 4-methoxycinnamic acid and 4-ethoxy-4-aminoben-zonitrile (28), see Table 3.4. The melting point of these three single components is very high. Therefore, prototype GH-LCDs had to be operated and evaluated at very high temperatures. Thermal decomposition of the mixtures led sequentially to lower contrast, homeotropic orientation due to decomposition products and finally device breakdown. However, these initial experiments were sufficient to demonstrate the feasibility of this display type. 

The advent of the apparently superior White and Taylor device retarded the development of better guest-host materials and mixtures for this device type. 

This chapter concentrates on the design of efficient dipolar NLO chromophores and the different approaches for their incorporation in non-centrosymmetric materials, including guest-host polymer systems, chromophore-functionalized polymers (side-chain and main-chain), cross-linked chromophore-macromolecule matrices, dendrimers, and intrinsically acentric self-assembled chromophoric superlattices. The different architectures will be compared together with the requirements (e.g., large EO coefficient, low optical absorption, high stability, and processability) for their incorporation into practical EO devices. First, a brief introduction to nonlinear optics is presented. 

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