Liquid crystalline host nonlinearity

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

Among all the experimental investigations performed in anthraquinone-doped nematic liquid crystals, two results are worth being noted here as they highlight some important features of the microscopic intermolecular interactions involved in this phenomenon. In the first experiment, we have found that slight changes in the molecular structure of relatively simple anthraquinone dyes may lead to dramatic modifications of the macroscopic nonlinear optical properties of the liquid-crystalline host [22]. This is shown in Fig. 5.2... 

Supramolecular structures obtained via self-assembly processes may be named as another future topic, which just starts an increasingly rapid development.294 By some examples it was already demonstrated how molecular chirality may by translated into supramolecular chirality in such self-assembled structures and this may lead not only to new, improved host-guest systems but also eventually to new materials in a more general sense. Liquid-crystalline systems or materials for nonlinear optics may be mentioned just as two examples. 

An earlier study of the molecularly doped polymer liquid crystalline system shown in Figure 6.9 revealed more complex behavior (27, 32). In this study, the amount of induced nonlinearity was path dependent, with the largest observed value occurring when poling was done near Tg ( 25 C). The host polymer exhibits a nematic-to-isotropic transition at = 100 C and a nematic texture between Tg and. Poling at 50 C produces a decrease in approximately 1 order of magnitude. The l/T dependence... 

As discussed earlier, while the scale of the fillers is substantially different, nanocomposite materials concepts and technology are very similar to those of conventional composite materials. This is clearly demonstrated in the case of new thermosets for nonlinear optical (NLO) applications, " " where nanocomposite of liquid crystalline thermosets, IPNs, and simple filled thermosets are evaluated. Tripathy et al. discussed four different ways to prepare nonlinear optical polymers. (1) The polymer matrix is doped with NLO moieties in a guest/host system (2) In side-chain polymer systems, NLO polymers with active moieties are covalently bonded as pendant groups (3) In the main chain polymer, the chromo-phores are incorporated as parts of the main polymer backbone to enhance the temporal stability of the NLO properties and (4) Stability of the optical noninearity in sol-gel-based thermosets is related to... 

The nearly linear dependence of Tni on concentration has been used to extrapolate [Tni] for compounds that do not form liquid crystalline phases and also for several mesogens. "- - An example of such an analysis for 71 (B = A) is shown in Figure 14.24. It has been observed that some 10-v p-carborane derivatives have nonlinear behavior at low concentrations <10 mol%. This has been ascribed to the symmetry of the cluster and conformational properties of its derivatives (see Rgure 14.6), and, consequently lower compatibility with the host. A particularly striking example is 8E for... 

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