Non-linear optical behaviour

The last polymer mentioned in Table 10.9 is poly(p-phenylene terephthalamide). This polymer is spun from its solution in pure sulphuric acid (100%), a dope that exhibits mesomorphic (=liquid crystalline) behaviour it is optically anisotropic and is nematic in character. A number of other polymers, containing rigid elements in the chain have melts of polymer liquid crystals, with a high birefringence and a non-linear optical behaviour in electric fields. 

As the following measurements will show, the PSF model seems to describe best the non-linear optical behaviour of polydiacetylenes. A very important prediction of this model is the proportionality of x and a in the near-resonant frequency regime [115]. This behaviour was foimd in polydiacetylenes, strongly supporting the PSF theory [118]. 

The poly diacetylenes non-linear optical behaviour was found to be in accordance with the predictions of the PSF model that assigns the non-linearity to the bleaching of the exciton band. Theoretical approaches neglecting excitionic effects clearly fail. The exciton dominates the non-linear optics of polydiacetylenes [118],... 

The study of the changes of phase, frequency and intensity that the incident radiation undergoes on passing through the material constitutes the field of non-linear optics (NLO). It, in fact, focuses on the description of deviations from the linear behaviour predicted by the laws of classical optics. 

To account for the non-linear optical (NLO) behaviour, the foregoing expressions, relating the induced polarization with the applied electric field, are expressed as a power series of the electric field. 

Liquid crystals with a large molecular mass are able to form a glassy state with mesomorphic behaviour. Yitzchaik and coworkers38 report about a new class of copolymers with non-linear optical properties, and a monomeric representative with a glassy metastable... 

Clathrates are useful for the control of the stereo- and regiospecificity of intra-cavity chemical reactions and can be used to engineer materials properties such as a polarity leading to non-linear optical materials, and ferroelastic or ferroelectric behaviour. 

The simplest model consists of two centres, one donor (D) and one acceptor (A), separated by a distance I and contains two electrons. Here we consider this simple system to illustrate some general relations between charge transfer, transition intensities and linear as well as non-linear optical polarizabilities. We will show below that the electro-optic parameters and the molecular polarizabilities may be described in terms of a single parameter, c, that is a measure of the extent of coupling between donor and acceptor. Conceptually, this approach is related to early computations on the behaviour of inorganic intervalence complexes (Robin and Day, 1967 Denning, 1995), Mulliken s model for molecular CT complexes (Mulliken and Pearson, 1969) and a two-form/two-state analysis of push-pull molecules (Blanchard-Desce and Barzoukas, 1998). 

Trifunctionnal alkoxides, R Si(OR)3, are usual precursors to introduce organic groups within an inorganic network. The organic goup R are introduced for their non-linear optical properties, their hydrophobic behaviour or as chemical sensors. 

Non-linear optical activity - Materials that exhibit non-linear optical (NLO) behaviour are used in second-harmonic generation (SHG) which doubles the frequency of light that is passed through the sample. In order for the network to display NLO behaviour, the structure must be non-centrosymmetric (diamondoid networks are often used) and it must contain suitable chro-mophores. 

Semmler, K. J. Dingemans, T. J. Samulski, E. T. Biaxial smectic phases in non-linear optical properties and phase behaviour of an oxadiazole liquid crystal. Liq. Cryst. 1998, 24, 799-803. 

A number of rmsaturated polyazamacrocyclic complexes and their derivatives exhibit unusual properties, such as high metal-like conductivities or, conversely, semiconductor behaviour [90-96], Metallophthalocyanines and subphthalocyanines are promising non-linear optical materials for application in optoelectronic devices [97, 98] Porphyrins have been used in organic light-emitting diodes [99],... 

For an organic material to show electro-optical activity it must exhibit high second order non-linear behaviour. This is one of the terms in the equation that defines the polarisation (u) of an organic molecule as shown below, where E is the perturbing electric field. 

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