Electro-Optic Response in Polymers

In PR crystals, the refractive index is altered mainly by the hnear EO effect, which results in a refractive index modulation of the form given in Equation 8.2 in response to the space charge field. 

The parameters that can be changed at the molecular level are the relative electron affinities of the donor and acceptor groups and the length as well as the nature of the conjugated segment connecting the donor to the acceptor. The chemical structures of some of the NLO chromophores such as 4-(dimethyl-amino)-4 -nitrostilbene (DANS) (a). Disperse Red 1 (b), l-(2 -ethylhexyloxy)-2, 5-dimethyl-4-(4 -nitrophenylazo)benzene (EHDNPB) (c), N,N-diethyl-substituted 

5-dimethyl-4-(p-nitrophenylazo)anisole (f) 4-piperidin-4-ylbenzylidenemalononitrile (g) 4-N,N-diethylaminocinnamonitrile and (h) 3-fluoro-4-N,N-diethylamino-jd-nitro-styrene. 

At the microscopic level, the dependence of the dipole moment of such a molecule on an electric field is given by Equation 8.3 

Instead of using a passive molecule such as dioctylphthalate, active plasticizers that may contribute to either the charge-transport or EO effect can be utihzed. The molecule ECZ, widely used in PR polymers, is a plasticizer that contributes to charge transport as well. It has been reported that an inert plasticizer reduces the Tg more efficiently than a charge-transporting plasticizer, but reduces the charge-carrier mobility and the PR phase shift. The molecule 4-4 -n-pentylcyanobiphenyl (5CB) has been reported to lower the Tg of a PVK-based PR system from 230 to 25 °C 

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