Electro-optical properties of waveguides based on a main-chain nonlinear optical polyamide

Polyamides based on 2',5'-diamino-4-(dimethylamino)-4'-nitrostilbene (DDANS) and linear aliphatic diacids represent a class of polymers with large and stable second-order nonlinear optical properties, where the nonlinear optical units are fixed in the polymer backbone with their dipole moments oriented transversely to the main chain. Such NLO polymers were processed by spin-coating into planar waveguides ranging from 500 to 1500 nm in thickness. Subsequently, a macroscopic noncentrosymmetric orientation of the NLO-phores was induced by a corona-discharge poling process. The refractive indices, n(TM) and n(TE), and the electro-optic coefficients, r(33) and r(13), were determined by measurement of the waveguiding modes of such devices at a wavelength of 785nm. The electro-optic coefficient r(33) of 12 pm/V compares favorably with state-of-the-art polymeric electro-optic materials. It also equals approximately the electronic contribution to the electro-optic effect which can be calculated from the nonlinear optical susceptibility dss. The ratio of electro-optic coefficients r(33)/r(31) was found to be approximate to 3, independent of the magnitude of the NLO-phore orientation. This behavior is in perfect agreement with the predictions from the isotropic model.