Electric field dependence of poling response for nematic liquid crystalline main chain polymers with large second order optical nonlinearities

We report on the characterisation of the second order nonlinear optical properties of a semiflexible main chain random copolymer with a head to tail structure synthesised from 4-carboxy-4′-[N-(11-hydroxyundecyl)-N-methylamino]-azobenzene and hydroxybenzoic acid. This polymer exhibits a nematic liquid crystal mesophase with a nematic-isotropic transition temperature of 200°C. We find a complicated phase behaviour for the polymer and show that the polymer films be readily annealed into large homogeneously aligned monodomains in device configurations. We have carried out measurements of the field dependent poling behaviour by means of an ellipsometric set-up after Teng and Man. We find three distinctive field regions corresponding to different mechanisms in the response of the polymer chains. No saturation in the poling response can be detected for fields up to 108 V/m. Absolute values for χ(2) reach 80 pm/V. We analysed any potential χ(3) contribution to the total NLO signal and showed that this contribution can be neglected for our measurements.