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Concentration dependence of photoinduced birefringence and second-order susceptibility in all-optical poling

We study photoinduced molecular reorientation of the azobenzene derivative Disperse Red 1 embedded in poly(4-vinylpyridine) polymer matrix. Photoinduced axial order leading to birefringence and polar order leading to second-order nonlinear optical (NLO) response are induced by purely optical means. These two photoinduced properties are found to exhibit markedly different dependences on the chromophore concentration: the photoinduced second-order NLO response reaches its peak already at 23 wt. % concentration while the photoinduced birefringence increases up to 51 wt. % concentration. The results show that chromophore-chromophore intermolecular interactions work against polar order already at modest concentration. The axial order, on the other hand, is not as easily affected by such interactions.