Nonlinear optical properties of poly-p-(phenylene terephthalates) with side group chromophores

The linear and nonlinear optical (NLO) properties are reported for a series of wholly aromatic polyesters containing NLO chromophores in the side group through a different alkyl tether length. For the first time, we have observed the domain structures of NLO chromophores for the poled thin-film samples of the polymers using atomic force microscopy. Measurements on the refractive indices of the transverse magnetic and transverse electric modes exhibited considerable anisotropy for the films as spin-coated and poled. The measured positive birefringences were greater for the polymer with shorter tether length, indicating a more pronounced orientation of the optically anisotropic groups in the direction of the film plane. SHG measurements also showed that small changes in the number of methylene units in the side spacer result in large changes in the second-order NLO properties. The tensor ratios of the NLO components were revealed to be smaller than d(33)/d(31) = 3, the predicted value by the isotropic model for a purely electronic response. This behavior was understood in terms of the different orientations of the chromophore through the sample thickness, that is, a surface domain layer with normal orientation and a compressed inner layer with tilted orientation.