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 grou p through a different alkyl tether length. For the first time, we have obse rved the domain structures of NLO chromophores for the poled thin-film samp les of the polymers using atomic force microscopy. Measurements on the refr active indices of the transverse magnetic and transverse electric modes exh ibited 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 an isotropic groups in the direction of the film plane. SHG measurements also showed that small changes in the number of methylene units in the side spac er result in large changes in the second-order NLO properties. The tensor r atios 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 respo nse. This behavior was understood in terms of the different orientations of the chromophore through the sample thickness, that is, a surface domain la yer with normal orientation and a compressed inner layer with tilted orient ation.