Optical anisotropy in films of photoaddressable polymers

The photoinduced anisotropy in polymers and copolymers with a mesogenic and a nonmesogenic azobenzene moiety in the side chain were investigated. The results of the light-induced optical anisotropy study in thin films obtained by means of polarized W-vis and IR absorption spectroscopy are presented. Starting from in-plane optically isotropic samples, irradiation with polarized laser at 514 nm induces significant and long-term stable optical anisotropy. The build-up of the optical anisotropy has been followed by transient absorption experiments with time resolution in the range of seconds. The coupling of photoinduced alignment between the two different azobenzenes could be proven by irradiation in the absorption band of the nonmesogenic chromophore. In- and out-of-plane refractive indices were determined using attenuated total reflection waveguide spectroscopy. High values of the in-plane light-induced birefringence of up to 0.23 depending on the relative content of the mesogenic and the nonmesogenic azobenzene moiety in polymer were found. The pronounced dependence of the birefringence on probe wavelength suggests a resonant enhancement of refractive index in the visible region. Refractive indices as a function of wavelength were evaluated from the measured polarized absorption spectra using the Kramers-Kronig relationship. Comparison with the results from waveguide spectroscopy showed that the anisotropy and dispersion of the refractive index below the absorption could almost entirely be explained by the reorientation of the two side-chain groups of the copolymer.