The effects of external applied voltage on the nonlinear refractive index and the birefringence of a dye-doped nematic mixture

The nonlinear refractive index, n₂, and the birefringence, Δn, of a nematic liquid crystal mixture (denoted as 1294-1b) doped with a anthraquinone derivative (denoted as AQ) dye were measured as a function of an external ac applied voltage. The self-phased modulation effect was used to measure the nonlinear refractive index of homeotropical-aligned samples. The change of the dyed liquid crystal nonlinearity, which is the direct impact of the applied voltage on the dye molecules orientation, was not observed for the nonlinear response of the pure liquid crystal. Planar (homogeneous) aligned samples were used for birefringence measurements by taking into account the absorption coefficients of the dye. The birefringence decreased to a limiting value in the high voltage region for both pure and dye-doped samples, although in the presence of the absorbing dye the birefringence of the liquid crystal was not approximately changed.The polarized absorption spectra of the dye in nematic mixture were recorded in parallel-aligned liquid crystal cell and its dichroic ratio R and the order parameter S_d were obtained. The electro-optical effect of the guest-host system was also using polarized spectroscopic method.