Quadratic electroabsorption studies of third-order susceptibility mechanisms in dye-doped polymers

Abstract

We apply quadratic electroabsorption to thin-film solid solutions of squarylium dye molecules in poly(methyl methacrylate) polymer to study the electronic and reorientational mechanisms in the third-order nonlinear-optical susceptibility. To interpret the data, we have generalized quadratic electro-optic response theory to include both electronic and hindered-molecular-reorientation mechanisms. Such a theory predicts that the ratio of two independent third-order susceptibility tensor components, namely, χ3333(3)/χ1133(3), characterizes the relative contribution of each mechanism. We have designed and built an experiment that measures this ratio as a function of temperature and wavelength. Results show that at room temperature and near the first electronic transition wavelength the response is dominated by the electronic mechanism and that the reorientational contribution dominates when the sample is heated above its glass-transition temperature. Furthermore, far off resonance, the sign of the imaginary part of the third-order susceptibility is positive and is consistent with the two-level model. We also discuss the possible role of electrostriction in our analysis. Quadratic electroabsorption is thus shown to be a versatile tool for measuring the imaginary part of the third-order nonlinear-optical susceptibility, which yields information about the role of the interaction of the polymer host with the dopant molecule