Mechanisms of the nonlinear optical properties of squaraine dyes in poly(methyl methacrylate) polymer

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Abstract
We report on the nonlinear optical characterization experiments of squaraine dyes in solid poly(methyl methacrylate) solutions. We find that a quantum three-level model of a squaraine best agrees with the quadratic electroabsorption measurements when the isomer composition is taken into account. We compare the nonlinear response of several dyes with the three- and four-level models to help determine which models are most suitable for predicting nonlinear behavior. Four-level models that include either two one-photon states or two two-photon states are inconsistent with quadratic electroabsorption spectroscopy. Furthermore, we find that the effect of isomers is more significant than molecular reorientation at room temperature and that the isomer model best agrees with the experimental results.