CHARACTERIZATION OF THE EXCITED-STATES OF A SQUARAINE MOLECULE WITH QUADRATIC ELECTROABSORPTION SPECTROSCOPY

We apply quadratic electroabsorption spectroscopy (QES) to thin-film s olid solutions of squarylium dye molecules in poly(methyl methacrylate ) polymer to study the dye's electronic excited states and to investig ate the importance of these states with regard to their contribution t o the third-order nonlinear-optical susceptibility. We first show that the room-temperature tensor ratio a = chi3333(3)/chi1133(3) almost-eq ual-to 3 throughout most of the visible region to establish that the e lectronic mechanism dominates. Because QES is a third-order nonlinear- optical susceptibility measurement, it can be used to identify two pho ton states. By obtaining good agreement between the quadratic electroa bsorption spectrum and a three level model, we conclude that there are two dominant states that contribute to the near-resonant third-order susceptibility: a one-photon state and a two-photon state that are sep arated by less than 0.2 eV in energy. QES is thus shown to be a versat ile tool for measuring the nature of excited states in a molecule. Fur thermore, by applying a Kramer-Kronig transformation to determine the real part of the response, we are able to assess the two-photon all-op tical device figure of merit of these materials. Such an understanding is important for designing molecules for all-optical device applicati ons.