# Dispersion of third order nonlinear optical properties of squaraines

#### Abstract

The effect of centrosymmetry on the third order nonlinear optical properties of most organic molecules remains unknown. General findings concerning symmetry effects applicable to all molecules is a crucial step towards the establishment of qualitative relationships between a molecule's structure and its third order properties. These structure-property relationships will guide the search for technologically promising materials. Previous theoretical studies on the effect of lowered symmetry on polyenes, one dimensional conjugated linear chains, has provided great insight into the basic origin of third order nonlinear optical effects. These studies showed that the important virtual excitation sequences can be classified into distinct types. The effect of symmetry on the largest of these terms will be discussed in detail. The work presented in this thesis addresses the effects of lowered symmetry on a new class of organic molecules, squaraines. Calculations of $\gamma({-}3\omega; \omega, \omega, \omega$) of squaraines employing the same methods which successfully described the polyenes, are presented. A complete test of these calculations requires measurements of the dispersion of $\gamma({-}3\omega; \omega, \omega, \omega$) at frequencies throughout the visible and near IR regions. These measurements have been performed for a centrosymmetric squaraine (BSQ) and a noncentrosymmetric squaraine (2-hydroxy-4-N,N-diethylaminophenyl) (4$\sp\prime$-isopentyloxyphenyl) squaraine (UNSQA). Comparison of the dispersion measurements for BSQ with the results for UNSQA reveals that lowered symmetry greatly affects $\gamma({-}3\omega; \omega, \omega, \omega$). The sign of $\gamma({-}3\omega; \omega, \omega, \omega$) below all resonances is negative for BSQ and positive for UNSQA. The dispersions are also markedly different. BSQ exhibits a strong, sharp resonance with a low lying two photon state in the near IR region, while UNSQA shows a much weaker and broader feature which likely results from a two photon resonance with a low lying state and a three photon resonance with a higher lying two photon state. Calculated dispersions are qualitatively correct, but do not always describe the resonances in a quantitatively correct fashion.

#### Subject Area

Optics|Molecules|Chemistry

#### Recommended Citation

Wu, Marvin H, "Dispersion of third order nonlinear optical properties of squaraines" (1995). Dissertations available from ProQuest. AAI9543157.
https://repository.upenn.edu/dissertations/AAI9543157

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