Enhancement of second order hyperpolarizabilities via SERS techniques in organic materials

Abstract

The usual strategy to improve molecular hyperpolarizabilities (β and γ) is the chemical functionalization of polyconjugated molecular chromophores (e.g. by increasing the number of delocalized π electrons or adding suitable electron-donating or electron-withdrawing functional groups) with the result of an increased molecular polarizability. In this work, we show that it is possible to increase the molecular polarizability in the presence of metal nanoparticles. The vibrational contribution to molecular polarizabilities has been evaluated by using experimental Raman intensities. SERS (surface enhanced Raman spectroscopy) techniques can enhance such signals and therefore improve the second-order hyperpolarizability by orders of magnitude. In particular we have measured the mean value over the entire scattering volume of the second-order hyperpolarizability for some organic molecules in a silver colloidal solution. We have observed an increase of second-order hyperpolarizability of about six orders of magnitude reaching values of about 10−28 esu. This large value of γv has been reached in spite of the fact that molecules used in the experiments were not optimized for third order nonlinear optics as shown by the low value of the second order hyperpolarizability of the isolated molecule (≤10−34 esu). Copyright © 2005 John Wiley & Sons, Ltd.