Abstract
The third-order nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$(-3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}) of thin films of poly(p-phenylenevinylene) (PPV) and several corresponding oligomers (OPV-n) has been investigated by third-harmonic generation using variable laser wavelengths from 900 to 1520 nm. The oligomers show a single three-photon resonance of ${\mathrm{\ensuremath{\chi}}}^{(3)}$(-3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}) which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the ${\mathrm{\ensuremath{\chi}}}^{(3)}$ spectrum of PPV. They are assigned to three-photon resonances with the maximum of the exciton absorption and with the threshold of the continuum of states, which can be located at 3.2\ifmmode\pm\else\textpm\fi{}0.1 eV. This corresponds to an exciton binding energy of 0.7\ifmmode\pm\else\textpm\fi{}0.1 eV. We observe a general scaling behavior for PPV, OPV-n, and other one-dimensional conjugated \ensuremath{\pi}-electron systems in their neutral form. Their ${\mathrm{\ensuremath{\chi}}}^{(3)}$ values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship ${\mathrm{\ensuremath{\chi}}}^{(3)}$/${\mathrm{\ensuremath{\alpha}}}_{\mathrm{max}}$\ensuremath{\sim}${\ensuremath{\lambda}}_{\mathrm{max}}^{\mathit{x}}$, where ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{max}}$ and ${\ensuremath{\lambda}}_{\mathrm{max}}$ denote the absorption coefficient and wavelength of the low-energy absorption maximum. We obtain an exponent x=10\ifmmode\pm\else\textpm\fi{}1 which is much larger than expected from an earlier theory. Possible reasons for the difference between theory and experimental results are discussed. \textcopyright{} 1996 The American Physical Society.
Published Version
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