Third-harmonic-generation spectroscopy of poly(p-phenylenevinylene): A comparison with oligomers and scaling laws for conjugated polymers

The third-order nonlinear optical susceptibility ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-3ω;ω,ω,ω) 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 ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-3ω;ω,ω,ω) which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ 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±0.1 eV. This corresponds to an exciton binding energy of 0.7±0.1 eV. We observe a general scaling behavior for PPV, OPV-n, and other one-dimensional conjugated π-electron systems in their neutral form. Their ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$/${\mathrm{\alpha }}_{\max }$∼${\lambda }_{\max }^{\mathit{x}}$, where ${\mathrm{\alpha }}_{\max }$ and ${\lambda }_{\max }$ denote the absorption coefficient and wavelength of the low-energy absorption maximum. We obtain an exponent x=10±1 which is much larger than expected from an earlier theory. Possible reasons for the difference between theory and experimental results are discussed. © 1996 The American Physical Society.