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

The third-order nonlinear optical susceptibility chi((3))(-3 omega;omega,omega 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 chi((3))(-3 omega;omega,omega,omega) which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the 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+/-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 pi-electron systems in their neutral form. Their chi((3)) values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship chi((3))/alpha(max)similar to lambda(max)(x), where 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.