3RD-HARMONIC-GENERATION SPECTROSCOPY OF POLY(P-PHENYLENEVINYLENE) - ACOMPARISON WITH OLIGOMERS AND SCALING LAWS FOR CONJUGATED POLYMERS

The third-order nonlinear optical susceptibility chi((3))(-3 omega;ome ga,omega omega) of thin films of poly(p-phenylenevinylene) (PPV) and s everal 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 absorp tion spectrum. We can identify, however, two maxima in the chi((3)) sp ectrum of PPV. They are assigned to three-photon resonances with the m aximum of the exciton absorption and with the threshold of the continu um of states, which can be located at 3.2+/-0.1 eV. This corresponds t o an exciton binding energy of 0.7+/-0.1 eV. We observe a general scal ing behavior for PPV, OPV-n, and other one-dimensional conjugated pi-e lectron systems in their neutral form. Their chi((3)) values, evaluate d at comparable resonant or low-resonant conditions, follow an empiric al 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 exponen t x=10+/-1 which is much larger than expected from an earlier theory. Possible reasons for the difference between theory and experimental re sults are discussed.