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

MATHY, A.; UEBERHOFEN, K; SCHENK, R.; GREGORIUS, H.; GARAY, R. O.; MÜLLEN., K.; BUBECK, C.

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

American Physical Society

Lugar: New York; Año: 1996 vol. 53 p. 4367 - 4376

0163-1829

The third-order nonlinear optical susceptibility x~3~23 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 x~3!~23 which is closely related o the linear absorption spectrum. We can identify, however, two maxima in the x~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 3.260.1 eV. This corresponds to an exciton binding energy of 0.760.1 eV. We observe a general scaling behavior for PPV, OPV-n, and other one-dimensional conjugated p-electron systems in their neutral form. Their x~3 values,evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship x(3)/amax;lmax x , where amax and lmax denote the absorption coefficient and wavelength of the low-energyabsorption maximum. We obtain an exponent x51061 which is much larger than expected from an earliertheory. Possible reasons for the difference between theory and experimental results are discussed.