Abstract
The third-order nonlinear optical susceptibility (-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 (-3ω;ω,ω,ω) which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the 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 values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship /∼, where and 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.
- Received 23 May 1995
DOI:https://doi.org/10.1103/PhysRevB.53.4367
©1996 American Physical Society