Abstract
The two-level model has been used to describe the second-order polarizability (hyperpolarizability) of organic nonlinear dyes but has been shown to be wholly inadequate for the characterization of third-order nonlinearities in conjugated organic and polymeric materials. Most organic molecules studied have shown a positive value for γijkl(− ω4; ω1, ω2, ω3) when all frequencies are below the first absorption band; this is inconsistent with a two-level model and generally requires a two-photon state. Additionally, in polymeric materials substantial contributions from two-photon processes are required to explain the dispersion of the nonlinearity. Experimentally locating and characterizing the two-photon states by using all-optical nonlinear techniques is then of considerable interest for understanding such systems.
© 1994 Optical Society of America
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