Two-photon absorption and spectroscopy of the lowest two-photon transition in small donor-acceptor–substituted organic molecules

We determine the dispersion of the third-order polarizability of small donor-acceptor substituted organic molecules using wavelength-dependent degenerate four-wave mixing experiments in solutions with varying concentrations. We find that donor-acceptor--substituted molecules that are characterized by extremely efficient off-resonant nonlinearities also have a correspondingly high two-photon absorption cross section. The width and shape of the...