EXCITED-STATE ENHANCEMENT OF OPTICAL NONLINEARITIES IN LINEAR CONJUGATED MOLECULES

NONLINEAR optical phenomena form the basis for all-optical devices such as optically bistable switches and nonlinear directional couplers1. The suitability of a material for these device applications requires a large magnitude of the relevant nonlinear effect (in this case, the third-order optical susceptibility chi(3), which is related to the intensity-dependent refractive index) and a small signal attenuation arising from the linear optical absorption. Conjugated organic molecules and polymers are of particular interest in this context: the delocalized pi-electron systems of these materials give rise to relatively large values of chi(3), with extremely fast response times, in wavelength regimes where there is minimal background absorption. Previous theoretical studies2 suggested a new enhancement mechanism for the nonlinear optical processes in these materials through population of the electronic excited states. Here we show that by optically exciting a linear conjugated molecule at one wavelength into an electronic excited state for a sufficient length of time to perform the nonlinear optical measurement, the value of chi(3) can be enhanced by more than two orders of magnitude without increasing optical absorption at the probe wavelength.