Excited-state enhancement of optical nonlinearities in linear conjugated molecules

Abstract

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 & psi;(3), which is related to the intensity-dependent refractive index) and a small signal attenu-ation arising from the linear optical absorption. Conjugated organic molecules and polymers are of particular interest in this context: the delocalized π-electron systems of these materials give rise to relatively large values of & psi;(3) with extremely fast response times, in wavelength regimes where there is minimal background absorption. Previous theoretical studies2 suggested a new enhance-ment 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 & psi;(3) can be enhanced by more than two orders of magnitude without increasing optical absorption at the probe wavelength.