Nonlinear plexcitons: excitons coupled with plasmons in two-photon absorption

Abstract

The nonlinear optical properties of a D–A (donor–acceptor) conjugated organic molecule with polythiophene (PT) as the donor and indene-C₆₀ bisadduct (IC₆₀BA) as the acceptor are theoretically investigated, which exhibits a large two-photon absorption (TPA) cross-section up to 8000 GM at the wavelength of 780 nm. Combining surface plasmon resonances (SPRs) with nonlinear optics, nonlinear properties can be strongly enhanced. In this paper, an appropriate nonlinear plexciton method by the coupling of Au@Ag nanorods and an Ag film is designed, in which the TPA properties of the PT:IC₆₀BA complex can be increased by 10⁶ times. The angle dependence on polarization and incidence is investigated to obtain the maximum of plasmonic enhancement. Our results emphasize the physical mechanism of nonlinear plexcitons and provide a feasible method to improve the nonlinear properties of organic solar cell materials.