Ultrafast optical nonlinearities in hybrid metal-J-aggregate nanostructures

The strong coupling between excitons and surface plasmon polaritons (SPP) in metal-semiconductor and metal-dye nanostructures are interesting because in analogy to semiconductor microcavities may be of importance in designing novel nano-optic devices. So far, however, very little is known about such nonlinearities and rather weak effects have been observed. This paper reports on the first observation of ultrafast optical nonlinearities in metal-J-aggregate hybrid nanostructures. Results demonstrate that the strong coupling between surface plasmon polaritons and excitons drastically alters the polariton dynamics. A metal-J-aggregate, hybrid nanostructure consisting of an aggregated cyanine dye-polymer film on a gold grating, is investigated. Strong, linear exciton-SPP coupling in these nanostructures is verified by angle-resolved, p-polarized reflectivity measurements, revealing significant bending due to the SPP-exciton polariton formation. These linear spectra are well understood within a coupled oscillator model with a coupling strength of ~20 meV. Near SPP resonances, strong polariton nonlinearity is observed, having a dispersive lineshape even at probe wavelengths which are strongly detuned from the exciton-SPP crossing, accompanied by significant changes in the dynamics of the polariton nonlinearity.