Strong Dipole Interaction between Chlorophyll-a Molecules and Surface Plasmon Polaritons

The ability of surface plasmon polaritons (SPPs) to localize light on the nanoscale dimensions results in significant field enhancement enabling efficient intensification of light matter interactions. Here, we report observation of strong radiative dipole coupling between molecular excitons in chlorophyll-a (Chl-a) and propagating SPPs excited on gold thin films as well as localized SPPs of gold nanoparticles at room temperature. The optical response of the Chl-a-metal hybrid nanostructures is investigated in spectral as well as temporal domains by using various spectroscopy techniques, which reveal polariton formation with a very large coupling strength of similar to 100 meV (24.2 THz). This strong dipole interaction considerably enhances the Chl-a absorption over a much wider spectrum and gives rise to SPP-exciton polariton emission. The changes in the optical response are also accompanied by ultrafast relaxation dynamics as well as suppression of luminescence yield. Our results suggest that plasmonic nanostructures can significantly alter optical response of Chl-a molecules via vacuum field interactions.