High Order Gap Modes of Film-Coupled Nanospheres

Tailoring coupled plasmonic structures is an alternative way to obtain new optical properties of plasmonic materials. Recently, film-coupled nanoparticle systems with high stability and controllability have been used to probe the ultimate limits of field enhancement/confinement and near-field interaction with other quantum emitters. When the gap between particles and films is below a few nanometers, induced high order (HO) gap modes become significant. In this report, we investigate these HO modes associated with the system of a gold nanosphere positioned above a gold film, separated by a nanometer-scale spacer layer. The shift in far-field scattering profile under different excitation conditions and collection wavelengths indicates the influence of HO modes, and the results are compared to the corresponding simulations. In addition, the far-field scattering spectra/patterns by multipole expansion and the near-field distributions by FEM, both calculated with the dielectric function of the low damping factor, are utilized to resolve the individual HO modes. These findings not only identify the HO gap modes but also clarify their excitation conditions and far-field/near-field scattered field distribution. The effect of HO modes should be taken into account when the interaction between the gap field and quantum emitters nearby is investigated for active plasmonic devices.