Plasmonic metasurfaces with tunable gap and collective SPR modes

M. Haftel, C. Eagan, A.O. Pinchuk
University of Colorado at Colorado Springs,
United States

Keywords: plasmonics, metamaterials, metasurfaces, collective and gap modes

Summary:

Metasurfaces fabricated with plasmonic nanostructures are of interest for a wide range of potential applications in optics, photovoltaic, microscopy, biochemical sensing and surface enhanced spectroscopy. When placed in close proximity to a thin metal film, gap modes may be induced between plasmonic nanostructures and the metal film. The enhanced electromagnetic field between the plasmonic nanostructures and the film might be used for surface enhanced spectroscopy (e.g. SERS) or efficient generation of electron-hole pairs in solar cells. We present our recent results on the optical response of a plasmonic metamaterial consisting of a Au nanoparticle monolayer separated by a dielectric spacer layer from an aluminum film by calculating theoretically and measuring experimentally the extinction spectrum as functions of the geometries of the spacer layer and nanoparticle monolayer for wavelengths of 400-800 nm. The goal is to elucidate the roles of the collective surface plasmon resonance (SPR) mode and possible gap plasmon modes and their dependence on the spacer thickness, and nanoparticle spacing and size. With finite-difference-time-domain (FDTD) simulations we find that the SPR extinction peak first red-shifts then separates into two peaks, the second of which is at 620 nm or greater, as the gap mode gradually makes itself evident as the spacer layer decreases from 35 nm to 7.5 nm. The red-shifting and separation is accelerated for larger nanoparticle spacing and size. The FDTD results follow from an examination of the dispersion curves of the related multilayer system. The computational results reproduce the experimental trends with respect to the spacer layer thickness.