Accessing plasmonic hotspots using nanoparticle-on-foil constructs

Metal–insulator–metal (MIM) nanogaps in the canonical nanoparticle-on-mirror geometry (NPoM) provide deep-subwavelength confinement of light with mode volumes smaller than V/Vλ < 10–6. However, access to these hotspots is limited by the impendence mismatch between the high in-plane k∥ of trapped light and free-space plane-waves, making the in- and out-coupling of light difficult. Here, by constructing a nanoparticle-on-foil (NPoF) system with thin metal films, we show the mixing of insulator–metal–insulator (IMI) modes and MIM gap modes results in MIMI modes. This mixing provides multichannel access to the plasmonic nanocavity through light incident from both sides of the metal film. The red-tuning and near-field strength of MIMI modes for thinner foils is measured experimentally with white-light scattering and surface-enhanced Raman scattering from individual NPoFs. We discuss further the utility of NPoF systems, since the geometry allows tightly confined light to be accessed simply through different ports.