Hybridization of Gap Plasmons and Grating‐Coupled Surface Plasmon Polaritons in Metal Nanoparticle Cluster Arrays on a Mirror

ABSTRACT Metallic nanoparticles exhibit unique plasmonic properties, enabling the confinement of electromagnetic fields at the nanoscale. In the nanoparticle‐on‐mirror (NPoM) geometry, nanoparticles vertically coupled to a metallic mirror across a dielectric spacer sustain tightly confined gap plasmons with limited direct far‐field access. Combining these cavities with periodic arrays allows for the excitation of diffractively excited resonances that act as radiative gateways, enabling efficient far‐field excitation and enhanced coupling to the gap modes. However, preparing such hybrid metasurfaces remains challenging because of the disparate length scales involved and the reliance on poorly scalable top‐down nanofabrication techniques. Here, we address these challenges by using a template‐assisted self‐assembly technique with metal colloids resulting in a hybrid plasmonic metasurface, a periodic array of clustered metal nanoparticles separated from a metallic film by a dielectric spacer. Our nanostructure allows the interplay of the grating‐coupled Surface Plasmon Polaritons (SPP), the Surface Lattice Resonance (SLR) and the gap mode through a variety of geometrical parameters enabling hybrid mode engineering and offer a route toward advanced photonic devices with tailored optical responses.