Spectral Tuning of High Order Plasmonic Resonances in Multimodal Film-Coupled Crystalline Cavities

Sub-micrometric and ultrathin gold cavities sustain several high order planar plasmon resonances in the visible to near-infrared spectral window that open new perspectives for the realization of self-assembled metasurfaces or integrated components for nano-optics. This article investigates in detail the far-field spectral features of these multimodal crystalline gold nanoprisms, deposited on either dielectric (glass) or metallic substrates (Au, Al) by dark-field scattering spectroscopy. Relying on the computation of the plasmonic density of states, the signature of each planar resonance is de-convoluted from the experimental global response of single cavities as a function of the cavity size and the substrate nature. The redshifting dispersion of each resonance is extracted from this decomposition analysis and agrees with predictions from a Green dyadic method based numerical tool. In addition, in the presence of a metallic film, a characteristic redshift or blueshift of the global response is observed for each cavity that results from a metal-specific redistribution of the resonances. The fine spectral tuning of high order plasmonic resonances achieved here reveals the potential of the metal-insulator-metal gap geometry for a static modal engineering within ultrathin gold cavities.