Spectral and spatial mode engineering of plasmonic nanocavities: Subradiant modes and tunable Fano resonances

Plasmonic nanostructures can serve as unit cells of new types of optical metamaterials with carefully engineered optical properties. For example, hybridized plasmonic systems consisting of several metallic subunits separated by nanoscale gaps can exhibit superradient and subradient modes due to dipolar coupling between the individual units. Additionally, in nanostructures with broken symmetry, Fano resonances can arise due to the interaction of narrow dark modes with broad bright modes. This paper presents an experimental demonstration of Fano resonances in two plasmonic systems: firstly a dimer/monomer slab slab structure ("dolmen") and secondly for a side-by-side arrangement of a Au ring and a Au disk (ring near disk cavity, RNDC), both fabricated using e-beam lithography.The dependence of the Fano lineshapes on the polarization of the incident light, and of the strength of the feature on the separation between individual nanoscale sub-units, is analyzed. Results show that for significantly broad dipolar modes, multiple Fano resonances can arise. The observed Fano-type dispersive features constitute a classical analogue to the well-known phenomenon of electromagnetically induced transparency (EIT).