Spectral and Spatial Mode Engineering of Plasmonic Nanocavities: Subradiant Modes and Tunable Fano Resonances

摘要

Plasmonic nanostruetures 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 nanostruetures with broken symmetry, Fano resonances can arise due to the interaction of narrow dark modes with broad bright modes. We have recently identified non-concentric ring/disk cavities as a system with a highly tunable Fano resonance and exceptionally large refractive index sensitivity and localized surface plasmon resonance figure of merit [1]. Here we present an experimental demonstration of Fano resonances in two plasmonic systems: firstly a dimer/monomer slab slab structure ("dolmen"), first proposed by Zhang et al [2], 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. We analyze the dependence of the Fano lineshapes on the polarization of the ineident light, and of the strength of the feature on the Separation between individual nanoscale sub-units. We further show that for significantly broad dipolar modes, multiple Fano resonances can arise. The observed Fano-type dispersive features constitute a classical analogue [3] to the well-known phenomenon of electromagnetically induced transparency (EIT).