Edge-Plasmon Whispering-Gallery Modes in Nanoholes

Metallic apertures remain one of the most intensively investigated optical structures due to their rich behavior. This encompasses physical phenomena such as Bragg scattering, surface plasmons, or localized modes, offering great potential for many applications in the growing fields of nanophotonics and plasmonics. Here, we show that isolated metallic nanoholes can additionally support edge plasmons in the form of whispering-gallery modes and we demonstrate that, although high-order modes remain dark at normal incidence, they can be excited at oblique illumination. As revealed by simulations, the existence of these modes in nanohole arrays is fundamental to understanding complex resonant phenomena and to explaining previous unexpected experimental results found in metamaterials. Notably, the symmetries of the employed lattice determine the excited mode orders, which, together with a strong dependence on the film thickness, yields an easily and highly tailorable optical response. Furthermore, some modes display unusually narrow line widths (approximately 7 nm) and a high sensitivity to the environment, resulting in one of the highest sensing figures of merit (approximately 80) reported for a localized plasmon. These singular features may find application in the construction of special polarizers and filters, single-layer zero-transmittance absorbers, and high-performance nanosensors.