Coupling of plasmonic nanopore pairs: facing dipoles attract each other

Control of the optical properties of nano-plasmonic structures is essential for next-generation optical circuits and high-throughput biosensing platforms. Realization of such nano-optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as dark modes, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well studied, couplings of inverse structures, that is, holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti-symmetric coupling modes has not been performed. Here we present, for the first time, a systematic study of the symmetric and anti-symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy and electromagnetic simulation. The symmetric coupling mode, approximated as a pair of facing dipoles, is observed at a lower energy than that of the anti-symmetric coupling mode, indicating that the facing dipoles attract each other. The anti-symmetric coupling mode splits into the inner- and outer-edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as inverses of coupled disks. Symmetric and anti-symmetric coupling modes are also observed in a short-range ordered pore array, where one pore supports multiple local resonance modes, depending on the distance to the neighboring pore. Accessibility to the observed symmetric modes by far field is also discussed, which is important for nanophotonic device applications.