Coupling of plasmonic nanopore pairs: facing dipoles attract each other

The coupling between two closely spaced nanopores has been analysed by cathodoluminescence scanning transmission electron microscopy. While the coupling behaviours between plasmonic nanoparticles have been well studied, that between their inverse structures — nanoapertures (or nanopores) — has been relatively neglected. Now, Takumi Sannomiya of Tokyo Institute of Technology and co-workers both imaged and simulated the symmetric and anti-symmetric modes of coupled 135-nanometre-diameter holes etched in a trilayer nanomembrane for hole spacings in the range 50-200 nanometres. They found that the symmetric coupling mode exhibits a lower energy resonance than the anti-symmetric mode, indicating that the facing dipoles attract each other. They also discovered that the resonance wavelength of the anti-symmetric mode increases as the gap distance grows. The insights are expected to be useful for designing plasmonic sensors and sub-wavelength photonic devices.