Mirror-coupled plasmonic nanostructures for enhanced in-plane magnetic dipole emission

Abstract Self-assembling of plasmonic nanoparticles into an oligomer can produce optical nanoantennas with resonant magnetic responses, which constituents a promising platform to study magnetic light-matter interactions on the nanoscale. However, these self-assembled magnetic nanostructures typically feature a flat-lying geometry configuration, giving rise to out-of-plane magnetic dipole moments that cannot couple to the far-field light efficiently. Herein, we proposed a new geometric configuration of plasmonic nanoantennas to realize in-plane magnetic dipole responses. This was achieved by elegantly coupling a plasmonic nanoparticle oligomer to one or more adjacent metal mirrors, virtually forming a vertically standing nanoparticle tetramer or trimer yet with in-plane magnetic dipole moments. We verified this design strategy by numerically simulating the resonance responses of three typical mirror-coupled nanostructures, all exhibiting pronounced resonant magnetic dipole characters. Furthermore, optical magnetic emitters can be readily coupled to these plasmonic nanoantennas and gain an emission enhancement with two orders of magnitude, while simultaneously featuring a high emission directionality (collection efficiency up to ~70% evaluated with common microscopy optics). We believe these mirror-enabled magnetic nanoantennas could lead to novel nanophotonic devices fully exploiting the magnetic nature of light.