Unidirectional Emission from a Single Upconversion Nanoparticle Optically Trapped in a Plasmonic Nanoantenna

A compatible and directional optical interface to route omni-directional light emission from nanoemitters into a specific path is essential for the construction of integrated photonic devices. An optical antenna as a converter between localized energy and propagating light introduces such a bridge to tailor the light-matter interaction in well-defined directions at the nanoscale. A well-controlled emission directivity relies on a precise local assembly between the nanoantenna and the emitting source (typically a semiconducting nanocrystal), but existing assemblies are largely based on stochastic means. We utilized optical nanotweezers to deterministically position a single rare-earth doped upconversion nanoparticle (UCNP) at the hotspot formed in the feed of a plasmonic Yagi-Uda antenna. The overlap of the optical force equilibrium point with the local hotspot naturally allows a precise construction of the UCNP-nanoantenna assembly, and the assembly process is under real-time monitoring to further ensure precision. The optical-force-driven assembly demonstrates not only a 50 times emission enhancement but, more importantly, a radiative front-to-back ratio of 7.5 dB. Our results suggest the significant potential of bifunctional optical antennas to deterministically position single nanoparticles for broad applications in nanophotonic structures for on-chip light-routing architectures.