Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity.

The negatively charged boron vacancy (V B - ) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, V B - preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of V B - defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of V B - with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled V B - defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single V B - defects.