Coupling Nanostructured Plasmon–Strain Microwave Waveguide to Spin Defects in Hexagonal Boron Nitride for High‐Sensitivity Quantum Sensors

Despite seamless integration of hexagonal boron nitride (hBN) with on-chip devices, the intrinsically low optical quantum yield of spin-active boron vacancy ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>V</mml:mi> <mml:mi>B</mml:mi> <mml:mo>-</mml:mo></mml:msubsup> <mml:annotation>${\mathrm{V}}_{\mathrm{B}}^ - $</mml:annotation></mml:semantics> </mml:math> ) defects remains a significant limitation to the sensitivity of hBN-based quantum sensors. Here, we demonstrate an hBN quantum sensor with enhanced quantum yield and high DC magnetic field sensitivity (η_DC), achieved by coupling <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>V</mml:mi> <mml:mi>B</mml:mi> <mml:mo>-</mml:mo></mml:msubsup> <mml:annotation>${\mathrm{V}}_{\mathrm{B}}^ - $</mml:annotation></mml:semantics> </mml:math> defects in hBN with a nanostructured plasmon-strain microwave waveguide architecture.This platform is realized by fabricating arrays of alumina-coated gold nanopillars, or plasmonic nanoresonators (PNRs), onto the constricted region of a microwave-efficient, single-port gold coplanar waveguide. The alumina coating acts as a dielectric barrier that suppresses photoluminescence (PL) quenching, while gold nanopillars enhance local electromagnetic fields and induce strain-driven perturbations of the defect energy levels, causing accelerated photo-emission. This synergistic effect results in a ∼tenfold enhancement in PL and improves optically detected magnetic resonance to -17% for on-PNR regions, exceeding comparable prior works by over an order of magnitude. Consequently, we achieve an η_DC of 9.4 µT/√Hz, approaching the highest reported values for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>V</mml:mi> <mml:mi>B</mml:mi> <mml:mo>-</mml:mo></mml:msubsup> <mml:annotation>${\mathrm{V}}_{\mathrm{B}}^ - $</mml:annotation></mml:semantics> </mml:math> defects. This research establishes a strategy for designing and fabricating highly sensitive quantum sensors that operate at room temperature without requiring extensive optimization of laser or microwave fields.