Quantum Sensing of Opaque Materials with Plasmonically Enhanced Hexagonal Boron Nitride Spin Defects

Spin defects in hexagonal boron nitride (hBN) are emerging platforms for quantum sensing. The negatively charged boron vacancy ($V_B^{-}$) is widely studied due to its robust spin properties, but its low brightness often requires plasmonic enhancement, limiting sensing in optically opaque or scattering environments, such as batteries. Here, we design and nanofabricate a coplanar waveguide integrated with nanoslit arrays to enable back-side excitation and photoluminescence collection from hBN spin defects. Using a neon focused ion beam, we pattern periodic nanoslits through a thin gold film, allowing simultaneous microwave delivery and optical access through the substrate. We demonstrate device performance via T₁ relaxometry and magnetic field mapping of nickel nanoparticles. This platform enables plasmonically enhanced quantum sensing in opaque materials and liquids, expanding applications beyond conventional transparent systems.