  Van der Waals (vdW) materials, including hexagonal boron nitride (hBN), are
layered crystalline solids with appealing properties for investigating
light-matter interactions at the nanoscale. hBN has emerged as a versatile
building block for nanophotonic structures, and the recent identification of
native optically addressable spin defects has opened up exciting possibilities
in quantum technologies. However, these defects exhibit relatively low quantum
efficiencies and a broad emission spectrum, limiting potential applications.
Optical metasurfaces present a novel approach to boost light emission
efficiency, offering remarkable control over light-matter coupling at the
sub-wavelength regime. Here, we propose and realise a monolithic scalable
integration between intrinsic spin defects in hBN metasurfaces and high quality
(Q) factor resonances leveraging quasi-bound states in the continuum (qBICs).
Coupling between spin defect ensembles and qBIC resonances delivers a 25-fold
increase in photoluminescence intensity, accompanied by spectral narrowing to
below 4 nm linewidth facilitated by Q factors exceeding $10^2$. Our findings
demonstrate a new class of spin based metasurfaces and pave the way towards
vdW-based nanophotonic devices with enhanced efficiency and sensitivity for
quantum applications in imaging, sensing, and light emission.
