High contrast shallow spin defects in hexagonal boron nitride enhanced by surface plasmon for quantum sensing

The recently discovered spin defects in hexagonal boron nitride (hBN) are emerging candidates for quantum sensing. However, the sensitivity of negatively charged boron vacancy (VB-) defects is limited by its weak photoluminescence and relatively low contrast of the optically detected magnetic resonance (ODMR). Here, we report a simultaneous enhancement of the photoluminescence of hBN spin defects by up to 17-fold by the surface plasmon of a gold-film microwave waveguide. We observed a record-high ODMR contrast of 46% at room temperature.  We also explore the effects of microwave and laser powers on the ODMR, and improve the sensitivity of hBN spin defects for magnetic field detection. Our samples were created by low-energy He+ ion implantation which generates shallow spin defects close to the hBN surface. We show that T1 and T2 relaxation times are independent of the ion energy, indicating that the spin properties of VB- defects are nearly independent of the depth.