Direct Measurement of the Singlet Lifetime and Photoexcitation Behavior of the Boron Vacancy Center in Hexagonal Boron Nitride

Optically active electronic spin defects in van der Waals (vdW) materials provide a promising platform for quantum sensing, as they can enable shorter standoff distances compared to defects in diamond, leading to sensitivity advantages. Perhaps the most studied defect in a vdW material is the negatively charged boron vacancy center ($V^{-}_{B}$) in hexagonal boron nitride (hBN). However, many of the $V^{-}_{B}$ electronic and spin transition rates and branching ratios are not fully known. Here, we use time-resolved photoluminescence (PL) measurements with a nanosecond rise-time 515 nm laser to determine directly the singlet state lifetime of a $V^{-}_{B}$ ensemble in neutron-irradiated, sub-micron-size flakes of hBN. We perform this measurement on 16 different hBN flakes at room temperature and obtain an average lifetime of 15(3) ns. Additionally, we probe the PL dynamics of thermal and optically polarized electronic spin distributions of the $V^{-}_{B}$ ensemble in a single sub-micron hBN flake, and fit our results to a 9-level model to extract the electronic transition rates. Lastly, we present PL measurements that potentially indicate optically-induced conversion of $V^{-}_{B}$ to another electronic state, or possibly the neutral charge state (V$^{0}_{B}$), in neutron-irradiated hBN flakes of size $>$ 1 $\mu$m.