Direct Measurement of the Singlet Lifetime and Photoexcitation Behavior of the Boron Vacancy Center in Hexagonal Boron Nitride
Optically active spin defects in van der Waals (vdW) materials are a promising platform for quantum sensing, potentially enabling shorter standoff distances than defects in diamond and thus improved measurement signal-to-noise ratio (SNR) and spatial resolution. The most studied such defect is the negatively charged boron vacancy center ($V^{-}_{B}$) in hexagonal boron nitride (hBN), yet many of its electronic and spin transition rates and branching ratios remain unknown. Here, we use time-resolved photoluminescence (PL) measurements with a nanosecond rise-time 515 nm laser to directly measure the singlet state lifetime of a $V^{-}_{B}$ ensemble in neutron-irradiated, sub-micron flakes of hBN. We perform this measurement on 16 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 sub-micron hBN flake, and fit our results to a 9-level model to extract 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 $μ$m.