Intermediate excited state relaxation dynamics of boron vacancy spin defects in hexagonal boron nitride

Optically addressable spin defects in hexagonal boron nitride offer promising potential for 2D quantum sensing, although excited-state dynamics remain poorly understood. In particular, the nonradiative relaxation paths from the excited triplet states to the ground state, especially those involving a shelving intermediate state (IS), remain largely hypothetical, and the rate constants have yet to be directly measured. In this work, we investigate the relaxation dynamics of the IS in the optical pumping cycle in a broad temperature range. We measure a 24.0(3)-nanosecond relaxation time from IS to the ground state at room temperature, which approximately doubles at low temperatures. Simulations reveal how spin populations and ground-state polarization evolve with varying excitation rates. Accordingly, we optimize optically detected magnetic resonance pulse sequences to account for the effects of IS relaxation. This considerably enhances spin manipulation efficiency, allowing substantial optimization of the quantum sensor's sensitivity based on boron vacancies.