Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride

Negatively charged boron vacancy (V₁^-) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D₄ₒ=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^-) centers in diamond, the ODMR contrast of V₁^- centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V₁^- defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology.