Excited-State Spectroscopy of Spin Defects in Hexagonal Boron Nitride.

A negatively charged boron vacancy (VB-) color center in hexagonal boron nitride has recently been proposed as a promising quantum sensor due to its excellent properties. However, the spin level structure of the VB- color center is still unclear, especially for the excited state. Here we measured and confirmed the excited-state spin transitions of VB- using an optically detected magnetic resonance (ODMR) technique. The zero-field splitting of the excited state is 2.06 GHz, the transverse splitting is 93.1 MHz, and the g factor is 2.04. Moreover, negative peaks in fluorescence intensity and ODMR contrast at the level anticrossing point were observed, and they further confirmed that the spin transitions we measured came from the excited state. Our work deepens the understanding of the excited-state structure of VB- and promotes VB--based quantum sensing applications.