Enhanced readout contrast of V2 ensembles in 4H-SiC through resonant optical excitation
The V2 silicon vacancy defect in 4H-SiC has emerged as a promising system for quantum technologies due to its favorable optical and spin properties and the advantages of the SiC host. However, the readout contrast - an important benchmark for quantum sensing - of V2 ensembles for optically-detected magnetic resonance (ODMR) is relatively low, usually <1$\%$ at room temperature. To overcome this challenge, we resonantly excite the V2 ensembles at cryogenic temperatures and compare the results with the off-resonant case. We report a maximum ODMR contrast of 50$\%$ with only 2 $\mu$W of resonant laser power, almost 100 times improvement over off-resonant excitation. We attribute this high readout contrast to a subset of V2 centers that have one spin-selective optical transition resonant with the laser. The ODMR contrast decreases with temperature, approaching the non-resonant contrast by 60 K, likely due to broadening of the optical transition linewidths. We achieve a maximum sensitivity of 100 nT/$\sqrt{Hz}$ with a resonant laser power of 300 $\mu$W, while 100 times more non-resonant excitation power is needed to achieve comparable sensitivity.