Ultrafast near-infrared laser probing of intersystem crossing dynamics in single nitrogen-vacancy center

The intersystem crossing (ISC) processes in negatively charged nitrogen-vacancy (NV⁻) centers have been extensively studied yet remain incompletely understood. Recent studies have demonstrated that near-infrared (NIR) laser modulation operates independently of both ISC and charge conversion processes. Leveraging this finding, we employ an ultrafast NIR pulsed laser as an optical probe to examine the ISC processes in a stable single NV⁻center. By developing a comprehensive six-level model that incorporates NIR modulation and spin-selective transitions, we quantitatively characterize the influence of excitation laser power on ISC dynamics across an extensive power range, complementing prior research that focused primarily on microwave field and temperature effects. Furthermore, our model successfully explains the previously unresolved NIR-induced fast quenching phenomenon. Numerical solutions of the rate equations yield ISC rates that show excellent agreement with both experimental data and theoretical predictions. In contrast to prior studies, our results reveal that the ISC shelving rates follow a power-law dependence on excitation laser field, providing insights into excitation and relaxation dynamics in NV⁻centers. The NIR modulation methodology presented in this work establish a reliable framework for spin state manipulation and charge state control, with applications in quantum sensing and characterization of diverse color center systems.