Charge-State Dynamics under Phonon-Assisted Excitation in Diamond NV Centers: Implications for Optical Cooling

Anti-Stokes optical cooling in diamond nitrogen-vacancy (NV) centers is investigated using photoluminescence (PL) spectroscopy, time-resolved measurements, and rate-equation modeling. Under excitation below the zero-phonon line, anti-Stokes emission from the negatively charged NV– state appears, but optical excitation also induces charge-state conversion between NV– and neutral NV0. Excitation-dependent PL dynamics reveal a reduction of the NV– population under strong excitation. A minimal rate-equation model constrained by nanosecond and millisecond PL measurements predicts that photoionization suppresses the NV–-mediated cooling contribution and leads to self-limiting cooling behavior. The calculations further show that net cooling requires exceptionally high quantum efficiency and does not increase monotonically with excitation density. These results highlight photoinduced charge-state conversion as a key photophysical bottleneck for defect-based anti-Stokes optical cooling.