We studied the spin depolarization of ensembles of nitrogen-vacancy (NV) centers in nitrogenrich single crystal diamonds. We found a strong dependence of the evolution of the polarized state in the dark on the concentration of NV centers. At low excitation power, we observed a simple exponential decay profile in the low-density regime and a paradoxical inverted exponential profile in the high-density regime. At higher excitation power, we observed complex behavior, with an initial sharp rise in luminescence signal after the preparation pulse followed by a slower exponential decay. Magnetic field and excitation laser power-dependent measurements suggest that the rapid initial increase of the luminescence signal is related to recharging of the nitrogen-vacancy centers (from neutral to negatively charged) in the dark. The slow relaxing component corresponds to the longitudinal spin relaxation of the NV ensemble. The shape of the decay profile reflects the interplay between two mechanisms: the NV charge state conversion in the dark and the longitudinal spin relaxation. These mechanisms, in turn, are influenced by ionization, recharging and polarization dynamics during excitation. Interestingly, we found that charge dynamics are dominant in NV-dense samples even at very feeble excitation power. These observations may be important for the use of ensembles of NV centers in precession magnetometry and sensing applications. arXiv:1801.00047v1 [cond-mat.mes-hall]

Sensing of a few unpaired electron spins, such as metal ions and radicals, is a useful but difficult task in nanoscale physics, biology, and chemistry. Single nitrogen-vacancy (NV) centers in diamond offer high sensitivity and spatial resolution in the optical detection of weak magnetic fields produced by a spin bath, but often require long acquisition times, of the order of seconds. Here we use a novel approach, based on coupled spin and charge dynamics in dense NV ensembles in strongly fluorescent nanodiamonds (NDs) to sense external magnetic dipoles. We apply this approach to various paramagnetic species, including gadolinium complexes, magnetite nanoparticles and hemoglobin in whole blood. Taking advantage of the high NV density, we demonstrate detection of subnanomolar amounts of Gd spins with ultrashort acquisition time approaching 10 ms. Strong luminescence, high sensitivity and short acquisition time make dense NVensembles in NDs a powerful tool for biosensing and bioimaging applications.

Nitrogen-vacancy (NV) centers in diamond have attracted considerable interest in sensing of weak magnetic fields, such as those created by biological systems. Detecting such feeble signals requires near-surface NV centers, to reduce the distance between NVs and sources. Moreover, dense ensembles of NVs are highly desirable to reduce measurement time. However, robust charge state switching is often observed in these systems, resulting in a complex interplay between charge and spin dynamics that can reduce the attainable level of spin polarization, and consequently, sensitivity. Understanding the mechanisms behind charge state switching is, therefore, crucial to developing NV based sensors. Here, we demonstrate a novel method to selectively measure charge dynamics in an ensemble of NVs by quenching the spin polarization using an off-axis magnetic field. Utilizing this technique, we show that, in nanodiamonds, charge state instability increases with increasing NV density. In the case of bulk single crystal diamond, we show that NV centers located near the surface are more stable in the neutral (NV 0 ) charge state, while the negatively charged (NV − ) form is more stable in bulk. arXiv:1812.02702v1 [cond-mat.mes-hall]