The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is important not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation without the presence of mobile carriers remain unclear. Here, we elucidate this issue for the case of photoionization of NV center ensembles. Using pump-probe spectroscopy, we ionize negatively charged NV centers and monitor the recovery of NV^{-} on timescales of up to several seconds. We find that the recovery rate depends strongly on the concentration of surrounding nitrogen donors. Remarkably, the equilibration dynamics exhibit no discernible dependence on temperature, ruling out thermally activated processes. We develop a multiphonon-assisted electron-tunneling model, supported by density-functional calculations, that explains the measurements and identifies tunneling as the equilibration mechanism. Because tunneling rates depend exponentially on donor separation, the random distribution of nitrogen donors spreads recovery times from nanoseconds to hours or longer.
The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is important not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation without the presence of mobile carriers remain unclear. Here, we elucidate this issue for the case of photoionization of NV center ensembles. Using pump-probe spectroscopy, we ionize negatively charged NV centers and monitor the recovery of NV^{-} on timescales of up to several seconds. We find that the recovery rate depends strongly on the concentration of surrounding nitrogen donors. Remarkably, the equilibration dynamics exhibit no discernible dependence on temperature, ruling out thermally activated processes. We develop a multiphonon-assisted electron-tunneling model, supported by density-functional calculations, that explains the measurements and identifies tunneling as the equilibration mechanism. Because tunneling rates depend exponentially on donor separation, the random distribution of nitrogen donors spreads recovery times from nanoseconds to hours or longer.
The charge degree of freedom in solid-state defects fundamentally underpins the electronic spin degree of freedom, a workhorse of quantum technologies. Here we measure, analyze, and control charge-state behavior in individual near-surface nitrogen-vacancy (NV) centers in diamond, where NV^{-} hosts the metrologically relevant electron spin. We find that NV^{-} initialization fidelity varies between individual centers and over time; we alleviate the deleterious effects of reduced NV^{-} initialization fidelity via logic-based initialization. Importantly, we also show that NV^{-} can ionize in the dark on experimentally relevant timescales, and we introduce measurement protocols that mitigate the compromising effects of charge conversion on spin measurements. We identify tunneling to a single local electron trap as the mechanism for ionization in the dark, and we develop novel NV-assisted techniques to control and read out the trap charge state. Our understanding and command of the NV's local electrostatic environment will simultaneously guide materials design and provide unique functionalities with NV centers.
Nitrogen-vacancy (NV) centers in diamond have been exploited as reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers: NV0 and NV–, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, the understanding of the dynamics between the different states still remains challenging. Here we show that electron excitation induces the conversion from NV– to NV0 through electron-induced carrier generation. We present for the first time ultrafast pump-probe cathodoluminescence spectroscopy, with electron pulses as pump, and laser pulses as probe, to prepare and read out the NV states. The experimental data is explained with a model considering carrier dynamics (0.8 ns), NV0 spontaneous emission (20 ns) and NV0NV– back transfer (500 ms). Our results provide new insights into the NV–NV0 conversion dynamics, and into the use of pump-probe cathodoluminescence as a nanoscale NV characterization tool.
Abstract Nitrogen-vacancy (NV) defect centers in diamond are key to applications in quantum sensing and quantum computing. They create localized electronic states in the diamond lattice with distinct population relaxation pathways following photoexcitation that ultimately enable its unique properties. The defect is known to exist in two charge states: neutral and negative, with respectively one and two known optically-active electronic transitions. Here, we report on the observation of a large number of hitherto undiscovered excited electronic states in both charge states as evidenced by distinct optical transitions in the infrared to ultraviolet part of the spectrum. These transitions are observed by monitoring the electronic relaxation of NV centers after photoexcitation using transient absorption spectroscopy, directly probing transient phenomena occurring on timescales from femtoseconds to microseconds. We also for the first time probed the electron transfer dynamics from the 3 E state of NV− to nearby single-substitutional nitrogen defects (N s ) that leads to the well-known effect of NV photoluminescence quenching.
Abstract Nitrogen-vacancy (NV) centers in diamond are reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers, NV0 and NV–, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, an understanding of the dynamics between the different states still remains challenging. Here, conversion from NV– to NV0 due to electron-induced carrier generation is shown. Ultrafast pump–probe cathodoluminescence spectroscopy is presented for the first time, with electron pulses as pump and laser pulses as probe, to prepare and read out the NV states. The experimental data are explained with a model considering carrier dynamics (0.8 ns), NV0 spontaneous emission (20 ns), and NV0 → NV– back transfer (500 ms). The results provide new insights into the NV– → NV0 conversion dynamics and into the use of pump–probe cathodoluminescence as a nanoscale NV characterization tool.
In recent years, defect centers in wide band gap semiconductors such as diamond, have received significant attention. Defects offer great utility as single photon emitters, nanoscale sensors, and quantum memories and registers for quantum computation. Critical to the utility of these defects, is their charge state. In this dissertation, experiments surrounding the charge state dynamics and the carrier dynamics are performed and analyzed. Extensive studies of the ionization and recombination processes of defects in diamond, specifically, the Nitrogen Vacancy (NV) center, have been performed. Diffusion of ionized charge carriers has been imaged indirectly through the recapture of said carriers by optically active defects such as the NV center and the Silicon Vacancy (SiV) center. With proper understanding of the carrier dynamics, diamond stands to be a strong competitor in the field of spintronics for quantum information processing. Additionally, the understanding of these charge state dynamics is utilized in a novel proof of principle experiment, showing that the NV center defect’s charge state could serve as an ultra-dense 3D memory platform. The NV center has also been used as a nanoscale magnetometer. The high degree of spin polarization and the ease of manipulation of the NV allows us to transfer this polarization to other spins in the diamond and assists in the detection of spins outside of the diamond. Ensembles of Nitrogen Vacancy centers can be used to perform NMR spectroscopy of sample volumes not achievable through traditional methods. The operating mechanisms of the magnetic resonance aspects of the NV center will be discussed in depth. Work surrounding the control and polarization of the NV center’s host nitrogen spin will be covered. The topic of sensitivity and methods to improve that sensitivity will be covered as well.
Since nitrogen-vacancy (NV) centers in diamond have emerged as crucial quantum sensors in physics and biology, the massive preparation of high-quality and high-concentration shallow NV center samples has become urgent and demanding. As NV centers approach the surface and their concentration increases, charge state instability can degrade coherence and introduce additional noise. This problem is further complicated in high-pressure and high-temperature (HPHT) diamonds due to the presence of unclear impurities and inhomogeneous strain. Through optically detected magnetic resonance testing on various implanted samples, we analyzed the spin and charge state properties of shallow NV center ensembles. Our results reveal that the doping dose significantly influences the quantity of NV−, charge, and charge stability. Our findings indicate that over-implanting nitrogen as electron donors before starting graphitization helps stabilize the charge environment of the shallow NV center ensembles. By controlling the nitrogen ion implantation dose, we optimized charge state stability under photoionization and prepared high-quality NV center ensemble samples in HPHT diamonds at the optimal implantation dose range of 2×1013–8×1013 ions/cm2. This method, which relies solely on nitrogen ion implantation and annealing on HPHT diamond, significantly reduces the cost of sensor samples, greatly benefiting the promotion of quantum sensing.