Ambacher, OliverFiederle, MichaelLuo, TingpengTingpengLuo2023-06-142023-06-142023https://publica.fraunhofer.de/handle/publica/44277010.6094/UNIFR/233418The nitrogen-vacancy (NV) center in diamond is a promising quantum platform for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels even at room temperature. In particular, NV-ensembles in bulk diamonds are favored for a considerably improved signal-to-noise ratio and sensitivity. Key material requirements for general NV-ensemble-based applications are a high NV⁻ concentration, a long spin coherence time, and a stable charge state. Additionally, for specific applications that require large detection volumes, for example, the multi-pass readout or cavity coupling, a low optical loss in the material is also an essential need, calling for a low diamond absorption and a low birefringence. These requirements, however, are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. Therefore, better understanding the correlation between these material properties and finding their balances are crucial for improved sensitivity from the material side. Chemical vapor deposition (CVD) diamonds typically exhibit NV concentrations below 10 parts per million (ppm), but often show a high homogeneity in the NV distribution. Moreover, the nitrogen incorporation during the CVD synthesis can be precisely controlled. With these advantages, the CVD diamond attracts more and more interest in NV research. In contrast, high-pressure high-temperature (HPHT) synthesis with higher NV concentrations (up to dozens of ppm) is also of great interest. However, its inhomogeneity in the nitrogen distribution and less controllability of the nitrogen concentration raise challenges when applying it to the sensing systems. In this thesis, the author investigates optical, NV and spin properties of diamonds, specifically for CVD diamonds with a wide variety of nitrogen densities but also in comparison with HPHT diamonds. This thesis studies the optimal process in the creation of NV centers and the link to optical properties. The author develops novel optical methods in this thesis to determine the defect concentrations, which are more widely accessible and easier to implement than the conventional methods. Additionally, the author establishes various characterization protocols to systematically study NV and diamond properties. Based on these methods, CVD diamond series with varied nitrogen flow over 4 orders of magnitude are investigated, to understand the incorporation of single substitutional nitrogen atoms (P1 centers) and NV creation during the growth. For a fixed nitrogen concentration, varied electron-irradiation fluences are investigated and optimized for two different accelerated electron energies. Defect transformations during the irradiation and annealing treatments are studied via optical characterizations. The author points out that with increasing fluences a turning point exists, above which mainly the undesirable NV charge state (NV⁰) is being created, indicating an optimum that balances the high conversion efficiency and charge stability. A general approach is suggested by the author to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. Optimizing the treatment, this thesis achieves spin-spin coherence times T₂ ranging from 45.5 to 549 μs for CVD diamonds containing 168 to 1~parts per billion (ppb) NV⁻ centers, respectively. This enables better combinations of high NV concentrations and long coherence times in bulk diamonds compared to previous works. Diamond is an excellent host for advanced optical/photonic applications, however, doping can compromise the optical properties significantly. Therefore, the author further investigates relationships and ways of combining high NV concentrations with improved optical properties, specifically absorption and birefringence. Based on this, high temperature (HT) treatments are introduced as a promising candidate to reduce optical loss, while not conflicting with the requirement for high NV⁻ concentrations. This thesis shows a pathway to engineering properties of NV-doped CVD diamonds for improved sensitivity.endoctoral thesis