Diamond color centers such as the nitrogen-vacancy (NV) center have been extensively studied as a popular platform that enables quantum sensing and computing applications. For highly sensitive sensing NV ensembles have significantly stronger signals than individual NV centers and can create strong light fields, which are also essentially required by novel concepts that need a large sensing volume, such as cavity coupling and readout, laser threshold magnetometry and multi-pass geometries. On top of strong signals, low optical material losses are the other key consideration for these techniques, by means of low optical absorption of signal light and low birefringence of the diamond crystal. In this work, we study systematically the behavior of absorption in the NV emission range around 700nm and birefringence with increasing nitrogen- and NV-doping, as well as their behavior during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen-doping yet single substitutional nitrogen atoms (Ns0 or P1 centers) do not seem to be the direct absorber. The high nitrogen doping level in the growth leads to high absorption in the whole UV-Vis range, which is the main contributors to the high absorption at 700 nm. When treating the diamond for an enhanced NV concentration, ‘over irradiating’ the diamond with too high fluence is harmful in the sense of creating neutral vacancies that increase absorption at the wavelength range of interest. Opposite to the absorption, birefringence reduces with increasing nitrogen doping and shows better homogeneity. After-growth treatments show a minor and if any a positive (i.e. reducing) influence on diamond birefringence. Multiple crystal defect concentrations and their changes are identified via absorption spectroscopy during the material processing steps and thus we identify potential causes of absorption and birefringence. From this we suggest strategies to fabricate CVD diamonds with high NV density yet low absorption and low birefringence
Diamond color centers such as the nitrogen-vacancy (NV) center have been extensively studied as a popular platform that enables quantum sensing and computing applications. For highly sensitive sensing NV ensembles have significantly stronger signals than individual NV centers and can create strong light fields, which are also essentially required by novel concepts that need a large sensing volume, such as cavity coupling and readout, laser threshold magnetometry and multi-pass geometries. On top of strong signals, low optical material losses are the other key consideration for these techniques, by means of low optical absorption of signal light and low birefringence of the diamond crystal. In this work, we study systematically the behavior of absorption in the NV emission range around 700nm and birefringence with increasing nitrogen- and NV-doping, as well as their behavior during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen-doping yet single substitutional nitrogen atoms (Ns0 or P1 centers) do not seem to be the direct absorber. The high nitrogen doping level in the growth leads to high absorption in the whole UV-Vis range, which is the main contributors to the high absorption at 700 nm. When treating the diamond for an enhanced NV concentration, ‘over irradiating’ the diamond with too high fluence is harmful in the sense of creating neutral vacancies that increase absorption at the wavelength range of interest. Opposite to the absorption, birefringence reduces with increasing nitrogen doping and shows better homogeneity. After-growth treatments show a minor and if any a positive (i.e. reducing) influence on diamond birefringence. Multiple crystal defect concentrations and their changes are identified via absorption spectroscopy during the material processing steps and thus we identify potential causes of absorption and birefringence. From this we suggest strategies to fabricate CVD diamonds with high NV density yet low absorption and low birefringence
The use of diamond colour centres such as the nitrogen-vacancy (NV) centre is increasingly enabling quantum sensing and computing applications. Novel concepts like cavity coupling and readout, laser-threshold magnetometry and multi-pass geometries allow significantly improved sensitivity and performance via increased signals and strong light fields. Enabling material properties for these techniques and their further improvements are low optical material losses via optical absorption of signal light and low birefringence. Here, we study systematically the behaviour of absorption around 700 nm and birefringence with increasing nitrogen- and NV-doping, as well as their behaviour during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen doping yet substitutional nitrogen does not seem to be the direct absorber. Birefringence reduces with increasing nitrogen doping. We identify multiple crystal defect concentrations via absorption spectroscopy and their changes during the material processing steps and thus identify potential causes of absorption and birefringence as well as strategies to fabricate chemical vapour deposition diamonds with high NV density yet low absorption and low birefringence. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.
Because of their high sensitivity, quantum materials are extremely promising for new technologies. The main challenge lies in maintaining a sufficiently quantum coherence times for quantum computation or quantum sensing systems. NV centers in diamond which are point defects consisting of a substitutional nitrogen atom neighboring an atomic vacancy, where an electron pair can reside in a triplet state – are ideal candidates for the development of industrial-scale quantum systems. These spin states exhibit ms-long coherence times at room temperature and can be manipulated optically, hence opening interesting perspectives for sending magnetic fields with high sensitivity1.At HiQuTe Diamond, we take up the challenge of mastering the entire nitrogen-doped diamond production chain synthesized by Microwave Plasma-Assisted Chemical Vapor Deposition (MPACVD). We aim at creating affordable diamonds specially engineered to achieve long coherence times with high NV density and high doping uniformity. This quest require to overcome the impact of several external factors that contribute to electron spin decoherence. First, the chemical composition of the diamond, specifically the types and concentrations of impurities within the diamond matrix (such as substitutional nitrogen NS, NV centres, mono-vacancies and clusters, hydrogen, etc.), as well as their electrical charge states (NS⁰, NS⁺, NV⁰, NV⁻) should be carefully controlled. Thanks to extensive scientific research conducted over the past decade, combined with our own R&D efforts, we are now able to quantify these parameters using experimental techniques compatible with industrial applications (Raman and UV-Vis spectroscopy) and compare our results with key performance indicators2. Second, we study the relationship between coherence (T2*) and strain inhomogeneities in our diamond layers, which we analyse using birefringence measurements. Our findings demonstrate our capability to produce CVD diamond layers with reduced strain and extended defect and confirm that strain could be a critical limiting factor for T2*.In this poster, I will review the efforts carried out at HiQuTe Diamond to develop specific material that is well suited for quantum sensing applications.References1.Barry, J. F., Schloss, J. M., Bauch, E., Turner, M. J., Hart, C. A., Pham, L. M., & Walsworth, R. L. (2020). Sensitivity optimization for NV-diamond magnetometry. Reviews of Modern Physics, 92(1), 015004.2.Langer, J. (2022). Quantum-grade diamond for cavity-based solutions. Fraunhofer Verlag.
Nitrogen-vacancy (NV) centers in single-crystal CVD diamond are promising candidates for quantum sensing due to their remarkable optical and spin properties [1]. These defects exhibit stable photoluminescence (PL), long spin coherence times, and high sensitivity to external fields [2]. In this study, we present a room-temperature PL characterization of NV centers excited with a 532 nm laser. The setup was optimized for high signal-to-noise ratio and spectral resolution. Key parameters analyzed include emission intensity, zero-phonon line (ZPL) at ~637 nm, phonon sidebands, and the charge state distribution between NV⁰ and NV⁻. The NV⁻ state, crucial for spin-based sensing, is evaluated to understand its stability under different nitrogen doping conditions [3]. The relative intensities of NV⁰ and NV⁻ provide insights into nitrogen incorporation and vacancy formation. These results contribute to optimizing synthesis conditions for enhanced NV⁻ concentration, supporting the advancement of diamond-based quantum sensing technologies.
We systematically investigate the NV center creation and properties in bulk chemical vapor deposition (CVD) diamond, throughout growth, electron-beam irradiation, and annealing steps. We study diamond series grown with a varying nitrogen flow over 4 orders of magnitude, resulting in a broad range of single substitutional nitrogen (P1 center) densities from 0.2 to 20 ppm. We investigate in-situ P1 and NV creations, as well as NV charge states distributions. As the next step, we optimize electron-irradiation fluences for a fixed nitrogen concentration, with two different accelerated electron energies. We bring up a general approach to determine the optimal irradiation conditions, and discuss criteria for P1 to NV conversion ratios in the context of stable NV charge states. Through all these optimizations, we enhance the NV- concentration remarkably in our CVD diamond samples, without sacrificing their Hahn-echo coherence time T2. We achieve NV concentrations from 1 to 168 ppb with T2 from 549 to 45.5 µs respectively. This study shows a pathway to engineer targeted, optimized properties of NV-ensembles in bulk CVD diamonds for improved sensitivity.
Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.