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arXiv:2104.04256 · Journal of Applied Physics

Bright ab initio photoluminescence of NV+ in diamond

Akib Karim, Igor Lyskov, Salvy P. Russo, Alberto Peruzzo

Published Dec 17, 2021• 12 citations
arXivHTMLPDF

Abstract

The positively charged nitrogen vacancy (NV+) center in diamond has been traditionally treated as a dark state due to the experimental lack of an optical signature. Recent computational studies have shown that it is possible for the NV+ defect to have an excited state transition equivalent to that of the negatively charged (NV−) center, but no photoluminescence (PL) predictions have been reported so far. We report the first ab initio calculation showing that the NV+ center presents quantum emission, with zero phonon line at 765 nm and a non-zero transition dipole moment, approximately one quarter of the transition dipole moment of NV−. We calculate the energy levels of the multielectron states under the time-dependent density functional theory (singlet and triplet E states), and using our recently developed frequency cutoff method, we predict the full PL spectrum. Our results suggest that this state cannot be considered intrinsically “dark” and charge specific quenching mechanisms should be investigated as the cause of the lack of optical activity in experimental characterizations.

DOI
10.1063/5.0068470

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Physical review. B./Physical review. B•Sep 10, 2019•10 citations
The positively charged nitrogen vacancy (NV+) center in diamond has been traditionally treated as a dark state due to the experimental lack of an optical signature. Recent computational studies have shown that it is possible for the NV+ defect to have an excited state transition equivalent to that of the negatively charged (NV−) center, but no photoluminescence (PL) predictions have been reported so far. We report the first ab initio calculation showing that the NV+ center presents quantum emission, with zero phonon line at 765 nm and a non-zero transition dipole moment, approximately one quarter of the transition dipole moment of NV−. We calculate the energy levels of the multielectron states under the time-dependent density functional theory (singlet and triplet E states), and using our recently developed frequency cutoff method, we predict the full PL spectrum. Our results suggest that this state cannot be considered intrinsically “dark” and charge specific quenching mechanisms should be investigated as the cause of the lack of optical activity in experimental characterizations.
The nitrogen-vacancy (NV) lattice defect in diamond, consisting of an N substitutional atom and an adjacent C vacancy, is commonly observed in two charge states, negative (NV−) and neutral (NV0). The NV− defect exhibits spin state-dependent fluorescence and is, therefore, amenable to optical methods for spin-state readout, while the NV0 is not. Hence, the NV− defect is much more useful for quantum sensing and quantum information processing. However, only NV0 electroluminescence has been observed, even from centers showing NV− in photoluminescence. In the present work, we use first-principles electronic structure calculations to determine adiabatic charge transition levels for the conversion of NV− to NV0 in the presence of substitutional N or P impurities, which provide the charge of the NV− center. We find that the adiabatic charge transition levels in the presence of such impurities lie at energies close to or lower than the zero-phonon line of the NV− center and that these energies only decrease as the concentration of N donors increases. This work, therefore, elucidates the absence of observed electroluminescence from the NV− and proposes a path toward observation of the phenomenon.
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.
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.
Nitrogen vacancy (NV) centers in diamond are at the heart of many emerging quantum technologies, all of which require control over the NV charge state. Hence, methods for quantification of the relative photoluminescence intensities of the NV0 and NV− charge states, i.e., a charge state ratio, are vital. Several approaches to quantify NV charge state ratios have been reported but are either limited to bulk-like NV diamond samples or yield qualitative results. We propose an NV charge state quantification protocol based on the determination of sample- and experimental setup-specific NV0 and NV− reference spectra. The approach employs blue (400–470 nm) and green (480–570 nm) excitation to infer pure NV0 and NV− spectra, which are then used to quantify NV charge state ratios in subsequent experiments via least squares fitting. We test our dual excitation protocol (DEP) for a bulk diamond NV sample and 20 and 100 nm nanodiamond particles and compare results with those obtained via other commonly used techniques such as zero-phonon line fitting and non-negative matrix factorization. We find that DEP can be employed across different samples and experimental setups and yields consistent and quantitative results for NV charge state ratios that are in agreement with our understanding of NV photophysics. By providing robust NV charge state quantification across sample types and measurement platforms, DEP will support the development of NV-based quantum technologies.
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.
A quantum-chemical study of the positive charge-state of the nitrogen-vacancy center in diamond is presented. Charge control of this promising qubit candidate is a focus of diamond quantum technology research, as currently charge stability relating to surfaces and nearby defects causes some difficulties for quantum applications. To demonstrate full charge control over the nitrogen vacancy, all three charge states should be identified and the processes that lead to charge state changes understood. However, experimental markers for the positive state remain elusive compared to the readily detectable zero-phonon lines of the neutral and negative. In this work we present predicted hyperfine and zero-field splitting tensors as clear signatures of the normally spinless NV+(1A1{\mathrm{NV}}^{+}\phantom{\rule{4pt}{0ex}}(^{1}\mathrm{A}_{1}NV+(1A1​ ground state) by probing a long-lived spin-triplet excited-state $\ensuremath{\sim}0.7$ eV above the NV+{\mathrm{NV}}^{+}NV+ ground state. We find a relatively narrow excitation energy range of approximately 0.7--1.1 eV between excitation into an 3E^{3}\mathrm{E}3E state and conversion into the neutral charge state. To provide insight into the thermal stability of the positive charge state, we have calculated binding and diffusion energies for both charged and uncharged systems. We predict, given that the activation energies are only weakly charge state dependent, all three charge states would diffuse in the 1600--$1900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\text{C}$ range, but the positive state has a significantly lower binding energy, suggesting that it will dissociate at temperatures around $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ rather than migrate.