We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by constructing dynamical matrices of large supercells containing tens of thousands of atoms. The main goal of the paper is to calculate luminescence and absorption lineshapes due to coupling to vibrational degrees of freedom. The coupling to symmetric $a_1$ modes is computed via the Huang-Rhys theory. Importantly, to include a nontrivial contribution of $e$ modes we develop an effective methodology to solve the multi-mode $E \otimes e$ Jahn-Teller problem. Our results show that for NV centers in diamond a proper treatment of $e$ modes is particularly important for absorption. We obtain good agreement with experiment for both luminescence and absorption. Finally, the remaining shortcomings of the theoretical approach are critically reviewed. The presented theoretical approach will benefit identification and future studies of point defects in solids.
Silicon-vacancy qubits in silicon carbide (SiC) are emerging tools in quantum technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in 4H SiC. We apply density functional theory beyond the Born-Oppenheimer approximation to describe the temperature dependent mixing of electronic excited states assisted by phonons. We obtain polaronic gap around 5 and 22~meV for V1 and V2 centers, respectively, that results in significant difference in…
The nitrogen-vacancy colour centre in diamond is emerging as one of the most important solid-state quantum systems. It has applications to fields including high-precision sensing, quantum computing, single photon communication, metrology, nanoscale magnetic imaging and biosensing. For all of these applications, a high quantum yield of emitted photons is desirable. However, diamond samples engineered to have high densities of nitrogen-vacancy centres show levels of brightness varying significantly within single batches, or even within the same sample. Here we show that nearby nitrogen impuritie…
The conversion of neutral nitrogen-vacancy centers to negatively charged nitrogen-vacancy centers is demonstrated for centers created by ion implantation and annealing in high-purity diamond. Conversion occurs with surface exposure to an oxygen atmosphere at 465 C. The spectral properties of the charge-converted centers are investigated. Charge state control of nitrogen-vacancy centers close to the diamond surface is an important step toward the integration of these centers into devices for quantum information and magnetic sensing applications.
We report on wide-field optically detected magnetic resonance imaging of nitrogen-vacancy centers (NVs) in type IIa polycrystalline diamond. These studies reveal a heterogeneous crystalline environment that produces a varied density of NV centers, including preferential orientation within some individual crystal grains, but preserves long spin coherence times. Using the native NVs as nanoscale sensors, we introduce a 3-dimensional strain imaging technique with high sensitivity ( $< 10^{-5}$ Hz$^{-1/2}$) and diffraction-limited resolution across a wide field of view.
We show a marked reduction in the emission from nitrogen-vacancy (NV) color centers in single crystal diamond due to exposure of the diamond to hydrogen plasmas ranging from 700°C to 1000°C. Significant fluorescence reduction was observed beneath the exposed surface to at least 80mm depth after ~10 minutes, and did not recover after post-annealing in vacuum for seven hours at 1100°C. We attribute the fluorescence reduction to the formation of NVH centers by the plasma induced diffusion of hydrogen. These results have important implications for the formation of nitrogen-vacancy centers for quan…
Experiments show that shallow nitrogen implantations (<10keV) result in a negatively charged nitrogen-vacancy center (NV-) yield of 0.01-0.1%. The most succesful technique for introducing NV- centers in the carbon matrix is ion implantation followed by annealing at 1100K. We investigated the influence of channeling effects during shallow implantation and statistical diffusion using molecular dynamics (MD) and Monte Carlo (MC) approaches. Energy barriers for the diffusion process were calculated using the density functional theory (DFT). Our simulations show a significant difference in the NV y…
Nanodiamond crystals containing single color centers have been grown by chemical vapor deposition (CVD). The fluorescence from individual crystallites was directly correlated with crystallite size using a combined atomic force and scanning confocal fluorescence microscope. Under the conditions employed, the optimal size for single optically active nitrogen-vacancy (NV) center incorporation was measured to be 60 to 70 nm. The findings highlight a strong dependence of NV incorporation on crystal size, particularly with crystals less than 50 nm in size.
The optical transition linewidth and emission polarization of single nitrogen-vacancy (NV) centers are measured from 5 K to room temperature. Inter-excited state population relaxation is shown to broaden the zero-phonon line and both the relaxation and linewidth are found to follow a T^5 dependence for T up to 100 K. This dependence indicates that the dynamic Jahn-Teller effect is the dominant dephasing mechanism for the NV optical transitions at low temperatures.
Etching experiments were performed that reveal the vertical distribution of optically active nitrogen-vacancy (NV) centers in diamond created in close proximity to a surface through ion implantation and annealing. The NV distribution depends strongly on the native nitrogen concentration, and spectral measurements of the neutral and negatively-charged NV peaks give evidence for electron depletion effects in lower-nitrogen material. The results are important for potential quantum information and magnetometer devices where NV centers must be created in close proximity to a surface for coupling to…
Spin ensembles of nitrogen vacancy (NV) centers in diamond are emerging as powerful spin-based sensors for magnetic, electric and thermal field imaging with high spatial and temporal resolution. Here we characterize the formation of depth-confined NV center ensembles, activated by electron irradiation in diamond layers grown by plasma enhanced chemical vapor deposition with nitrogen co-doping. To do so, we exploit the high magnetic sensitivity of ensembles of NV centers to probe their spin environment as a function of growth and irradiation parameters. We engineer an NV ensemble whose magnetic…
We investigate the effect of surface termination on the charge state of nitrogen vacancy centers, which have been ion-implanted few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV- centers convert into NV0 and, subsequently, into an unknown non-fluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the dissappearance of NV- in the vicinity of the hydrogen-terminated surface. The band bending, which induces a p-type surface…
Photochromism in single nitrogen-vacancy optical centers in diamond is demonstrated. Time-resolved optical spectroscopy shows that intense irradiation at 514 nm switches the nitrogen-vacancy defects to the negative form. This defect state relaxes back to the neutral form under dark conditions. Temporal anticorrelation of photons emitted by the different charge states of the optical center unambiguously indicates that the nitrogen-vacancy defect accounts for both 575 nm and 638 nm emission bands. Possible mechanism of photochromism involving nitrogen donors is discussed.
We report depth-resolved photoluminescence measurements of nitrogen-vacancy (NV$^-$) centers formed along the tracks of swift heavy ions (SHIs) in type Ib synthetic single crystal diamonds that had been doped with 100 ppm nitrogen during crystal growth. Analysis of the spectra shows that NV$^-$ centers are formed preferentially within regions where electronic stopping processes dominate and not at the end of the ion range where elastic collisions lead to formation of vacancies and defects. Thermal annealing further increases NV yields after irradiation with SHIs preferentially in regions with …
Nitrogen-vacancy (NV) centers in diamond can be used for nanoscale sensing with atomic resolution and sensitivity; however, it has been observed that their properties degrade as they approach the diamond surface. Here we report that in addition to degraded spin coherence, NV centers within nanometers of the surface can also exhibit decreased fluorescence contrast for optically detected electron spin resonance (OD-ESR). We demonstrate that this decreased OD-ESR contrast arises from charge state dynamics of the NV center, and that it is strongly surface-dependent, indicating that surface enginee…
The negatively charged nitrogen-vacancy defect (NV)$^-$ in diamond has attracted much attention in recent years in qubit and biological applications. The negative charge is donated from nearby nitrogen donors that could limit or stem the successful application of (NV)$^-$. In this Letter, we unambiguously identify the \emph{neutral} nitrogen-vacancy defect (NV$^0$) by \emph{ab initio} supercell calculations. Our analysis shows that i) the spin state can be \emph{selectively} occupied optically, ii) the electron spin state can be manipulated by time-varying magnetic field, and iii) the spin sta…
Symmetry considerations are used in presenting a model of the electronic structure and the associated dynamics of the nitrogen-vacancy center in diamond. The model accounts for the occurrence of optically induced spin polarization, for the change of emission level with spin polarization and for new measurements of transient emission. The rate constants given are in variance to those reported previously.
We investigated the effect of fluorine-terminated diamond surface on the charged state of shallow nitrogen vacancy defect centers (NVs). Fluorination is achieved with CF4 plasma and the surface chemistry is confirmed with x-ray photoemission spectroscopy. Photoluminescence of these ensemble NVs reveal that fluorine-treated surfaces lead to a higher negatively charged nitrogen vacancy (NV-) population than oxygen-terminated surfaces. Using surface chemistry to control NV charges, in particular increasing the density of NV- centers, is an important step towards improving the optical and spin pro…
We present systematic measurements of longitudinal relaxation rates ($1/T_1$) of spin polarization in the ground state of the nitrogen-vacancy (NV$^-$) color center in synthetic diamond as a function of NV$^-$ concentration and magnetic field $B$. NV$^-$ centers were created by irradiating a Type 1b single-crystal diamond along the [100] axis with 200 keV electrons from a transmission electron microscope with varying doses to achieve spots of different NV$^-$ center concentrations. Values of ($1/T_1$) were measured for each spot as a function of $B$.
Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, Ω-shaped microwave antenna that enables to reliably manipulate NV spins. We find an optimal antenna design using finite integral simulations. We fabricate our antennas on low-cost, transparent glass substrate. We demonstrate highly uniform microwave fields in areas of roughly 400 x 400 μm^2 …
We demonstrate precise control over the zero-phonon optical transition energies of individual nitrogen-vacancy (NV) centers in diamond by applying multiaxis electric fields, via the dc Stark effect. The Stark shifts display surprising asymmetries that we attribute to an enhancement and rectification of the local electric field by photoionized charge traps in the diamond. Using this effect, we tune the excited-state orbitals of strained NV centers to degeneracy and vary the resulting degenerate optical transition frequency by >10 GHz, a scale comparable to the inhomogeneous frequency distributi…
We report the first observation of multi-photon photoluminescence excitation (PLE) below the resonant energies of nitrogen vacancy (NV) centers in diamond. The quadratic and cubic dependence of the integrated fluorescence intensity as a function of excitation power indicate a two-photon excitation pathway for the NV- charge state and a three-photon process involved for the neutral NV0 charge state respectively. Comparing the total multi-photon energy with its single-photon equivalent, the PLE spectra follows the absorption spectrum of single photon excitation. We also observed that the efficie…
We report measurements of the optical properties of the 1042 nm transition of negatively-charged Nitrogen-Vacancy (NV) centers in type 1b diamond. The results indicate that the upper level of this transition couples to the m_s=+/-1 sublevels of the {^3}E excited state and is short-lived, with a lifetime <~ 1 ns. The lower level is shown to have a temperature-dependent lifetime of 462(10) ns at 4.4 K and 219(3) ns at 295 K. The light-polarization dependence of 1042 nm absorption confirms that the transition is between orbitals of A_1 and E character. The results shed new light on the NV level s…
In this paper, starting from a quantum master equation, we discuss the interaction between two negatively charged Nitrogen-vacancy color centers in diamond via exciton-polaritons propagating in a two-dimensional transition metal dichalcogenide layer in close proximity to a diamond crystal. We focus on the optical 1.945 eV transition and model the Nitrogen-vacancy color centers as two-level (artificial) atoms. We find that the interaction parameters and the energy levels renormalization constants are extremely sensitive to the distance of the Nitrogen-vacancy centers to the transition metal dic…
We determine and analyze the charge and spin density distributions of nitrogen-vacancy (N-V) center in diamond for both the ground and excited states by \emph{ab initio} supercell calculations. We show that the hyperfine tensor of $^{15}$N nuclear spin is negative and strongly anisotropic in the excited state, in contrast to previous models used extensively to explain electron spin resonance measurements. In addition, we detect a significant redistribution of the spin density due to excitation that has serious implications for the quantum register applications of N-V center.
Individual, luminescent point defects in solids so called color centers are atomic-sized quantum systems enabling sensing and imaging with nanoscale spatial resolution. In this overview, we introduce nanoscale sensing based on individual nitrogen vacancy (NV) centers in diamond. We discuss two central challenges of the field: First, the creation of highly-coherent, shallow NV centers less than 10 nm below the surface of single-crystal diamond. Second, the fabrication of tip-like photonic nanostructures that enable efficient fluorescence collection and can be used for scanning probe imaging bas…
Vacancy-mediated nitrogen diffusion and aggregation via high-fluence electron beam irradiation in HPHT synthesized diamond crystal
National Institute for Materials Science（物质材料研究机构）；University of Tsukuba（筑波大学）；National Institutes for Quantum Science and Technology（量子科学技术研究开发机构）；Tohoku University（东北大学）
This work explores the possibility of increasing the density of negatively charged nitrogen-vacancy centers [NV-] in nanodiamonds using nitrogen-rich type Ib diamond powders as the starting materials. The nanodiamonds (10 - 100 nm in diameters) were prepared by ball-milling of microdiamonds, in which the density of neutral and automatically dispersed nitrogen atoms [N0] was measured by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). A systematic measurement for the fluorescence intensities and lifetimes of the crushed monocrystalline diamonds as a function of [N0] indicate…
We report on the implementation of a scanning nitrogen-vacancy (NV) magnetometer in a dry dilution refrigerator. Using pulsed optically detected magnetic resonance combined with efficient microwave delivery through a co-planar waveguide, we reach a base temperature of 350 mK, limited by experimental heat load and thermalization of the probe. We demonstrate scanning NV magnetometry by imaging superconducting vortices in a 50-nm-thin aluminum microstructure. The sensitivity of our measurements is approximately 3 μT per square root Hz. Our work demonstrates the feasibility for performing non-inva…
Optical and microwave double resonance techniques are used to obtain the excited state structure of single nitrogen-vacancy centers in diamond. The excited state is an orbital doublet and it is shown that it can be split and associated transition strengths varied by external electric fields and by strain. A group theoretical model is developed. It gives a good account of the observations and contributes to an improved understanding of the electronic structure of the center. The findings are important for quantum information processing and other applications of the center.
Nitrogen-vacancy (NV-) color centers in diamond were created by implantation of 7 keV 15N (I = 1/2) ions into type IIa diamond. Optically detected magnetic resonance was employed to measure the hyperfine coupling of the NV- centers. The hyperfine spectrum from 15NV- arising from implanted 15N can be distinguished from 14NV- centers created by native 14N (I = 1) sites. Analysis indicates 1 in 40 implanted 15N atoms give rise to an optically observable 15NV- center. This report ultimately demonstrates a mechanism by which the yield of NV- center formation by nitrogen implantation can be measured…
The implementation of quantum networks involving quantum memories and photonic channels without the need for cryogenics would be a major technological breakthrough. Nitrogen-vacancy centers have excellent spin properties even at room temperature, but phonon-induced broadening makes it challenging to interface these spins with photons at non-cryogenic temperatures. Inspired by recent progress in achieving ultra-high mechanical quality factors, we propose that this challenge can be overcome by spin-opto-mechanical transduction. We quantify the coherence of the interface by calculating the indist…
Although chemical vapor deposition (CVD) is one of the preferred routes to synthetic diamond crystals, a full knowledge of the point defects produced during growth is still incomplete. Here we exploit the charge and spin properties of nitrogen-vacancy (NV) centers in type-1b CVD diamond to expose an optically and magnetically dark point defect, so far virtually unnoticed despite an abundance comparable to (if not greater than) that of substitutional nitrogen. Indirectly-detected photo-luminescence spectroscopy indicates a donor state 1.6 eV above the valence band, although the defect's microsc…
We demonstrate magnetometry by detection of the spin state of high-density nitrogen-vacancy ensembles in diamond using optical absorption at 1042 nm. With this technique, measurement contrast, and collection efficiency can approach unity, leading to an increase in magnetic sensitivity compared to the more common method of collecting red fluorescence. Working at 75 K with a sensor with effective volume $50 \times 50 \times 300$ microns^3, we project photon shot-noise limited sensitivity of 5 pT in one second of acquisition and bandwidth from dc to a few megahertz. Operation in a gradiometer con…
The localized spin triplet ground state of a nitrogen vacancy (NV) center in diamond can be used in atomic-scale detection of local magnetic fields. Here we present a technique using these defects in diamond to image fields around magnetic structures. We extract the local magnetic field vector by probing resonant transitions of the four fixed tetrahedral NV orientations. In combination with confocal microscopy techniques, we construct a 2-dimensional image of the local magnetic field vectors. Measurements are done in external fields less than 50 G and under ambient conditions.
The nitrogen-vacancy (NV) center occurs in GaAs bulk crystals doped or implanted with nitrogen. The local vibration of nitrogen gives rise to a sharp infrared absorption band at 638 cm$^{-1}$, exhibiting a fine structure due to the different masses of neighboring $^{69}$Ga and $^{71}$Ga host isotopes. Piezospectroscopic investigations in the crystallographic <100> direction prove that the center has C$_\text{3v}$ point symmetry, which is weakly perturbed by the isotope effect. The stress-induced shifts of some band components show an unusual non-linear behavior that can be explained by couplin…
We present a beam-directed chemical technique for controlling the charge states of near-surface luminescence centers in semiconductors. Specifically, we fluorinate the surface of H-terminated diamond by electron beam irradiation in the presence of NF3 vapor. The fluorination treatment acts as a local chemical switch that alters the charge state of nitrogen-vacancy luminescence centers from the neutral to the negative state. The electron beam fluorination process is highly localized and can be used to control the emission spectrum of individual nanodiamonds and surface regions scanned by the el…
We report a systematic investigation on the spectral splitting of negatively charged, nitrogen-vacancy (NV-) photo-luminescent emission in single crystal diamond induced by strain engineering. The stress fields arise from MeV ion-induced conversion of diamond to amorphous and graphitic material in regions proximal to the centers of interest. In low-nitrogen sectors of a HPHT diamond, clearly distinguishable spectral components in the NV- emission develop over a range of 4.8 THz corresponding to distinct alignment of sub-ensembles which were mapped with micron spatial resolution. This method pr…
We show that the orientation of nitrogen-vacancy (NV) defects in diamond can be efficiently controlled through chemical vapor deposition (CVD) growth on a (111)-oriented diamond substrate. More precisely, we demonstrate that spontaneously generated NV defects are oriented with a ~ 97 % probability along the [111] axis, corresponding to the most appealing orientation among the four possible crystallographic axes. Such a nearly perfect preferential orientation is explained by analyzing the diamond growth mechanism on a (111)-oriented substrate and could be extended to other types of defects. Thi…
Exposure to beams of low energy electrons (2 to 30 keV) in a scanning electron microscope locally induces formation of NV-centers without thermal annealing in diamonds that have been implanted with nitrogen ions. We find that non-thermal, electron beam induced NV-formation is about four times less efficient than thermal annealing. But NV-center formation in a consecutive thermal annealing step (800C) following exposure to low energy electrons increases by a factor of up to 1.8 compared to thermal annealing alone. These observations point to reconstruction of nitrogen-vacancy complexes induced …
Nitrogen-vacancy (NV) centers in diamond have attracted a great deal of attention because of their possible use in information processing and electromagnetic sensing technologies. We examined theatomistic generation mechanism for the NV defect aligned in the [111] direction of C(111) substrates. We found that N is incorporated in the C bilayers during the lateral growth arising from a sequence of kink propagation along the step edge down to [-1,-1,2]. As a result, the atomic configuration with the N-atom lone-pair pointing in the [111] direction is formed, which causes preferential alignment o…
Fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy (NV$^-$) centers are promising for a wide range of applications, such as for sensing, as fluorescence biomarkers, or to hyperpolarize nuclear spins. NV$^-$ centers are formed from substitutional nitrogen (P1 centers) defects and vacancies in the diamond lattice. Maximizing the concentration of NVs is most beneficial, which justifies the search for methods with a high yield of conversion from P1 to NV$^-$. We report here the characterization of surface cleaned fluorescent micro- and nanodiamonds, obtained by irradiation of …
The nitrogen-vacancy center in diamond is a promising candidate for realizing the spin qubits concept in quantum information. Even though this defect is known for a long time, its electronic structure and other properties have not yet been explored in detail. We study the properties of the nitrogen-vacancy center in diamond through density functional theory within the local spin density approximation, using supercell calculations. While this theory is strictly applicable for ground state properties, we are able to give an estimate for the energy sequence of the excited states of this defect. W…
Fluorescent nanodiamonds are attracting major attention in the field of bio-sensing and biolabeling. In this work we demonstrate a robust approach to surface functionalize individual nanodiamonds with metal-phenolic networks that enhance the photoluminescence from single nitrogen vacancy (NV) centers. We show that single NV centres in the coated nanodiamonds also exhibit shorter lifetimes, opening another channel for high resolution sensing. We propose that the nanodiamond encapsulation suppresses the non-radiative decay pathways of the NV color centers. Our results provide a versatile and ass…
A theory of magnetoacoustic resonance is proposed to measure quadrupole-strain couplings in a spin-triplet state with the $C_{3v}$ point group symmetry, considering the spin-strain interaction in a diamond nitrogen-vacancy (NV) center. Based on the Floquet theory, we demonstrate how the single- and two-phonon transition probabilities depend on the change in the longitudinal and transverse quadrupole couplings, which can be controlled by rotating an applied magnetic field, around the threefold axis. The obtained quadrupole dynamics results are useful for realizing mechanical or ac strain-contro…
