Charge State Dynamics Research Articles

Vanadium in silicon carbide: telecom-ready spin centres with long relaxation lifetimes and hyperfine-resolved optical transitions

Vanadium in silicon carbide (SiC) is emerging as an important candidate system for quantum technology due to its optical transitions in the telecom wavelength range. However, several key characteristics of this defect family including their spin relaxation lifetime (T1), charge state dynamics, and level structure are not fully understood. In this work, we determine the T1 of an ensemble of vanadium defects, demonstrating that it can be greatly enhanced at low temperature. We observe a large spin contrast exceeding 90% and long spin-relaxation times of up to 25 s at 100 mK, and of order 1 s at 1.3 K. These measurements are complemented by a characterization of the ensemble charge state dynamics. The stable electron spin furthermore enables high-resolution characterization of the systems’ hyperfine level structure via two-photon magneto-spectroscopy. The acquired insights point towards high-performance spin-photon interfaces based on vanadium in SiC.

Measurement of Charge State Dynamics in Nitrogen−Vacancy Centers Based on Microwave‐Pulses‐Assisted Longitudinal Relaxations Balancing of Spin Qutrit

AbstractThe nitrogen−vacancy (NV) center in diamond gains its versatility when negatively charged (NV−) but is mediocre when neutrally charged. Particularly, the charge states of NV centers are convertible under optical pumping and during the dark intervals, whose dynamics are mixed with the NV−s’ spin polarizations and relaxations, making them difficult to detect. Here, a microwave‐pulses‐assisted charge state dynamics (CSD) measurement method of NV centers in the dark time (DT) is proposed. The microwave pulses are designed to manipulate the populations of the NV−s’ ground state spin triplets (qutrit) to the equilibrium state before the DT. Thus, the longitudinal relaxations of the qutrit are balanced, and pure CSD can be detected. Interestingly, in an annealed bulk diamond, not only the traditional tunneling‐induced fast exponential CSD are observed, but also a slow and long‐term recharging process, which is probably attributed to the exchanging of the electrons between NV centers and the high‐energy‐level charge traps such as vacancy clusters. Furthermore, results demonstrate a 40% increase in NV−s’ contrast by properly extending the recharging DT. These results are significant for the in‐depth study of the NV centers’ CSD and can improve the sensing abilities of the NV− ensemble.

Photo-Induced Charge State Dynamics of the Neutral and Negatively Charged Silicon Vacancy Centers in Room-Temperature Diamond.

The silicon vacancy (SiV) center in diamond is drawing much attention due to its optical and spin properties, attractive for quantum information processing and sensing. Comparatively little is known, however, about the dynamics governing SiV charge state interconversion mainly due to challenges associated with generating, stabilizing, and characterizing all possible charge states, particularly at room temperature. Here, multi-color confocal microscopy and density functional theory are used to examine photo-induced SiV recombination - from neutral, to single-, to double-negatively charged - over a broad spectral window in chemical-vapor-deposition (CVD) diamond under ambient conditions. For the SiV0 to SiV- transition, a linear growth of the photo-recombination rate with laser power at all observed wavelengths is found, a hallmark of single photon dynamics. Laser excitation of SiV‒, on the other hand, yields only fractional recombination into SiV2‒, a finding that is interpreted in terms of a photo-activated electron tunneling process from proximal nitrogen atoms.

Probing NV and SiV charge state dynamics using high-voltage nanosecond pulse and photoluminescence spectral analysis

Nitrogen-vacancy (NV) and silicon-vacancy (SiV) color defects in diamond are promising systems for applications in quantum technology. The NV and SiV centers have multiple charge states, and their charge states have different electronic, optical and spin properties. For the NV centers, most investigations for quantum sensing applications are targeted on the negatively charged NV (NV−), and it is important for the NV centers to be in the NV− state. However, it is known that the NV centers are converted to the neutrally charged state (NV0) under laser excitation. An energetically favorable charge state for the NV and SiV centers depends on their local environments. It is essential to understand and control the charge state dynamics for their quantum applications. In this work, we discuss the charge state dynamics of NV and SiV centers under high-voltage nanosecond pulse discharges. The NV and SiV centers coexist in the diamond crystal. The high-voltage pulses enable manipulating the charge states efficiently. These voltage-induced changes in charge states are probed by their photoluminescence spectral analysis. The analysis result from the present experiment shows that the high-voltage nanosecond pulses cause shifts of the chemical potential and can convert the charge states of NV and SiV centers with the transition rates of ∼MHz. This result also indicates that the major population of the SiV centers in the sample is the doubly negatively charged state (SiV2−), which is often overlooked because of its non-fluorescent and non-magnetic nature. This demonstration paves a path for a method of rapid manipulation of the NV and SiV charge states in the future.

Multicolor-illuminated charge-state dynamics of the nitrogen-vacancy center in diamond

Charge-state conversion has been a very useful tool for improving the performance of nitrogen-vacancy (NV) centers in diamond in many application scenarios. Lasers of different colors are employed to manipulate the NV charge, ranging from blue to infrared. Here, we focus on the study of the charge-state dynamics under illumination of multicolors and their combination with infrared (IR) light, and the results are directly presented in terms of charge-state population. It is shown that each color of light serves some kind of superiority over others in charge-state conversion and a greatly improved conversion rate can be induced by the additional IR illumination. We report that the steady-state ${\mathrm{NV}}^{\ensuremath{-}}$ population is increased up to 82% by IR light, and it presents basically the same response trend to infrared power as that of fluorescence counts. We also report a fast-oscillating fluorescence time trace by modulating the IR power. We believe that our work offers some straightforward results that might be of interest to researchers and lead to a clearer understanding of the NV charge-state dynamics.

Low-Temperature Photophysics of Single Nitrogen-Vacancy Centers in Diamond.

We investigate the magnetic field dependent photophysics of individual nitrogen-vacancy (NV) color centers in diamond under cryogenic conditions. At distinct magnetic fields, we observe significant reductions in the NV photoluminescence rate, which indicate a marked decrease in the optical readout efficiency of the NV's ground state spin. We assign these dips to excited state level anticrossings, which occur at magnetic fields that strongly depend on the effective, local strain environment of the NV center. Our results offer new insights into the structure of the NVs' excited states and a new tool for their effective characterization. Using this tool, we observe strong indications for strain-dependent variations of the NV's orbital g factor, obtain new insights into NV charge state dynamics, and draw important conclusions regarding the applicability of NV centers for low-temperature quantum sensing.

Magnetic-Field-Assisted Spectral Decomposition and Imaging of Charge States of N - V Centers in Diamond

With the advent of quantum technology, nitrogen vacancy ($\mathrm{N}$-$V$) centers in diamond turn out to be a frontier that provides an efficient platform for quantum computation, communication, and sensing applications. Due to the coupled spin-charge dynamics of the $\mathrm{N}$-$V$ system, knowledge of $\mathrm{N}$-$V$ charge-state dynamics can help to formulate efficient spin-control sequences strategically. Here, we report two spectroscopy-based deconvolution methods to create charge-state mapping images of ensembles of $\mathrm{N}$-$V$ centers in diamond. First, relying on the fact that an off-axis external magnetic field mixes the electronic spins and selectively modifies the photoluminescence (PL) of $\mathrm{N}$-${V}^{\ensuremath{-}}$, we perform decomposition of the optical spectrum for an ensemble of $\mathrm{N}$-$V$ and extract the spectra for $\mathrm{N}$-${V}^{\ensuremath{-}}$ and $\mathrm{N}$-${V}^{0}$ states. Next, we introduce an optical-filter-based decomposition protocol and perform PL imaging for $\mathrm{N}$-${V}^{\ensuremath{-}}$ and $\mathrm{N}$-${V}^{0}$. Previously obtained spectra for $\mathrm{N}$-${V}^{\ensuremath{-}}$ and $\mathrm{N}$-${V}^{0}$ states are used to calculate their transmissivities through a long-pass optical filter. These results help us to determine the spatial distribution of the $\mathrm{N}$-$V$ charge states in a diamond sample.

Photoionization of negatively charged NV centers in diamond: Theory and ab initio calculations

We present ab-initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond from the ground $^{3}\!A_2$ and the excited $^{3}\!E$ states. We show that after the ionization from the $^{3}\!E$ level the NV center transitions into the metastable $^{4}\!A_2$ electronic state of the neutral defect. We reveal how spin polarization of $\mathrm{NV}^{-}$ gives rise to spin polarization of the $^{4}\!A_2$ state, providing an explanation of electron spin resonance experiments. We obtain smooth photoionization cross sections by employing dense $k$-point meshes for the Brillouin zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of $\mathrm{NV}^{-}$. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers.

Charge-state dynamics of heavy-ion beams penetrating a hydrogen plasma

The dynamics of charge-state fractions of Au- and U-ion beams, penetrating hydrogen plasmas, is investigated for 1.4 and 3.6 MeV/u ion energies and plasma electron temperatures Te = (0.6 − 3.0) eV and densities Ne =(1016 − 1020) cm−3. Calculations of the fractions as a function of the plasma thickness are performed by solving the balance rate equations with calculated ionization and recombination rates including those for dielectronic recombination (DR). The DR rates are calculated by the recently created computer code DRIMP using the fully relativistic multi-configuration approach and the so-called shifted Maxwellian velocity distribution function. Calculated by this method DR rates of Au- and U-ion beams passing through a hydrogen plasma are presented for the first time. Using the results obtained, the temperatures Te and densities Ne are found at which the equilibrium mean charges q̄ of Au- and U-ion beams are maximal for hydrogen-plasma length L≤ 100 cm. Dependencies of the plasma equilibrium thicknesses Xeq on the initial charge q0 of incident ions are investigated, and it is found that Xeq values are minimal at q0≈q̄.

Charge state dynamics and optically detected electron spin resonance contrast of shallow nitrogen-vacancy centers in diamond

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 engineering will be critical for nanoscale sensing applications based on color centers in diamond.

Negative charge enhancement of near-surface nitrogen vacancy centers by multicolor excitation

Nitrogen-Vacancy (NV) centers in diamond have been identified over the past few years as promising systems for a variety of applications, ranging from quantum information science to magnetic sensing. This relies on the unique optical and spin properties of the negatively charged NV. Many of these applications require shallow NV centers, i.e. NVs that are close (a few nm) to the diamond surface. In recent years there has been increasing interest in understanding the dynamics of NV centers under various illumination conditions, specifically under infra-red (IR) excitation, which has been demonstrated to have significant impact on the NV centers' emission and charge state. Nevertheless, a full understanding of all experimental data is still lacking, with further complications arising from potential differences between the photo-dynamics of bulk vs. shallow NVs. Here we suggest a generalized quantitative model for NV center spin and charge state dynamics under both green and IR excitation. We experimentally extract the relevant transition rates, providing a comprehensive model which reconciles all existing results in the literature. Moreover, we identify key differences between the photo-dynamics of bulk and shallow NVs, and use them to significantly enhance the initialization fidelity of shallow NVs to the useful negatively-charged state.

Charge-state dynamics during excitation and depletion of the nitrogen-vacancy center in diamond

The charge state dynamics of the nitrogen-vacancy (NV) center in diamond play a key role in a wide range of applications, yet remain imperfectly understood. Using single ps-pulses and pulse pairs, we quantitatively investigate the charge dynamics associated with excitation and fluorescence depletion of a single NV center. Our pulsed excitation approach permits significant modeling simplifications, and allows us to extract relative rates of excitation, stimulated emission, ionization, and recombination under 531 nm and 766 nm illumination. By varying the duration between paired pulses, we can also investigate ionization and recombination out of metastable states. Our results are directly applicable to experiments employing stimulated emission-depletion imaging, and can be used to predict optimal operating regimes where excitation and stimulated emission are maximized relative to charge-state-switching processes.

Charge-state dynamics of 1.4- and 11-MeV/u uranium ions penetrating H2 and He gas targets

Abstract Theoretical description and interpretation are presented of the recent experimental data on stripping of fast 238U ions, penetrating H2 and He gaseous targets: obtained in GSI, Darmstadt, Germany at 1.4 MeV/u with a H2 stripper, and in RIKEN, Saitama, Japan at 11 MeV/u with H2 and He strippers. Electron-loss and capture cross sections for uranium ions, interacting with H2 and He targets are calculated and used as input data in the BREIT code to obtain dynamic characteristics of uranium-ion beams: non-equilibrium and equilibrium charge-state fractions, mean and equilibrium charges, and equilibrium thicknesses. Special attention is paid for the calculation of the dynamic characteristics of 1.4-MeV/u uranium ions stripped by a H2 in a windowless gas target at GSI, using a newly designed pulsed gas cell with a pressure changing from ∼ 0.1 to ∼ 100 mbar along the ion beam. To explain the experimental data, a Gaussian pressure model for the H stripper is applied with account for the target-density effect. A possible deviation of the Bragg’s additivity rule for electron-capture cross sections for a H2 target is also considered and we predict that the equilibrium charge of a 1.4-MeV/u uranium beam in atomic hydrogen is larger than in molecular hydrogen, i.e., it is indicated that atomic hydrogen can be a more effective stripper than molecular hydrogen. Charge-changing cross sections and dynamic characteristics of 11-MeV/u uranium ions passing through He and H2 targets are calculated as well. Calculated equilibrium mean charge states q ¯ ( He ) = 67.8 and q ¯ ( H 2 ) = 75.7 are in agreement with RIKEN experimental data within 10%.

Charge state dynamics of the nitrogen vacancy center in diamond under 1064-nm laser excitation

The photophysics and charge state dynamics of the nitrogen vacancy (NV) center in diamond has been extensively investigated but is still not fully understood. In contrast to previous work, we find that NV$^{0}$ converts to NV$^{-}$ under excitation with low power near-infrared (1064 nm) light, resulting in $increased$ photoluminescence from the NV$^{-}$ state. We used a combination of spectral and time-resolved photoluminescence experiments and rate-equation modeling to conclude that NV$^{0}$ converts to NV$^{-}$ via absorption of 1064 nm photons from the valence band of diamond. We report fast quenching and recovery of the photoluminescence from $both$ charge states of the NV center under low power 1064 nm laser excitation, which has not been previously observed. We also find, using optically detected magnetic resonance experiments, that the charge transfer process mediated by the 1064 nm laser is spin-dependent.

Charge-state dynamics in electrostatic force spectroscopy

We present a numerical model that allows us to study the response of an oscillating probe in electrostatic force spectroscopy to charge switching in quantum dots at various time scales. The model provides more insight into the behavior of frequency shift and dissipated energy under different scanning conditions when measuring a temporarily charged quantum dot on a surface. Namely, we analyze the dependence of the frequency shift, the dissipated energy, and their fluctuations on the resonance frequency of the tip and on the electron tunneling rates across the tip–quantum dot and quantum dot–sample junctions. We discuss two complementary approaches to simulating the charge dynamics, a stochastic and a deterministic one. In addition, we derive analytic formulas valid for small amplitudes, describing relations between the frequency shift, dissipated energy, and the characteristic rates driving the charging and discharging processes.

Photo-induced ionization dynamics of the nitrogen vacancy defect in diamond investigated by single-shot charge state detection

The nitrogen–vacancy centre (NV) has drawn much attention for over a decade, yet detailed knowledge of the photophysics needs to be established. Under typical conditions, the NV can have two stable charge states, negative (NV−) or neutral (NV0), with photo-induced interconversion of these two states. Here, we present detailed studies of the ionization dynamics of single NV centres in bulk diamond at room temperature during illumination and its dependence on the excitation wavelength and power. We apply a recent method which allows us to directly measure the charge state of a single NV centre, and observe its temporal evolution. We find that the steady-state NV− population is always ⩽75% for 450–610 nm excitation wavelength. In combination with saturation measurements, we show that the optimal excitation wavelength is around 510–540 nm. Furthermore, the relative absorption cross-section of NV− is determined for 540–610 nm, revealing a double-peak structure. Finally, the energy of the NV− ground state of 2.6 eV below the conduction band is measured. These results reveal new insights into the charge state dynamics of the NV centre.

Numerical Rate Equation Approach to Picosecond Charge State Dynamics in CdTe/ZnTe Quantum Dots

The rate equation is used for description of photoluminescence dynamics after pulsed excitation of various states of quantum dots. The picosecond dynamics of averaged charge state of quantum dot is described. We compare our simulations with the experiment and come up with the conclusion that probability of carrier capture weakly depends on quantum dot charge state and that electrons and holes are captured non-synchronously.

Analytical approach to impurity transport studies: Charge state dynamics in tokamak plasmas

Ionization and recombination of plasma impurities govern their charge state kinetics, which is imposed upon the dynamics of ions that implies a superposition of the appropriate probabilities and causes an impurity charge state dynamics. The latter is considered in terms of a vector field of conditional probabilities and presented by a vector charge state distribution function with coupled equations of the Kolmogorov type. Analytical solutions of a diffusion problem are derived with the basic spatial and temporal dimensionless parameters. Analysis shows that the empirical scaling DA∝ne−1 [K. Krieger, G. Fussmann, and the ASDEX Upgrade Team, Nucl. Fusion 30, 2392 (1990)] can be explained by the ratio of the diffusive and kinetic terms, DA∕(nea2), being used instead of diffusivity, DA. The derived time scales of charge state dynamics are given by a sum of the diffusive and kinetic times. Detailed simulations of charge state dynamics are performed for argon impurity and compared with the reference modeling.

Muonium in InSb: Shallow acceptor versus deep trap or recombination center

The bound state of a muonium atom has been detected in both $n$-type and $p$-type InSb using a high-field $\ensuremath{\mu}\mathrm{SR}$ technique. The hyperfine constant obtained for this isotropic center $({A}_{\mathrm{T}}=2464\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz})$, roughly half that of a Mu atom in vacuum, is characteristic of deep level ${\mathrm{Mu}}^{0}$ centers at tetrahedral interstitial sites in other cubic semiconductors, which typically ionize above $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In contrast, the ${\mathrm{Mu}}^{0}$ signal in InSb begins to disappear near $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is more characteristic of ionization of a shallow level impurity. The charge-state dynamics of Mu in InSb can be understood in terms of the predicted shallow acceptor behavior for H in InSb versus a deep hole trap or recombination center, rather than as ionization via electron promotion to the conduction band.

Deep muonium state in InSb: Recombination center vs. trapping center

The bound state of an isotropic muonium atom has been detected in both n- and p-type InSb using a high-field μ SR technique. The hyperfine constant obtained for this center ( A = 2464 ± 1 MHz ) is characteristic of deep-level Mu 0 centers at tetrahedral interstitial sites in other cubic semiconductors, which typically ionize above 300 K . In contrast, the Mu 0 center in InSb disappears above about 30 K , which is more characteristic of ionization of a shallow-level impurity. The charge-state dynamics of Mu in InSb is discussed in terms of a deep trap or recombination center, rather than as electron ionization.