Optically Detected Magnetic Resonance Spectra Research Articles

Vector magnetometry in zero bias magnetic field using nitrogen-vacancy ensembles

Abstract The application of the vector magnetometry based on Nitrogen-Vacancy (NV) ensembles has been widely investigated in multiple areas. It has the superiority of high sensitivity and high stability in ambient conditions with microscale spatial resolution. However, a bias magnetic field is necessary to fully separate the resonance lines of optically detected magnetic resonance (ODMR) spectrum of NV ensembles. This brings disturbances in samples being detected and limits the range of application. Here, we demonstrate a method of vector magnetometry in zero bias magnetic field using NV ensembles. By utilizing the anisotropy property of fluorescence excited from NV centers, we analyzed the ODMR spectrum of NV ensembles under various polarized angles of excitation laser in zero bias magnetic field with a quantitative numerical model and reconstructed the magnetic field vector. The minimum magnetic field modulus that can be resolved accurately is down to ~0.64 G theoretically depending on the ODMR spectral line width (1.8MHz), and ~2 G experimentally due to noises in fluorescence signals and errors in calibration. By using 13C purified and low Nitrogen concentration diamond combined with improving calibration of unknown parameters, the ODMR spectral line width can be further decreased below 0.5 MHz corresponding to ~0.18 G minimum resolvable magnetic field modulus.

Optically detected magnetic resonance study of thermal effects due to absorbing environment around nitrogen-vacancy-nanodiamond powders

We implanted Fe+ ions in nanodiamond (ND) powder containing negatively charged nitrogen-vacancy (NV−) centers and studied their Raman spectra and optically detected magnetic resonance (ODMR) in various applied magnetic fields with green light (532 nm) excitation. In Raman spectra, we observed a blue shift of the NV− peak associated with the conversion of the electronic sp3 configuration to the disordered sp2 one typical for the carbon/graphite structure. In the ODMR spectra, we observed a red shift of the resonance position caused by local heating by an absorptive environment that recovers after annealing. To reveal the red shift mechanism in ODMR, we created a controlled absorptive environment around ND by adding iron-based Fe2O3 and graphitic sp2 powders to the ND suspension. This admixture caused a substantial increase in the observed shift proportional to the applied laser power, corresponding to an increase in the local temperature by 150–180 K. This surprisingly large shift is absent in non-irradiated NV-ND powders, is associated only with the modification of the local temperature by the absorptive environment of NV-NDs, and can be studied using ODMR signals of NV−.

Zero-Field ODMR and relaxation of Si-vacancy centers in 6H-SiC

Silicon vacancies in silicon carbide (SiC) have been proposed as interesting candidates for quantum technology applications such as quantum sensing and quantum repeaters. SiC exists in many polytypes with different plane stacking sequences, and in each polytype, the vacancies can occupy a variety of different lattice sites. In this work, we focus on the three important charged silicon vacancies in the 6H-SiC polytype. We record the photoluminescence and continuous-wave optically detected magnetic resonance (ODMR) spectra at different radio-frequency power levels and different temperatures. We individually select the zero-phonon lines of the different silicon vacancies at low temperatures and record the corresponding ODMR spectra. ODMR allows us to correlate optical and magnetic resonance spectra and thereby separate signals from V 1 and V 3. The results also explain the observed sign change of the ODMR signal as a function of temperature.

Optically detected magnetic resonance spectroscopic analyses on the role of magnetic ions in colloidal nanocrystals.

Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier-magnetic ion interactions. Various host materials, including the II-VI group, halide perovskites, and I-III-VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.

Optically detected magnetic resonance of silicon vacancies in 4H-SiC at elevated temperatures toward magnetic sensing under harsh environments

Negatively charged silicon vacancy (VSi−) defects in silicon carbide are expected to be used for magnetic sensors under harsh environments, such as space and underground due to their structural stability and potential for high-fidelity spin manipulation at high temperatures. To realize VSi− based magnetic sensors operating at high temperatures, the temperature dependence of optically detected magnetic resonance (ODMR) in the ground states of VSi− defects, which is the basic principle of magnetic sensing, should be systematically understood. In this work, we demonstrate the potential of VSi− magnetic sensors up to at least 591 K by showing the ODMR spectra with different temperatures. Furthermore, the resonance frequency of the ground level was independent of temperature, indicating the potential for calibration-free magnetic sensors in temperature-varying environments. We also characterize the concentration of VSi− defects formed by electron irradiation and clarify the relationship of magnetic sensing sensitivity to VSi− concentration and find that the sensing sensitivity increases linearly with VSi− concentration up to at least 6.0 × 1016 cm−3. The magnetic sensitivity at a temperature above 549 K was reduced by half as compared to that at 300 K. The results pave the way for the use of a highly sensitive VSi−-based magnetic sensor under harsh environments.

High Sensitivity Spin Defects in hBN Created by High‐Energy He Beam Irradiation

AbstractRecently the negatively charged boron vacancies () in hexagonal boron nitride (hBN) have been shown as spin defects that have great potential in quantum sensing. However, so far the sensitivity is limited by either photoluminescence (PL) intensity or the optically detected magnetic resonance (ODMR) contrast, and linewidth. In this work, the generation of these spin defects is demonstrated using high‐energy helium ion beams, and ODMR measurements with different laser and microwave powers are performed. The spin defects generated by high‐energy helium ions exhibit a high PL intensity and ODMR contrast while keeping a small linewidth, hence a good sensitivity. By comparing different fluences of helium irradiations, an optimal fluence is determined which is sufficient in creating spin defects without damaging the overall crystal lattice structure. With this optimal fluence, a high signal‐to‐noise ratio ODMR spectrum can be obtained with an accurate measurement of zero‐field splitting frequency, and a best sensitivity as . Moreover, with a focused beam, such spin defects can be created deterministically with nanometer precision.

Optically Detected Magnetic Resonance Spectroscopy of Cu-Doped CdSe/CdS and CuInS2 Colloidal Quantum Dots.

Copper-doped II-VI and copper-based I-III-VI2 colloidal quantum dots (CQDs) have been at the forefront of interest in nanocrystals over the past decade, attributable to their optically activated copper states. However, the related recombination mechanisms are still unclear. The current work elaborates on recombination processes in such materials by following the spin properties of copper-doped CdSe/CdS (Cu@CdSe/CdS) and of CuInS2 and CuInS2/(CdS, ZnS) core/shell CQDs using continuous-wave and time-resolved optically detected magnetic resonance (ODMR) spectroscopy. The Cu@CdSe/CdS ODMR showed two distinct resonances with different g factors and spin relaxation times. The best fit by a spin Hamiltonian simulation suggests that emission comes from recombination of a delocalized electron at the conduction band edge with a hole trapped in a Cu2+ site with a weak exchange coupling between the two spins. The ODMR spectra of CuInS2 CQDs (with and without shells) differ significantly from those of the copper-doped II-VI CQDs. They are comprised of a primary resonance accompanied by another resonance at half-field, with a strong correlation between the two, indicating the involvement of a triplet exciton and hence stronger electron-hole exchange coupling than in the doped core/shell CQDs. The spin Hamiltonian simulation shows that the hole is again associated with a photogenerated Cu2+ site. The electron resides near this Cu2+ site, and its ODMR spectrum shows contributions from superhyperfine coupling to neighboring indium atoms. These observations are consistent with the occurrence of a self-trapped exciton associated with the copper site. The results presented here support models under debate for over a decade and help define the magneto-optical properties of these important materials.

Deep-Learning-Enhanced Single-Spin Readout in Silicon Carbide at Room Temperature

Defect spin qubits in silicon carbide have recently drawn widespread attention. Extraction of spin information in the presence of noise always requires multiple repeated measurements, which consumes a large amount of time resources. In this paper, we propose a deep-learning-enhanced method to extract effective parameters from continuous-wave optically detected magnetic resonance (ODMR) spectra and Rabi oscillations with less time consumption. Even if the signal-to-noise ratio is reasonably low, a well-trained convolutional neural network (CNN) can predict the resonance peaks of ODMR spectra or the period of Rabi oscillations. Because of the fast output of predictions by the CNN, this method can be used to sense the magnetic field in the environment and microwave intensities in real time.

Optically detected magnetic resonance of nitrogen-vacancy centers in vertical diamond Schottky diodes

Diamond solid-state devices are very attractive to electrically control the charge state of nitrogen-vacancy (NV) centers. In this work, p-type vertical diamond Schottky diodes (VDSDs) are introduced as a platform to electrically control the interconversion between the neutral charge NV (NV0) and negatively charged NV (NV−) centers. The photoluminescence of NV centers generated by ion implantation in VDSDs shows an increase in NV− zero phonon line (ZPL) and phonon sideband intensities with reverse voltage, whereas the NV0 ZPL intensity decreases. Thus, NV centers embedded in VDSDs are converted into NV− under reverse bias voltage. Moreover, the optically detected magnetic resonance (ODMR) of NV− exhibits an increase in the ODMR contrast with reverse bias voltage and splitting of the resonance dips. Since no magnetic field is applied, the dip splitting in the ODMR spectrum is ascribed to the Stark effect induced by the interaction of NV− with the electric field existing within the depletion region of VDSDs.

Towards ab initio identification of paramagnetic substitutional carbon defects in hexagonal boron nitride acting as quantum bits

Paramagnetic substitutional carbon (${\mathrm{C}}_{\text{B}},{\mathrm{C}}_{\text{N}}$) defects in hexagonal boron nitride are discussed as candidates for quantum bits. Their identification and suitability are approached by means of photoluminescence (PL), charge transitions, electron paramagnetic resonance, and optically detected magnetic resonance (ODMR) spectra. Several clear trends in these are revealed by means of an efficient plane wave periodic supercell ab initio density functional theory approach. In particular, this yields insight into the role of the separation between ${\mathrm{C}}_{\text{B}}$ and ${\mathrm{C}}_{\text{N}}$. In most of the cases, the charge transition between the neutral and a singly charged ground state of a defect is predicted to be experimentally accessible, since the charge transition level position lies within the band gap. A posteriori charge corrections are also discussed. A near-identification of an experimentally isolated single spin center as the neutral ${\mathrm{C}}_{\text{B}}$ point defect was found via comparison of results to recently observed PL and ODMR spectra.

Optimizing Optical Tweezers Experiments for Magnetic Resonance Sensing with Nanodiamonds

In this Article we explore the requirements for enabling high quality optically detected magnetic resonance (ODMR) spectroscopy in a conventional gradient force optical tweezers using nanodiamonds containing nitrogen-vacancy (NV-) centers. We find that modulation of the infrared (1064 nm) trapping laser during spectroscopic measurements substantially improves the ODMR contrast compared with continuous wave trapping. The work is significant as it allows trapping and quantum sensing protocols to be performed in conditions where signal contrast is substantially reduced. We demonstrate the utility of the technique by resolving NV- spin projections within the ODMR spectrum. Manipulating the orientation of the nanodiamond via the trapping laser polarization, we observe changes in spectral features. Theoretical modeling then allows us to infer the crystallographic orientation of the NV-. This is an essential capability for future magnetic sensing applications of optically trapped nanodiamonds.

Effect of metal electrodes on optically detected magnetic resonance of nitrogen vacancy centers in diamond

Operando detection of various physical quantities in electronic devices under operation has been demonstrated by optically detected magnetic resonance (ODMR) using nitrogen vacancy (NV) centers in diamond. However, the ODMR spectrum may also be affected by the existence of the metal electrodes and wirings of the electronic device, which limits the accurate measurement of the objective physical quantity. In this paper, we report the effect of metal electrodes on the ODMR spectrum of NV centers. It was found that the ODMR contrast increased in the vicinity of the metal electrode fabricated on diamond with NV centers. The microwave concentration at the edge of the metal electrode is a plausible cause for the increase in the ODMR contrast. Our results suggest that it is necessary to recognize that the ODMR spectrum may change near the metal electrode and wirings in the ODMR-based operand analysis of electronic devices.

Features of High-Frequency EPR/ESE/ODMR Spectroscopy of NV-Defects in Diamond

The methods of high-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE) and optically detected magnetic resonance (ODMR) are used to study the unique properties of nitrogen-vacancy (NV) defects in diamond in strong magnetic fields. It is shown that in strong magnetic fields (∼3–5 T) there occurs an effective optically induced alignment of populations of spin levels resulting in filling the level MS = 0 and emptying of the levels MS = ±1, that makes it possible to record ODMR using the change in the intensity of photoluminescence which reaches 10% at resonance. It is demonstrated that the efficiency of the alignment has the same order as in zero and low magnetic fields. The samples were preliminary studied by the ODMR method in zero magnetic fields that allowed accurate determination of the main parameters of the fine structure and hyperfine interactions with nitrogen nuclei, as well as dipole-dipole interactions between the NV center and deep nitrogen donors in the form of a nitrogen atom replacing carbon, N0. Hyperfine interactions with the nearest carbon atoms (isotope 13C) were observed in the high-frequency ODMR spectra, that opens up opportunities for measuring the processes of dynamic polarization of carbon nuclei in strong magnetic fields using optical methods. It is assumed that narrow ODMR lines in strong magnetic fields can be used to measure these fields with submicron spatial resolution. A new method for recording ODMR of NV centers with microwave frequency modulation has been developed, which simplifies the technique of measuring high magnetic fields. A significant increase in the intensity of the ODMR signal was demonstrated when a strong magnetic field was oriented along the symmetry axis of the NV center.

Insight into the Spin Properties in Undoped and Mn-Doped CdSe/CdS-Seeded Nanorods by Optically Detected Magnetic Resonance.

Controlling the spin degrees of freedom of photogenerated species in semiconductor nanostructures via magnetic doping is an emerging scientific field that may play an important role in the development of new spin-based technologies. The current work explores spin properties in colloidal CdSe/CdS:Mn seeded-nanorod structures doped with a dilute concentration of Mn2+ ions across the rods. The spin properties were determined using continuous-wave optically detected magnetic resonance (ODMR) spectroscopy recorded under variable microwave chopping frequencies. These experiments enabled the deconvolution of a few different radiative recombination processes: band-to-band, trap-to-band, and trap-to-trap emission. The results uncovered the major role of carrier trapping on the spin properties of elongated structures. The magnetic parameters, determined through spin-Hamiltonian simulation of the steady-state ODMR spectra, reflect anisotropy associated with carrier trapping at the seed/rod interface. These observations unveiled changes in the carriers' g-factors and spin-exchange coupling constants as well as extension of radiative and spin-lattice relaxation times due to magnetic coupling between interface carriers and neighboring Mn2+ ions. Overall, this work highlights that the spin degrees of freedom in seeded nanorods are governed by interfacial trapping and can be further manipulated by magnetic doping. These results provide insights into anisotropic nanostructure spin properties relevant to future spin-based technologies.

Optically Detected Magnetic Resonance Study of 3D Arrayed Silicon Vacancies in SiC pn Diodes

We demonstrated optically detected magnetic resonance (ODMR) measurements using three-dimensional (3D) arrayed silicon vacancies (VSis) in in-plane SiC pn diodes. Proton beam writing successfully created 3D arrayed VSis using different ion (proton) energies. The results of PL mapping analysis indicate that the features of luminescent spot such as size and depth can be estimated by a Monte Carlo simulation (SRIM). This suggests that diagnosis at any locations in SiC devices can be realized using VSi quantum sensors. Luminescent spots with different depth ranging 4-60 μm showed similar ODMR spectra including its contrast, which means that a similar sensor sensitivity is expected. The results suggest that 3D arrayed VSi can act as quantum sensor elements with uniform sensitivity in SiC devices.

Electronic structure of non-KramersTb3+centers in garnet crystals and evidence of their energy and spin transfer toCe3+emitters

The electronic structure of non-Kramers ${\mathrm{Tb}}^{3+}$ centers in single crystals of yttrium aluminum garnet (YAG) was studied using a new-generation high-frequency magnetic resonance spectrometer that allows measurements of electron paramagnetic resonance (EPR), electron spin echo (ESE), the photon echo on hyperfine components of Stark levels in zero magnetic field, and optically detected magnetic resonance (ODMR). It was created on the basis of a highly stable microwave bridge operating in the near-terahertz region (0.094 or 0.130 THz), both in continuous-wave (cw) and pulse modes, a nonresonant system for supplying microwave power to a sample, and a cryogen-free magneto-optical cryostat. EPR, ESE, and cw measurements at two frequencies allowed us to reliably and with high accuracy determine the parameters of ${\mathrm{Tb}}^{3+}$ centers that occupy yttrium dodecahedral positions in YAG: ${g}_{||}=15.8\ifmmode\pm\else\textpm\fi{}0.2$, ${g}_{\ensuremath{\perp}}\ensuremath{\approx}0$ (|| corresponds to one of the $\ensuremath{\langle}100\ensuremath{\rangle}$ crystal axes), the zero-field splitting $\mathrm{\ensuremath{\Delta}}=2.705\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, and the hyperfine interaction constant $A=0.197\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. In addition to the ${\mathrm{Tb}}^{3+}$ centers in a regular environment, EPR spectra with lower intensity and a resolved hyperfine structure were found for at least three types of additional ${\mathrm{Tb}}^{3+}$ centers. They have symmetry, $g$ factors, and hyperfine interaction constants which are close to those of the main ${\mathrm{Tb}}^{3+}$ centers but differ in the zero-field splitting parameters \ensuremath{\Delta} that strongly depend on the crystal field. They were ascribed to ${\mathrm{Tb}}^{3+}$ centers with nearby antisite ${\mathrm{Y}}_{\mathrm{Al}}$ defects. Similarity with several types of ${\mathrm{Ce}}^{3+}$ centers in YAG crystals, the types, and relative concentrations of antisite defects are discussed. For one of the additional ${\mathrm{Tb}}^{3+}$ centers, splitting of the energy levels in the zero field turned out to be close to an energy of 94 GHz microwave quanta, and intense echo signals were observed in weak magnetic fields and in the zero field corresponding to the EPR transitions between the hyperfine components of the ${\mathrm{Tb}}^{3+}$ spin levels. ODMR spectra of ${\mathrm{Tb}}^{3+}$ centers in YAG crystals, containing Ce and Tb, were obtained by monitoring the intensity of the ${\mathrm{Ce}}^{3+}$ luminescence, which implies the transfer of energy from ${\mathrm{Tb}}^{3+}$ ions to ${\mathrm{Ce}}^{3+}$ ions with conservation of spin. This result seems to be important, since these systems are of interest for quantum communication and computations. Taking into account that cerium does not have isotopes with a nuclear spin and that terbium has 100% $^{159}\mathrm{Tb}$ having nuclear spin $I=3/2$ and a sufficiently large nuclear magnetic moment, the ${\mathrm{Tb}}^{3+}\ensuremath{-}{\mathrm{Ce}}^{3+}$ system in garnet crystals can be promising for coherent information processing. ${\mathrm{Tb}}^{3+}$ ions can play a role of qubits and ${\mathrm{Ce}}^{3+}$ ion with a lifetime of about 50 ns, and almost unity quantum efficiency of optical transitions can be used as a single-ion readout arrangement. The prospects for using this system as hardware for quantum communication and computations are discussed.

Evidence of the Excitation of Mn2+ Spin-Dependent Photoluminescence in Manganese-Doped Yttrium Aluminum Garnets

Mn2+ ions in yttrium positions have been studied by means of optically detected magnetic resonance (ODMR) via Mn2+ spin-dependent emission in manganese-doped yttrium aluminum garnet crystals. It was shown that the intensity of photoluminescence excited by circularly polarized light depends on the population of the spin sublevels of the Mn2+ ground state and therefore can be used to study the ODMR. EPR measurements have confirmed that Mn2+ ions in the crystals under study occupy preferentially dodecahedral positions in the YAG lattice. Observation of forbidden transitions in the ODMR spectra has proved that the observed ODMR signals belong to Mn2+. Thus the wavelength dependence of the ODMR amplitude reveals the emission band of Mn2+ ions at dodecahedral positions in YAG:Mn.

Quantum Sensing in a Physiological-Like Cell Niche Using Fluorescent Nanodiamonds Embedded in Electrospun Polymer Nanofibers.

Fluorescent nanodiamonds (fNDs) containing nitrogen vacancy (NV) centers are promising candidates for quantum sensing in biological environments. This work describes the fabrication and implementation of electrospun poly lactic-co-glycolic acid (PLGA) nanofibers embedded with fNDs for optical quantum sensing in an environment, which recapitulates the nanoscale architecture and topography of the cell niche. A protocol that produces uniformly dispersed fNDs within electrospun nanofibers is demonstrated and the resulting fibers are characterized using fluorescent microscopy and scanning electron microscopy (SEM). Optically detected magnetic resonance (ODMR) and longitudinal spin relaxometry results for fNDs and embedded fNDs are compared. A new approach for fast detection of time varying magnetic fields external to the fND embedded nanofibers is demonstrated. ODMR spectra are successfully acquired from a culture of live differentiated neural stem cells functioning as a connected neural network grown on fND embedded nanofibers. This work advances the current state of the art in quantum sensing by providing a versatile sensing platform that can be tailored to produce physiological-like cell niches to replicate biologically relevant growth environments and fast measurement protocols for the detection of co-ordinated endogenous signals from clinically relevant populations of electrically active neuronal circuits.

DC Magnetometer Based on Ensembles of Nitrogen Vacancy Centers in Diamond

The solid spin of ensembles of the nitrogen vacancy (NV) centers in diamond offers unique features to be employed as atomic-scale sensors for detection of weak magnetic field. Here, we mainly demonstrate two types of DC magnetometer based on ensembles of the nitrogen vacancy (NV) centers in diamond with the method of analyzing optically detected magnetic resonance (ODMR) spectra and microwave frequency modulation technology, respectively and experimentally. Firstly, we demonstrate ODMR spectra and resonance frequency of ODMR with the various external magnetic field magnitudes applied to the NV centers in diamond. The minimum detectable magnetic field of the magnetometer is 0.0498 Gauss, and a magnetic sensitivity of 5.22 μT/Hz obtained with the analysis of ODMR spectra. Further, we utilize microwave frequency modulation technology and analyze the power spectra of noise of demodulated signal of the Lock in Amplifier. The sensitivity of the magnetometer can reach as low as 14.3 nT/Hz with microwave frequency modulation. This study lays a foundation for the advancement of magnetic detection research with higher sensitivity based on ensembles of NV centers.

Spin colour centres in SiC as a material platform for sensing and information processing at ambient conditions

Atomic-scale colour centres in bulk and nanocrystalline SiC are promising systems for quantum photonics compatible with fiber optics, quantum information processing and sensing at ambient conditions. Colour centres which acts as stable single photon sources in SiC can be key elements for quantum photonics and communications. It has been shown that there are at least two families of colour centres in SiC with S = 1 and S = 3/2, which have the property of optical alignment of the spin levels even at room temperature and above. The spin state can be initialized, manipulated and readout by means of optically detected magnetic resonance (ODMR), level anticrossing and cross-relaxation. Recently, we observed the effects of “hole burning” in the ODMR spectra, which made it possible to narrow the ODMR line by approximately an order of magnitude, which substantially increases the possibilities of technological applications of spin centres.