Optically Detected Magnetic Resonance Research Articles

Ultra-Sensitive Two-Dimensional Integrated Quantum Thermometer Module Based on the Optical Detection of Magnetic Resonance Using Nitrogen-Vacancy Centers

In this paper, we propose a two-dimensional thermometer module based on the Nitrogen-Vacancy (NV) centers in diamonds. The thermometer integrates four modules on one Printed Circuit Board (PCB) including a laser module and a corresponding voltage regulator circuit, a microwave module, a focusing module, and an acquisition module, and it has a constructed area of 6.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\textit {cm}}^{{2}}$ </tex-math></inline-formula> . In the process of integrating equipment, we made a signal amplifier module to improve the signal-to-noise ratio (SNR). Through the amplifier module, we realize a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 25$ </tex-math></inline-formula> -fold increase in the fluorescence, and it successfully improved the SNR by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 8.9$ </tex-math></inline-formula> -fold. Fixed Frequency Optical Detection of Magnetic Resonance (FF-ODMR) measures the relationship between temperature (T) and photoluminescence (PL) by scanning the temperature field at 2.84GHz. Which is the most direct display of the thermometer. We developed an algorithm for analyzing the thermometer sensitivity based on PL the SNR analysis. The thermometer temperature-field sensitivity of approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${19}\textit {nk}/{\textit {Hz}}^{{1}/{2}}$ </tex-math></inline-formula> .

Robust magnetometry with single nitrogen-vacancy centers via two-step optimization

Shallow Nitrogen-Vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization/readout process. In a second step, we optimize microwave pulses for pulsed Optically Detected Magnetic Resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below 1$\mu$T\,Hz$^{-\frac{1}{2}}$ for an 83\% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100\,nT\,Hz$^{-\frac{1}{2}}$ giving a two-fold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications.

Towards the design and operation of a uniformly illuminated NV detector for magnetic field mapping applications

Detectors based on an ensemble of nitrogen-vacancy (NV) centres in diamond are the latest addition to the list of magnetometers capable of detecting weak magnetic fields. Here, we report the design, construction, and testing of an NV magnetometer with a wide spatial area excitation and collection capability. The system was developed for the purpose of magnetic field measurement in an Halbach magnet configuration. The designed system performance was comparable to a conventional NV confocal system in the low power excitation region. From the dip profiles observed in the optically detected magnetic resonance (ODMR) spectrum, each NV centre experienced a static magnetic field in the 2.14–6.07 mT range of the Halbach magnet. The sensitivity of the field mapping system using the continuous wave (CW)-ODMR approach was estimated and found to be approximately 0.2 μT/Hz at 170 MHz. Extensive details of the setup and the experimental procedures are presented, and the system can be easily reproduced by those who are not familiar with NV centre instrumentation. The results obtained pave way for the development of scalable NV sensors using wide area illumination and collection methods, especially for the mapping B0 and B1 field in magnetic resonance imaging (MRI) to facilitate non-FFT image reconstruction.

Excited-state spin-resonance spectroscopy of V{}_{{{{{{{{rm{B}}}}}}}}}^{-} defect centers in hexagonal boron nitride

The recently discovered spin-active boron vacancy (V{}_{{{{{{{{rm{B}}}}}}}}}^{-}) defect center in hexagonal boron nitride (hBN) has high contrast optically-detected magnetic resonance (ODMR) at room-temperature, with a spin-triplet ground-state that shows promise as a quantum sensor. Here we report temperature-dependent ODMR spectroscopy to probe spin within the orbital excited-state. Our experiments determine the excited-state spin Hamiltonian, including a room-temperature zero-field splitting of 2.1 GHz and a g-factor similar to that of the ground-state. We confirm that the resonance is associated with spin rotation in the excited-state using pulsed ODMR measurements, and we observe Zeeman-mediated level anti-crossings in both the orbital ground- and excited-state. Our observation of a single set of excited-state spin-triplet resonance from 10 to 300 K is suggestive of symmetry-lowering of the defect system from D3h to C2v. Additionally, the excited-state ODMR has strong temperature dependence of both contrast and transverse anisotropy splitting, enabling promising avenues for quantum sensing.

Low-temperature recombination luminescence of La-doped Ca2SnO4

Low-temperature ultraviolet-excited photoluminescence (PL) and recombination luminescence (RL) properties of La-doped Ca2SnO4 have been investigated by luminescence, electron paramagnetic resonance (EPR) and optically-detected magnetic resonance (ODMR) techniques. Two PL and RL bands at 340 nm and 450 nm have been observed. PL excitation spectra measurements with a synchrotron source showed a significant difference between the 450 nm and the 340 nm PL bands. The 450 nm band has a long-lasting hyperbolic decay, while the 350 nm band shows a fast decay. Assuming an excitonic nature of the 340 nm band, the band gap of the Ca2SnO4:La has been estimated to be approximately 5.5 eV. ODMR measurements suggest that the low-temperature RL band at 450 nm is caused by tunnelling recombination of electron trap and hole trap centres, and the recombination energy is transferred to Sn2+ luminescence centres.

TR12 centers in diamond as a room temperature atomic scale vector magnetometer

The family of room temperature atomic scale magnetometers is currently limited to nitrogen-vacancy centers in diamond. However, nitrogen-vacancy centers are insensitive to strong off-axis magnetic fields. In this work, we show that the well-known TR12 radiative defect in diamond, exhibits strong optically detected magnetic resonance (ODMR) signal under optical saturation. We also demonstrate that the spin system responsible for the magnetic resonance is an excited triplet state that can be coherently controlled at room temperature on a single defect level. The high optically detected magnetic resonance contrast, which is maintained even for strong off-axis magnetic fields, suggests that TR12 centers can be used for vector magnetometry even at high field.

Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology.

Sub-second temporal magnetic field microscopy using quantum defects in diamond

Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it’s current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50–200 frames per s (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy.

Violaxanthin and Zeaxanthin May Replace Lutein at the L1 Site of LHCII, Conserving the Interactions with Surrounding Chlorophylls and the Capability of Triplet-Triplet Energy Transfer.

Carotenoids represent the first line of defence of photosystems against singlet oxygen (1O2) toxicity, because of their capacity to quench the chlorophyll triplet state (3Chl) through a physical mechanism based on the transfer of triplet excitation (triplet–triplet energy transfer, TTET). In previous works, we showed that the antenna LHCII is characterised by a robust photoprotective mechanism, able to adapt to the removal of individual chlorophylls while maintaining a remarkable capacity for 3Chl quenching. In this work, we investigated the effects on this quenching induced in LHCII by the replacement of the lutein bound at the L1 site with violaxanthin and zeaxanthin. We studied LHCII isolated from the Arabidopsis thaliana mutants lut2—in which lutein is replaced by violaxanthin—and lut2 npq2, in which all xanthophylls are replaced constitutively by zeaxanthin. We characterised the photophysics of these systems via optically detected magnetic resonance (ODMR) and time-resolved electron paramagnetic resonance (TR-EPR). We concluded that, in LHCII, lutein-binding sites have conserved characteristics, and ensure efficient TTET regardless of the identity of the carotenoid accommodated.

Excited-State Spectroscopy of Spin Defects in Hexagonal Boron Nitride.

A negatively charged boron vacancy (VB-) color center in hexagonal boron nitride has recently been proposed as a promising quantum sensor due to its excellent properties. However, the spin level structure of the VB- color center is still unclear, especially for the excited state. Here we measured and confirmed the excited-state spin transitions of VB- using an optically detected magnetic resonance (ODMR) technique. The zero-field splitting of the excited state is 2.06 GHz, the transverse splitting is 93.1 MHz, and the g factor is 2.04. Moreover, negative peaks in fluorescence intensity and ODMR contrast at the level anticrossing point were observed, and they further confirmed that the spin transitions we measured came from the excited state. Our work deepens the understanding of the excited-state structure of VB- and promotes VB--based quantum sensing applications.

A pulsed lock-in method for DC ensemble nitrogen-vacancy center magnetometry

This article proposes a scheme for DC nitrogen-vacancy (NV) center magnetometry that combines the advantages of the lock-in detection and pulse-type schemes. The proposed pulsed lock-in scheme does not lead to optical power broadening of optically-detected magnetic resonance, resulting in smaller linewidth, higher contrast, and better photon shot-noise-limited sensitivity compared with the conventional continuous-wave lock-in scheme. The optimal conditions, optimal sensitivity, and noise-suppression capability of the proposed method are compared with those of the conventional methods from both theoretical and experimental points of view. Through experimental measurements, an improvement by a factor of four in the sensitivity and an improvement by a factor of 60 in the minimum resolvable magnetic field (MRMF) compared with conventional method were obtained. Using a confocal experiment setup, the proposed scheme achieves an MRMF of 100 pT and a sensitivity of 3 nT·Hz-1/2 corresponding to a volume-normalized sensitivity of 2.5 pT/(Hz·mm−3)1/2, demonstrating the potential of this method in magnetic measurement applications with high sensitivity and high spatial resolution.

Multi-photon multi-quantum transitions in the spin- 32 silicon-vacancy centers of SiC

Silicon vacancy centers in silicon carbide are promising candidates for storing and manipulating quantum information. Implementation of fast quantum gates is an essential requirement for quantum information processing. In a low magnetic field, the resonance frequencies of silicon vacancy spins are in the range of a few MHz, the same order of magnitude as the Rabi frequencies of typical control fields. As a consequence, the rotating wave approximation becomes invalid and nonlinear processes like the absorption and emission of multiple photons become relevant. This work focuses on multi-photon transitions of negatively charged silicon vacancies driven by a strong RF field. We present continuous-wave optically detected magnetic resonance (ODMR) spectra measured at different RF powers to identify the 1-, 2-, and 3-RF photon transitions of different types of the silicon vacancy in the 6$H$-SIC polytype. Time-resolved experiments of Rabi oscillations and free induction decays of these multiple RF photon transitions were observed for the first time. Apart from zero-field data, we also present spectra in magnetic fields with different strength and orientation with respect to the system's symmetry axis.

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.

Angle-resolved optically detected magnetic resonance as a tool for strain determination in nanostructures

In this paper, we apply the angle-resolved optically detected magnetic resonance (ODMR) technique to study a series of strained (Cd, Mn)Te/(Cd, Mg)Te quantum wells (QWs) produced by molecular beam epitaxy. By analyzing characteristic features of ODMR angular scans, we determine the strain-induced axial-symmetry spin Hamiltonian parameter $D$ with neV precision. Furthermore, we use low-temperature optical reflectivity measurements and x-ray diffraction scans to evaluate the local strain present in the QW material. In our analysis, we take into account different thermal expansion coefficients of the GaAs substrate and CdTe buffer. The additional deformation due to the thermal expansion effects has the same magnitude as the deformation that originates from the different compositions of the samples. Based on the evaluated deformations and values of the strain-induced axial-symmetry spin Hamiltonian parameter $D$, we find the strain spin-lattice coefficient ${G}_{11}=(72.2\ifmmode\pm\else\textpm\fi{}1.9)$ neV for ${\mathrm{Mn}}^{2+}$ in CdTe and shear deformation potential $b=(\ensuremath{-}0.94\ifmmode\pm\else\textpm\fi{}0.11)$ eV for CdTe.

Optical interface for a hybrid magnon–photon resonator

We study optical detection of magnetic resonance of a ferrimagnetic sphere resonator, which is strongly coupled to a microwave loop gap resonator. Optical fibers are employed for coupling the sphere resonator with light in the telecom band. We find that magnetic resonance can be optically detected near the region of anti-crossing between the loop gap and the ferrimagnetic resonances. The detection bandwidth is found to be limited by a ferrimagnetic damping rate.

Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride

Optically addressable solid-state spins are important platforms for quantum technologies, such as repeaters and sensors. Spins in two-dimensional materials offer an advantage, as the reduced dimensionality enables feasible on-chip integration into devices. Here, we report room-temperature optically detected magnetic resonance (ODMR) from single carbon-related defects in hexagonal boron nitride with up to 100 times stronger contrast than the ensemble average. We identify two distinct bunching timescales in the second-order intensity-correlation measurements for ODMR-active defects, but only one for those without an ODMR response. We also observe either positive or negative ODMR signal for each defect. Based on kinematic models, we relate this bipolarity to highly tuneable internal optical rates. Finally, we resolve an ODMR fine structure in the form of an angle-dependent doublet resonance, indicative of weak but finite zero-field splitting. Our results offer a promising route towards realising a room-temperature spin-photon quantum interface in hexagonal boron nitride.

Excited State Spectroscopy of Boron Vacancy Defects in Hexagonal Boron Nitride Using Time-Resolved Optically Detected Magnetic Resonance.

We report optically detected magnetic resonance (ODMR) measurements of an ensemble of spin-1 negatively charged boron vacancies in hexagonal boron nitride. The photoluminescence decay rates are spin-dependent, with intersystem crossing rates of 1.02 ns-1 and 2.03 ns-1 for the mS = 0 and mS = ±1 states, respectively. Time gating the photoluminescence enhances the ODMR contrast by discriminating between different decay rates. This is particularly effective for detecting the spin of the optically excited state, where a zero-field splitting of |DES| = 2.09 GHz is measured. The magnetic field dependence of the photoluminescence exhibits dips corresponding to the ground (GSLAC) and excited-state (ESLAC) anticrossings and additional anticrossings due to coupling with nearby spin-1/2 parasitic impurities. Comparison to a model suggests that the anticrossings are mediated by the interaction with nuclear spins and allows an estimate of the ratio of the singlet to triplet spin-dependent relaxation rates of κ0/κ1 = 0.34.

Applicability of gas-jet MPCVD polycrystalline diamond films on silicon with NV centers in quantum magnetometry

Polycrystalline diamond film optical and electrical properties are investigated after the growth on <001> and <111> Si substrate by gas-jet MPCVD deposition in the presence of nitrogen in the gas mixture. Negatively charged NV− center formation was observed at the ~1.0 ppm level with the substitutional nitrogen concentration of 70 ppm. A comparison with the IIa type monocrystalline diamond plates with implanted and annealed nitrogen atoms at the 90 ppm concentration shows three times higher NV center formation efficiency by gas-jet MPCVD deposition than by ion implantation. CW optically detected magnetic resonance (ODMR) demonstrates the NV contented polycrystalline film application in a quantum magnetometry.

Optically detected magnetic resonance of indirect excitons in an ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots

The energy level structure as well as the exciton recombination and spin dynamics are studied in a dense ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots (QDs). The band alignment in the QDs is shown to have type-I, indirect character with the lowest electron state at the $X$ valleys of the conduction band and the top hole state in the $\mathrm{\ensuremath{\Gamma}}$ point of the valence band, so that indirect excitons are formed in the QDs. Time-resolved photoluminescence and magnetic-field-induced circular polarization allow us to distinguish electron states belonging to the QDs and the wetting layer. Suppression of the exciton migration within the QD ensemble and along the wetting layer in the magnetic field is found. A pronounced effect of applied microwave radiation on the recombination and spin polarization of the indirect excitons is observed in longitudinal magnetic fields. Optically detected magnetic resonance (ODMR) is detected in both the intensity and the circular polarization degree of the QD emission. The ODMR resonance corresponds to the $g$ factor of 1.97, associated with $X$-valley electrons. The spin relaxation time of the $X$-valley electrons is measured to be $600\ifmmode\pm\else\textpm\fi{}25$ ns.