Electron–electron double resonance detected NMR spectroscopy using ensemble NV centers at 230 GHz and 8.3 T
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-08-23 |
| Journal | Journal of Applied Physics |
| Authors | Benjamin Fortman, Laura Mugica-Sanchez, Noah Tischler, Cooper Selco, Yuxiao Hang |
| Institutions | University of Southern California, University of California, Los Angeles |
| Citations | 25 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”- High-Field NV Sensing Demonstrated: The research successfully implemented Optically Detected Magnetic Resonance (ODMR) on ensemble Nitrogen-Vacancy (NV) centers at the highest field and frequency reported to date: 8.3 Tesla (T) and 230 GHz.
- Novel NMR Detection Method: The first measurement of Electron-Electron Double Resonance Detected NMR (EDNMR) using the NV center was demonstrated, successfully detecting intrinsic 13C nuclear bath spins.
- Resolution Enhancement: Operating at 8.3 T corresponds to a proton NMR frequency of 350 MHz, significantly improving spectral resolution compared to traditional low-field NV-NMR techniques (typically < 0.1 T).
- T1 Limited Operation: Unlike traditional NV-detected NMR (which is T2 limited), EDNMR is limited by the longitudinal relaxation time (T1). The long T1 (measured at 3.9 ms) allows for the use of long High Turning Angle (HTA) pulses (500 µs), which is critical for achieving polarization transfer in high-field environments where microwave (MW) power is often restricted.
- Groundwork for Nanoscale NMR: This work establishes the necessary groundwork for performing nanoscale NMR of external spins (e.g., 1H, 19F) at 8.3 T and potentially higher magnetic fields, enabling structural analysis of complex molecules with high sensitivity.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Magnetic Field (B0) | 8.306 | T | Determined from ODMR transitions (Sample 1) |
| NV Larmor Frequency | 230 | GHz | Corresponding to the B0 field |
| Proton NMR Frequency Equivalent | 350 | MHz | Equivalent frequency for 1H NMR at 8.3 T |
| ODMR Lower Transition (ν0) | 229.953 | GHz | NV center ESR transition ( |
| ODMR Upper Transition | 235.687 | GHz | NV center ESR transition ( |
| Spin-Lattice Relaxation Time (T1) | 3.9 ± 0.2 | ms | Measured for ensemble NV centers (Sample 1) |
| Rabi π Pulse Length | 1.9 | µs | Used for MW1 detection pulse (Sample 1) |
| HTA Pulse Length (EDNMR) | 500 | µs | Chosen to minimize T1 influence after transfer |
| Detected 13C Signal Offset | ±88 | MHz | Double quantum transitions of 13C bath spins |
| Nearest Neighbor 13C Hyperfine Coupling | 129 | MHz | Consistent with 126-130 MHz literature values |
| P1 Center Concentration | ~70 | ppm | Estimated nitrogen concentration in diamond |
| Linewidth (Δω) | 2.3 to 3.7 | MHz | Measured 13C EDNMR linewidth (P1 limited) |
| Laser Excitation Power | ~4 | mW | Power at the sample stage |
| Detected Fluorescence (FL) | 1-2 | µW | Typical detected FL intensity |
Key Methodologies
Section titled “Key Methodologies”-
High-Field ODMR Spectrometer Setup:
- A home-built, high-field (HF) ODMR spectrometer was used, operating in the 215-240 GHz band.
- The diamond sample was mounted in a 12.1 T variable field superconducting magnet.
- Microwave (MW) excitation (230 GHz, 115 mW source power) was delivered via quasioptics and a corrugated waveguide.
- Optical initialization and readout used a solid-state single mode laser (4 mW) directed through an Acousto-Optic Modulator (AOM) and a microscope objective (Zeiss 100X, NA=0.8).
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Diamond Sample Preparation:
- Two high pressure, high temperature (HPHT) type Ib diamond samples were used (2.0 x 2.0 x 0.3 mm3 and 4.4 x 3.9 x 0.5 mm3).
- Samples were subjected to high energy (4 MeV) electron beam irradiation with a total fluence of 1.2 x 1018 e-/cm2.
- Post-irradiation annealing was performed at 1000 °C to create NV centers, resulting in a concentration greater than 1 ppm (P1 center concentration ~70 ppm).
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Pulsed ODMR and T1 Measurement:
- Pulsed ODMR was performed by monitoring relative fluorescence (FL) intensity while sweeping the MW frequency (MW1).
- T1 relaxation time was measured by varying the delay (τ) between the laser initialization pulse (20 µs) and the readout pulse (15 µs), with and without a MW π pulse, followed by fitting to a single exponential decay.
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EDNMR Pulse Sequence Implementation:
- The EDNMR sequence utilized two MW frequencies: MW1 (ν0) for detection and MW2 (ν1) for polarization transfer.
- Sequence: Laser Initialization (Init) → MW2 High Turning Angle (HTA) pulse (500 µs, frequency ν1) → MW1 π pulse (1.9 µs, fixed at ν0) → Laser Readout (RO).
- The frequency ν1 was swept relative to the central ESR transition ν0. Resonance with forbidden transitions (Δms = 1, ΔmI = 1) caused population transfer, resulting in an observable increase in FL intensity.
Commercial Applications
Section titled “Commercial Applications”The successful implementation of NV-detected EDNMR at high magnetic fields opens pathways for advanced quantum sensing and high-resolution spectroscopy:
- Nanoscale NMR and Structural Biology: Enables high-resolution NMR of external spins (e.g., 1H, 19F) on surfaces or within biomacromolecules, providing structural insights not feasible with low-field NV-NMR.
- Chemical Analysis and Pharmaceutical R&D: High-field operation improves spectral resolution, allowing clear identification of closely related chemical species and detection of low natural abundance nuclei (e.g., 17O NMR).
- Quantum Sensing and Metrology: Provides a robust, T1-limited sensing technique applicable in environments where MW power is limited, facilitating the development of high-field quantum registers and sensors.
- Diamond Material Science: The technique can be used to characterize paramagnetic impurities (like P1 centers) and nuclear bath spins (13C, 14N) in high-purity, isotopically engineered diamonds, which are essential for next-generation quantum devices.
- High-Field Magnet Technology: Establishes methods for combining NV ODMR systems with extreme high-field ESR systems (e.g., 35 T hybrid magnets), pushing the limits of magnetic resonance instrumentation.
View Original Abstract
The nitrogen-vacancy (NV) center has enabled widespread study of nanoscale nuclear magnetic resonance (NMR) spectroscopy at low magnetic fields. NMR spectroscopy at high magnetic fields significantly improves the technique’s spectral resolution, enabling clear identification of closely related chemical species. However, NV-detected NMR is typically performed using AC sensing through electron spin echo envelope modulation, a hyperfine spectroscopic technique that is not feasible at high magnetic fields. Within this paper, we have explored an NV-detected NMR technique for applications of high field NMR. We have demonstrated optically detected magnetic resonance with the NV Larmor frequency of 230 GHz at 8.3 T, corresponding to a proton NMR frequency of 350 MHz. We also demonstrated the first measurement of electron-electron double resonance detected NMR using the NV center and successfully detected 13C nuclear bath spins. The described technique is limited by the longitudinal relaxation time (T1), not the transverse relaxation time (T2). Future applications of the method to perform nanoscale NMR of external spins at 8.3 T and even higher magnetic fields are also discussed.