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6CCVD Research

Optical Dynamic Nuclear Polarization of 13C Spins in Diamond at a Low Field with Multi-Tone Microwave Irradiation

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-03-04
JournalMolecules
AuthorsVladimir Vladimirovich Kavtanyuk, Hyun Joon Lee, Sangwon Oh, Keunhong Jeong, Jeong Hyun Shim
InstitutionsKorea Military Academy, Korea Research Institute of Standards and Science
Citations3
AnalysisFull AI Review Included

Optical Dynamic Nuclear Polarization of 13C Spins in Diamond at a Low Field with Multi-Tone Microwave Irradiation

Section titled “Optical Dynamic Nuclear Polarization of 13C Spins in Diamond at a Low Field with Multi-Tone Microwave Irradiation”

Executive Summary

Section titled “Executive Summary”

This research demonstrates a robust, room-temperature, low-magnetic-field method for achieving high bulk 13C nuclear spin polarization in natural abundance diamond, circumventing the need for expensive helium cryogenics and high fields during the polarization phase.

  • Core Achievement: Bulk 13C nuclear polarization of 0.113% was achieved in HPHT diamond with natural 13C abundance.
  • Enhancement Factor: The polarization represents an enhancement of 90,000 times over the thermal equilibrium polarization achievable in situ at the DNP field (17.6 mT) and operating temperature (above 100 °C).
  • Methodological Novelty: Multi-tone microwave (MW) irradiation was successfully implemented, simultaneously exciting three frequencies corresponding to the 14N hyperfine triplet peaks.
  • Performance Gain: The triple-tone excitation increased the final 13C polarization by a factor of 1.7 compared to single-tone excitation.
  • Spectroscopic Insight: For the first time in optical DNP studies of diamond, a triplet structure was clearly observed in the 13C polarization spectrum, directly revealing the 14N hyperfine splitting (2.16 MHz).
  • Operational Parameters: Optimal DNP performance was achieved at a low magnetic field of 17.6 mT, using a laser power density of ~30 mW/mm2 and an MW power of ~10 W.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
DNP Magnetic Field (BEM)17.6mTLow field used for hyperpolarization.
NMR Readout Field (BSM)6THigh field used for 13C signal detection.
13C Resonance Frequency (6 T)64.237MHzReadout frequency in the superconducting magnet.
Maximum 13C Polarization0.113%Achieved using triple-tone MW irradiation.
Enhancement Factor (Δ)90,000TimesRelative to in situ thermal polarization at 17.6 mT.
MW Power (Optimal)~10WPower applied to the Helmholtz coil.
Laser Power Density (Optimal)~30mW/mm2Power density of the 532 nm pump laser.
DNP Duration120sContinuous irradiation time for hyperpolarization.
Diamond Temperature (TL)>100°CTemperature during DNP at optimal laser power density.
14N Hyperfine Splitting2.16MHzObserved splitting in the 13C polarization spectrum.
NV Center Concentration1.25ppmConcentration of negatively charged NV centers.
P1 Center Concentration~50ppmConcentration of substitutional nitrogen defects.
Diamond TypeHPHTN/AHigh-Pressure High-Temperature grown crystal.
Crystal Orientation(111)N/ASurface orientation aligned with the magnetic field.
Shuttling Time (DNP to NMR)2sTime required to move the sample 80 cm between magnets.

Key Methodologies

Section titled “Key Methodologies”

The experimental setup integrates a low-field DNP stage with a high-field NMR readout stage, connected by a rapid shuttling device.

  1. Diamond Preparation:

    • HPHT-grown diamond (42 mg, natural 13C abundance) was used.
    • NV centers (1.25 ppm) were created via 1 MeV electron irradiation followed by thermal annealing at 800 °C.

  2. DNP Setup (Low Field, 17.6 mT):

    • Continuous 532 nm green laser irradiation (up to 4 W output) was used for optical pumping of NV electronic spins.
    • MW signals (up to 50 W amplified) were delivered via a handmade Helmholtz coil installed within an electrical magnet.
    • The magnetic field (17.6 mT) was aligned parallel to the (111) NV center orientation.

  3. Optimization of DNP Parameters:

    • MW Power: Optimized to ~10 W. Higher power led to decreased net polarization due to off-resonant excitation inducing opposite polarization (transition from selective to A regime).
    • Laser Power Density: Optimized to ~30 mW/mm2. Higher density caused significant diamond heating (above 100 °C), leading to a decrease in polarization, likely due to thermally activated 13C depolarization.

  4. Multi-Tone MW Irradiation:

    • The 13C polarization spectrum was mapped, revealing three distinct peaks separated by 2.16 MHz (the 14N hyperfine splitting).
    • Three separate MW sources were synthesized and combined to simultaneously excite the three peak frequencies (f1, f2, f3).
    • This triple-tone excitation was applied continuously for 120 s, yielding the maximum polarization (0.113%).

  5. NMR Readout:

    • After DNP, the diamond was rapidly shuttled 80 cm in 2 s into the center of a 6 T superconducting magnet.
    • The 13C NMR signal was detected at 64.237 MHz using a handmade saddle coil and a commercial NMR console (LapNMR).
    • Residual 13C polarization was depleted using a series of 90 pulses before each new measurement to ensure zero baseline.

Commercial Applications

Section titled “Commercial Applications”

The ability to achieve high nuclear spin polarization in diamond at room temperature and low field opens pathways for advanced quantum technologies and biomedical imaging.

  • Quantum Metrology and Sensing:

    • High-Field Magnetometers: Hyperpolarized 13C spins can be used as highly sensitive probes for high-field magnetometry, leveraging the long coherence times of diamond nuclear spins.
    • Solid-State Nuclear Spin Gyroscopes: The long spin life times of 13C in diamond are crucial for developing robust, solid-state gyroscopes for navigation systems.

  • Biomedical Imaging (MRI/NMR):

    • Molecule-Targeted In Vivo Imaging: Hyperpolarized nanodiamonds, benefiting from long spin relaxation times (T1), are promising contrast agents for Magnetic Resonance Imaging (MRI).
    • Enhanced NMR Spectroscopy: Providing a massive signal-to-noise ratio (SNR) boost for analyzing chemical and biological processes at molecular resolution, particularly in solid-state and solution NMR.

  • Materials Science:

    • DNP Instrumentation: Development of novel, compact DNP systems that do not require expensive helium cryogenics, lowering the barrier to entry for advanced NMR/MRI research.
View Original Abstract

Majority of dynamic nuclear polarization (DNP) experiments have been requiring helium cryogenics and strong magnetic fields for a high degree of nuclear polarization. In this work, we instead demonstrate an optical hyperpolarization of naturally abundant 13C nuclei in a diamond crystal at a low magnetic field and the room temperature. It exploits continuous laser irradiation for polarizing electronic spins of nitrogen vacancy centers and microwave irradiation for transferring the electronic polarization to 13C nuclear spins. We have studied the dependence of 13C polarization on laser and microwave powers. For the first time, a triplet structure corresponding to the 14N hyperfine splitting has been observed in the 13C polarization spectrum. By simultaneously exciting three microwave frequencies at the peaks of the triplet, we have achieved 13C bulk polarization of 0.113 %, leading to an enhancement of 90,000 over the thermal polarization at 17.6 mT. We believe that the multi-tone irradiation can be extended to further enhance the 13C polarization at a low magnetic field.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2003 - Increase in signal-to-noise ratio of >10,000 times in liquid-state NMR [Crossref]
  2. 2006 - In Situ Temperature Jump High-Frequency Dynamic Nuclear Polarization Experiments: Enhanced Sensitivity in Liquid-State NMR Spectroscopy [Crossref]
  3. 2017 - Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR [Crossref]
  4. 2019 - Recent developments in MAS DNP-NMR of materials [Crossref]
  5. 2020 - Hyperpolarization-Enhanced NMR Spectroscopy with Femtomole Sensitivity Using Quantum Defects in Diamond
  6. 2012 - DNP in MRI: An in-bore approach at 1.5 T [Crossref]
  7. 2017 - Continuous-flow DNP polarizer for MRI applications at 1.5 T [Crossref]
  8. 2006 - Real-time metabolic imaging [Crossref]
  9. 2008 - High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR [Crossref]
  10. 2012 - Dynamic Nuclear Polarization NMR Spectroscopy of Microcrystalline Solids [Crossref]

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