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

At a Glance

Metadata	Details
Publication Date	2022-03-04
Journal	Molecules
Authors	Vladimir Vladimirovich Kavtanyuk, Hyun Joon Lee, Sangwon Oh, Keunhong Jeong, Jeong Hyun Shim
Institutions	Korea University of Science and Technology, Korea Research Institute of Standards and Science
Citations	3
Analysis	Full AI Review Included

Technical Documentation & Analysis: Optical Dynamic Nuclear Polarization in Diamond

This document analyzes the research paper “Optical Dynamic Nuclear Polarization of ¹³C Spins in Diamond at a Low Field with Multi-Tone Microwave Irradiation” to provide technical specifications and demonstrate how 6CCVD’s specialized MPCVD diamond materials and customization services can support and advance this critical quantum technology research.

Executive Summary

The research successfully demonstrates highly efficient Dynamic Nuclear Polarization (DNP) of bulk ¹³C nuclear spins in diamond under non-cryogenic, low-field conditions, opening pathways for portable hyperpolarization systems.

Record Enhancement: Achieved a ¹³C bulk nuclear spin polarization of 0.113% at room temperature and a low magnetic field (17.6 mT).
High Efficiency: This polarization level represents an enhancement factor of 90,000 times over the in situ thermal equilibrium polarization.
Novel Technique: The study pioneered the use of multi-tone microwave (MW) irradiation (three simultaneous frequencies) to match the observed triplet structure in the ¹³C polarization spectrum.
Performance Gain: Multi-tone irradiation resulted in a 1.7 times improvement in ¹³C polarization compared to single-tone excitation.
Fundamental Observation: For the first time in optical DNP studies of diamond, a clear triplet structure was observed in the ¹³C polarization spectrum, attributed to the 2.16 MHz ¹⁴N hyperfine splitting of the NV electronic spin.
Material Requirement: The experiment relied on HPHT diamond with precise concentrations of NV centers (1.25 ppm) and P₁ centers (50 ppm), highlighting the need for highly controlled material synthesis and post-processing.

Technical Specifications

The following hard data points were extracted from the research paper, detailing the experimental conditions and performance metrics.

Parameter	Value	Unit	Context
Material Type	HPHT Diamond	N/A	Natural abundance ¹³C
Crystal Orientation	(111)	N/A	NV center alignment
NV Center Concentration	1.25	ppm	Required for DNP
P₁ Center Concentration	~50	ppm	Estimated by ESR
DNP Magnetic Field (B_EM)	17.6	mT	Low-field hyperpolarization
NMR Readout Field (B_SM)	6	T	Superconducting magnet
¹³C NMR Frequency	64.237	MHz	Readout frequency at 6 T
MW Frequencies Used	3 (f₁, f₂, f₃)	N/A	Multi-tone excitation
¹⁴N Hyperfine Splitting	2.16	MHz	Observed in ¹³C spectrum
Optimal MW Power	~10	W	Applied via Helmholtz coil
Optimal Laser Power Density	~30	mW/mm²	532 nm green laser
Achieved ¹³C Polarization	0.113	%	Bulk nuclear spin polarization
Enhancement Factor (ε)	90,000	N/A	Over in situ thermal polarization
Temperature Rise (at optimal P)	> 100	°C	Laser-induced heating
Shuttling Time	2	s	Time to move diamond 80 cm (DNP to NMR)

Key Methodologies

The experimental success hinges on precise material preparation and the integration of low-field DNP with high-field NMR readout, facilitated by rapid sample shuttling.

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.
DNP Setup (Low Field, Room Temperature):
- The diamond was placed in a low magnetic field region (17.6 mT) generated by an electrical Helmholtz magnet.
- Continuous optical pumping was performed using a 532 nm green laser (4 W max power, 2 mm beam diameter).
- Continuous MW irradiation was applied via a handmade Helmholtz coil (13 mm diameter) powered by a 50 W amplifier.
Multi-Tone MW Excitation:
- Three separate MW sources (APSIN12G) were synthesized and combined to simultaneously excite the three hyperfine peaks (f₁, f₂, f₃) corresponding to the ¹⁴N splitting.
Optimization:
- MW power was optimized to ~10 W to avoid off-resonant excitation effects that reduce net polarization.
- Laser power density was optimized to ~30 mW/mm², balancing NV polarization against laser-induced heating (which raised the diamond temperature above 100 °C).
Readout System:
- A high-precision motion controller (Newport XPS-RL) rapidly shuttled the hyperpolarized diamond 80 cm in 2 s from the low-field DNP region to the center of a 6 T superconducting magnet (OXFORD).
- ¹³C NMR signals were detected at 64.237 MHz using a commercial NMR console (LapNMR) and a handmade saddle coil.

6CCVD Solutions & Capabilities

6CCVD specializes in providing the high-quality, customized MPCVD diamond required to replicate, optimize, and scale advanced quantum sensing and hyperpolarization experiments like the one detailed in this paper.

Applicable Materials

To replicate or significantly extend this research, 6CCVD recommends materials engineered for superior spin coherence and defect control:

Material Recommendation	6CCVD Specification	Application/Benefit
Optical Grade SCD	SCD, Ra < 1 nm polishing	Essential for minimizing laser scattering and maximizing optical pumping efficiency.
High-Purity ¹²C SCD	Isotopic purity > 99.99% ¹²C	While the paper used natural abundance ¹³C, high ¹²C purity is crucial for maximizing ¹³C spin coherence time (T₂) in future experiments requiring enhanced quantum control.
Custom ¹³C Enriched SCD	Custom ¹³C enrichment (e.g., 1% or 10%)	For studies requiring higher ¹³C signal density or investigating dipolar coupling effects (as mentioned in Ref. [19]), 6CCVD offers precise isotopic control during growth.
NV-Optimized Substrates	SCD plates, (111) orientation	We provide single crystal diamond (SCD) substrates with precise crystallographic orientation, matching the (111) orientation used in this study for optimal NV alignment.

Customization Potential

The success of DNP relies heavily on precise material engineering and integration. 6CCVD offers comprehensive services to meet the exact specifications required for advanced hyperpolarization research:

Defect Engineering: 6CCVD can supply high-purity diamond ready for post-processing (electron irradiation and annealing) to achieve precise NV and P₁ center concentrations (e.g., 1.25 ppm NV, 50 ppm P₁) necessary for efficient polarization transfer.
Custom Dimensions and Thickness: The paper used a 42 mg sample. 6CCVD provides SCD plates ranging from 0.1 µm up to 500 µm thick, and substrates up to 10 mm thick. Thinner plates can be supplied to mitigate the laser-induced heating (above 100 °C) observed in the study, thereby potentially increasing the maximum achievable polarization.
Advanced Polishing: We guarantee ultra-low surface roughness (Ra < 1 nm for SCD) critical for high-power optical setups and minimizing surface defects that can degrade NV center performance.
Metalization Services: Although not explicitly used for DNP in this paper, 6CCVD offers custom metalization (Au, Pt, Ti, W, Cu) for integrating on-chip MW delivery structures or thermal contacts, essential for scaling DNP systems.

Engineering Support

The observed limitations, such as the decrease in polarization due to laser-induced heating, require careful material selection. 6CCVD’s in-house PhD team can assist researchers with material selection for similar Optical Dynamic Nuclear Polarization (DNP) projects, focusing on optimizing thermal management and spin coherence.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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

Original Source

DOI: https://doi.org/10.3390/molecules27051700

References

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