[Paper Review] Room-Temperature in situ Nuclear Spin Hyperpolarization from Optically-Pumped Nitrogen Vacancy Centers in Diamond

Jonathan P. King, Keunhong Jeong+6 more|arXiv (Cornell University)|Jan 13, 2015

High-pressure geophysics and materials3 references4 citations

TL;DR

This paper demonstrates room-temperature, in situ hyperpolarization of 13C nuclear spins in diamond using optical pumping of nitrogen vacancy (NV) centers combined with dynamic nuclear polarization (DNP). By applying 532 nm laser light and microwave irradiation at 420 mT, the authors achieve a bulk nuclear spin polarization of 6%, detected via standard inductive NMR without sample shuttling or precise crystal alignment, enabling high-sensitivity NMR at arbitrary magnetic fields.

ABSTRACT

We report bulk, room-temperature hyperpolarization of 13C nuclear spins observed via high-field nuclear magnetic resonance (NMR). The hyperpolarization is achieved by optical pumping (OP) of nitrogen vacancy defect centers in diamond accompanied by dynamic nuclear polarization (DNP). The technique harnesses the large optically-induced spin polarization of NV- centers at room temperature, which is many orders of magnitude greater than thermal equilibrium polarization and typically achievable only at sub-Kelvin temperatures. Transfer of the spin polarization to the 13C nuclear spins is accomplished via a combination of OP and microwave irradiation. The OP/DNP is performed at 420 mT, where inductive detection of NMR is feasible, in contrast to the typically exploited level anticrossing regimes at 100 mT and 50 mT. Here, we report a bulk nuclear spin polarization of 6%. This polarization was generated in situ and detected with a standard, inductive NMR probe without the need for sample shuttling or precise crystal orientation. Hyperpolarization via OP/DNP should operate at arbitrary magnetic fields, enabling orders of magnitude sensitivity enhancement for NMR of solids and liquids at ambient conditions.

Motivation & Objective

• To develop a general method for hyperpolarizing nuclear spins at room temperature and ambient magnetic fields.
• To eliminate the need for cryogenic conditions, sample shuttling, or precise crystal orientation in NMR hyperpolarization.
• To enable bulk, in situ hyperpolarization of 13C spins using optically-polarized NV centers and DNP at fields compatible with standard NMR detection.
• To demonstrate that hyperpolarization can be generated and detected under identical experimental conditions, enhancing NMR sensitivity without specialized equipment.

Proposed method

• Optical pumping with a 532 nm laser selectively populates the ms=0 state of NV− centers, creating large electron spin polarization at room temperature.
• Microwave irradiation at 8896 MHz drives forbidden nuclear spin transitions via dipolar coupling, enabling polarization transfer from NV− to 13C nuclei through the solid effect or thermal mixing mechanisms.
• The experiment is conducted at 420 mT, where inductive NMR detection is feasible, avoiding the need for level anticrossing fields at 100 mT or 50 mT.
• Nuclear spin polarization is measured using a standard inductive NMR probe after 60 scans with a 60 s repetition time, with calibration against a Gd(III)-doped 13C-enriched DMSO sample.
• Laser intensity and microwave power are varied to optimize polarization transfer, with increased intensity and power showing slight improvements in polarization yield.
• The method operates without sample shuttling or precise crystal orientation, enabling hyperpolarization in situ under ambient conditions.

Experimental results

Research questions

• RQ1 Can bulk nuclear spin hyperpolarization be achieved at room temperature using NV centers in diamond without cryogenic cooling?
• RQ2 Can hyperpolarization be generated and detected in situ at a magnetic field strength compatible with standard inductive NMR detection?
• RQ3 To what extent do laser intensity and microwave power enhance the polarization transfer efficiency in OP/DNP at 420 mT?
• RQ4 Is the observed hyperpolarization due to the solid effect, thermal mixing, or a combination of both mechanisms?

Key findings

• A bulk nuclear spin polarization of 6% was achieved for 13C spins in a 4.5 mg diamond sample at room temperature and 420 mT magnetic field.
• The hyperpolarization was generated in situ and detected using a standard inductive NMR probe without sample shuttling or reorientation.
• The polarization level was confirmed by comparison to a Gd(III)-doped 13C-enriched DMSO sample, where 12,676 scans at 10 ms repetition time produced a signal lower by a factor of ~12.
• Laser intensity showed a slight positive correlation with polarization, indicating increased effective volume of highly polarized NV centers with higher power.
• Microwave power also enhanced polarization transfer, consistent with both solid effect and thermal mixing DNP mechanisms being active.
• The method operates at arbitrary magnetic fields and does not require level anticrossing conditions, enabling broad applicability to NMR in solids and liquids at ambient conditions.