Hyperpolarizing nuclear spins typically requires microwave irradiation, which complicates the experiment and limits applicability. In diamond, NV centers can be polarized with light at room temperature, but in powder the random orientation of centers blurs the resonance conditions. This work sits alongside studies of optical hyperpolarization in single crystals and protocols based on microwave pulses for micro- and nanodiamonds. Neighboring works use diamond NV centers for magnetometry, quantum information, and optical microscopy, but a purely optical protocol for powder had not been demonstrated before.
Two samples of diamond particles: submicron with 20% 13C and commercial 1.5–2.5 µm with natural abundance of 1.1%. NV centers were created in both by electron irradiation and annealing. Particles were placed in an NMR spectrometer rotor, spun at the magic angle of 54.74° at frequencies up to 6 kHz, and illuminated with a 532 nm laser through an optical fiber. The sequence included quenching of residual polarization, a light pumping period, and a 90° detection pulse. Defect concentrations were determined by EPR. To describe the mechanism, a minimal three-spin model NV–P1–13C was built, solving the Lindblad equation for statics and Landau–Zener equations for rotation.
The maximum 13C signal enhancement was 280-fold for the 20% enriched sample at a spinning frequency of 2 kHz and 411-fold for the natural sample at 1 kHz. In the steady state, absolute polarization reached 0.12% and 0.23%, respectively, nearly an order of magnitude higher than previous results for micro- and nanodiamonds. The dependence on spinning frequency showed a sharp rise to a maximum and a decline at higher speeds. Experiments varying the delay after a short light pulse revealed polarization transfer from nuclei near NV centers to distant ones via spin diffusion. The model reproduced the qualitative trend of the frequency dependence and predicted a positive sign for the averaged polarization.
This work differs from neighboring studies in that it is the first to achieve purely optical hyperpolarization in diamond powder without microwaves, using rotation to overcome the anisotropy of NV centers. Unlike static schemes, where only centers with favorable orientation participate in transfer, here more than 99.9% of NV centers geometrically cross the level anti-crossing regions. This removes a fundamental limitation for ensembles of randomly oriented nanoparticles and makes the protocol compatible with standard magic-angle spinning NMR techniques.
The immediate goal is to transfer polarization from 13C inside the diamond to external nuclei, such as protons on the surface. An estimate of the spin diffusion length of about 6 nm points to an optimal particle size below 30 nm. Direct transfer to surface protons via subsurface paramagnetic defects with strong hyperfine coupling may also be possible. The method could be extended to other systems with optically polarizable triplets, such as pentacene in molecular crystals.
The method could boost the sensitivity of solid-state NMR spectroscopy and magnetometry on hyperpolarized nuclei at room temperature. It also opens a route to analyzing substances adsorbed on the surface of diamond nanoparticles.
Optimize particle size and defect concentration for efficient polarization transport to the surface. Test transfer to external nuclei using cross-polarization and investigate direct polarization of surface protons.
In the field of magnetic resonance, the problem of hyperpolarizing arbitrary molecules at room temperature without microwave equipment remains unsolved. This work shows that optical pumping of defects combined with mechanical rotation could become a universal tool, but requires solving the problem of polarization transport across the phase boundary.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This work demonstrates purely optical hyperpolarization of 13C nuclei in diamond powder with NV centers under magic-angle spinning. Signal enhancement up to 280–411 times and absolute polarization up to 0.23% were measured, and a three-spin model explaining the effect through level crossings during rotation is proposed.
Where its strength is
For the first time, it is shown that sample rotation allows nearly all NV centers to participate in polarization transfer, lifting the anisotropy limitation for powders and paving the way for microwave-free ultrasensitive NMR.
Where it could go next
Worth trying
Why this may matter
Purely optical hyperpolarization in diamond powders could greatly simplify the instrumentation for enhancing NMR signals and make the method accessible to a wide range of applications, including chemical analysis and medical diagnostics.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.56, against a usual 0.64. It looks like few have drilled nearby.
The angle of 54.74° between the rotation axis and the magnetic field is called the magic angle because it nullifies dipole-dipole interactions between nuclei. It is this angle that made more than 99.9% of NV centers pass through the required level crossing points twice per rotor revolution.