Dynamics of frequency-swept nuclear spin optical pumping in powdered diamond at low magnetic fields

Abstract

A broad effort is underway to improve the sensitivity of nuclear magnetic resonance through the use of dynamic nuclear polarization. Nitrogen-vacancy (NV) centers in diamond offer an appealing platform because these paramagnetic defects can be optically polarized efficiently at room temperature. However, work thus far has been mainly limited to single crystals because most polarization transfer protocols are sensitive to misalignment between the NV and magnetic field axes. Here we study the spin dynamics of NV-13 C pairs in the simultaneous presence of optical excitation and microwave frequency sweeps at low magnetic fields. We show that a subtle interplay between illumination intensity, frequency sweep rate, and hyperfine coupling strength leads to efficient, sweep-directiondependent 13 C spin polarization over a broad range of orientations of the magnetic field. In particular, our results strongly suggest that finely-tuned, moderately coupled nuclear spins are key to the hyperpolarization process, which makes this mechanism distinct from other known dynamic polarization channels. These findings pave the route to applications where powders are intrinsically advantageous, including the hyper-polarization of target fluids in contact with the diamond surface or the use of hyperpolarized particles as contrast agents for in-vivo imaging. Nitrogen-vacancy center | hyperpolarization | diamond powder | optical spin pumping | Landau-Zener crossingsNuclear magnetic resonance (NMR) has proven to be a powerful tool in areas ranging from molecular analysis to biomedical imaging. Unfortunately, the attainable nuclear spin polarization is often a small fraction of the possible maximum, thus imposing strict constraints on the minimum sample size and acquisition time. Dynamic nuclear polarization (DNP), i.e, the transfer of magnetization from electron to nuclear spins (1), is a route of growing popularity that substantially mitigates this problem. Enhanced polarization can be attained, e.g., with the aid of dissolved molecular radicals, though the most efficient implementations often rely on freeze-thaw protocols and highfrequency microwave (MW) excitation, which are expensive and technically demanding (2).Adding to the library of DNP platforms, optically active spin-defects in semiconductors are attracting widespread attention as alternative hyperpolarization agents. Among them, the negatively-charged nitrogen vacancy center (NV) in diamond is arguably one of the most promising candidates, since it can be spin-polarized optically to a high degree with only modest illumination intensities and under ambient conditions (3). A variety of protocols have already been implemented to transfer NV spin polarization to surrounding nuclear spins including level-anti-crossing-mediated transfer in the NVground (4) and excited states (5), cross-relaxation with P1-centers (6-8), spin-swap and population trapping (9), amplitude-matched microwave excitation (10,11), and transfer via microwave sweeps (12,13). Despite this progress, however, effic...

Optically pumped spin polarization as a probe of many-body thermalization

“…Extensions of the ideas introduced herein can provide additional insights on the complex spin dynamics at play. For example, the use of chirped micro-wave (MW) pulses to induce nuclear spin polarization (36,37), away from the NV-P1 cross-relaxation condition, can be exploited to separate the roles of NVs and P1s during the spin diffusion process. Along the same lines, microwave manipulation of the electron spin bath should give us the opportunity to controllably reintroduce localization in the nuclear spin system or to count the number of correlated carbons as the polarization spreads (38,39).…”

Enhanced Optical 13C Hyperpolarization in Diamond Treated by High‐Temperature Rapid Thermal Annealing

“…Figure 1B shows typical hyperpolarization buildup curve, saturating in ≈90 s of optical pumping. Polarization transfer under the MW sweeps (Figure 1C) proceeds through a sequence of rotating frame Landau–Zener transitions [ 30 ] to relatively weakly coupled nuclei, and subsequent spin‐diffusion serves to homogenize the polarization in the bulk lattice. Interactions of the nuclei with other lattice spins, especially paramagnetic impurities, contribute to the leakage of polarization to unmeasurable degrees of freedom that effectively manifests as T 1 relaxation, bounding the overall polarization level.…”

Interference in Atomic Magnetometry

“…Notable examples include superconducting quantum interference devices (SQUIDs) [ 2,3 ] or superconducting qubit sensors, [ 4–6 ] atomic magnetometers, [ 7,8 ] and nitrogen‐vacancy centers. [ 9,10 ] Their capabilities of high sensitivity and spatial resolution provide new opportunities in applied physics and other areas of frontier science. For example, nitrogen‐vacancy centers have unlocked the door for nanoscale magnetic resonance.…”