Optical spin pumping of color centers promises to revolutionize the practice of nuclear magnetic resonance via efficient dynamic nuclear polarization (DNP) protocols. Significant progress has been attained with systems such as the nitrogen-vacancy (NV) center in diamond, but extreme spectral broadening and nuclear spin-lattice relaxation from co-existing paramagnetic defects are hurdles still difficult to circumvent. The technique we present in this manuscript addresses both issues through the implementation of magnetic-field-sweep-induced NV cross-relaxation with ancilla point defects, here acting as proxies to mediate the transfer of polarization to the nuclear spin ensemble. The governing spin dynamics is robust to field misalignment and ideally suited for shallow NVs, indirectly coupled to outside nuclei through paramagnetic defects at the diamond surface.

Optically-pumped color centers in semiconductor powders can potentially induce high levels of nuclear spin polarization in surrounding solids or fluids at or near ambient conditions, but complications stemming from the random orientation of the particles and the presence of unpolarized paramagnetic defects hinder the flow of polarization beyond the defect’s host material. Here, we theoretically study the spin dynamics of interacting nitrogen-vacancy (NV) and substitutional nitrogen (P1) centers in diamond to show that outside protons spin-polarize efficiently upon a magnetic field sweep across the NV–P1 level anti-crossing. The process can be interpreted in terms of an NV–P1 spin ratchet, whose handedness —and hence the sign of the resulting nuclear polarization — depends on the relative timing of the optical excitation pulse. Further, we find that the polarization transfer mechanism is robust to NV misalignment relative to the external magnetic field, and efficient over a broad range of electron-electron and electron-nuclear spin couplings, even if proxy spins feature short coherence or spin-lattice relaxation times. Therefore, these results pave the route towards the dynamic nuclear polarization of arbitrary spin targets brought in proximity with a diamond powder under ambient conditions.

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-¹³C 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-direction-dependent ¹³C 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.

Dr. Maria Belen Franzoni joins us from Cordoba, Argentina for a month-long stay in our group. Belen is an experimentalist with expertise in the physics and applications of solid-state magnetic resonance. She will be working with us on the use of NV centers in diamond for nuclear spin hyperpolarization. We are delighted to have you here, Belen!

The press release of our Optica article can be found here. A Phys.Org article can be seen here.