The nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has emerged as a candidate to noninvasively hyperpolarize nuclear spins in molecular systems to improve the sensitivity of nuclear magnetic resonance (NMR) experiments. Several promising proof-of-principle experiments have demonstrated small-scale polarization transfer from single $\mathrm{N}$-$V$ centers to hydrogen spins outside the diamond. However, the scaling up of these results to the use of a dense $\mathrm{N}$-$V$ ensemble, which is a necessary prerequisite for achieving realistic NMR sensitivity enhancement, has not yet been demonstrated. In this work, we present evidence for a polarizing interaction between a shallow $\mathrm{N}$-$V$ ensemble and external nuclear targets over a micrometer scale, and characterize the challenges in achieving useful polarization enhancement. In the most favorable example of the interaction with hydrogen in a solid-state target, a maximum polarization transfer rate of approximately $7500$ spins per second per $\mathrm{N}$-$V$ is measured, averaged over an area containing order ${10}^6$ $\mathrm{N}$-$V$ centers. Reduced levels of polarization efficiency are found for liquid-state targets, where molecular diffusion limits the transfer. Through analysis via a theoretical model, we find that our results suggest that implementation of this technique for NMR sensitivity enhancement is feasible following realistic diamond material improvements.

• Received 28 January 2021
• Revised 20 April 2021
• Accepted 22 April 2021

DOI:https://doi.org/10.1103/PhysRevApplied.15.054052