Prospects for nuclear spin hyperpolarization of molecular samples using nitrogen-vacancy centers in diamond

After initial proof-of-principle demonstrations, optically pumped nitrogen-vacancy (NV) centers in diamond have been proposed as a noninvasive platform to achieve hyperpolarization of nuclear spins in molecular samples over macroscopic volumes and enhance the sensitivity in nuclear magnetic resonance (NMR) experiments. In this work we model the process of polarization of external samples by NV centers and theoretically evaluate their performance in a range of scenarios. We find that average nuclear spin polarizations exceeding 10% can in principle be generated over macroscopic sample volumes $(\ensuremath{\gtrsim}\ensuremath{\mu}\mathrm{l})$ with a careful engineering of the system's geometry to maximize the diamond-sample contact area. The fabrication requirements and other practical challenges are discussed. We then explore the possibility of exploiting local polarization enhancements in nano/micro-NMR experiments based on NV centers. For micro-NMR we find that modest signal enhancements over thermal polarization (by 1--2 orders of magnitude) can in essence be achieved with existing technology, with larger enhancements achievable via microstructuring of the sample/substrate interface. However, there is generally no benefit for nano-NMR where the detection of statistical polarization provides the largest signal-to-noise ratio. This work will guide future experimental efforts to integrate NV-based hyperpolarization to NMR systems.