Maximising Dynamic Nuclear Polarisation via Selective Hyperfine Tuning

Dynamic nuclear polarisation (DNP) refers to a class of techniques used to increase the signal in nuclear magnetic resonance measurements by transferring spin polarisation from ensembles of highly polarised electrons to target nuclear analytes. These techniques, however, require the application of strong magnetic fields to maximise electron spin polarisation, limiting pathways for electron-nuclear (hyperfine) spin coupling and transfer. In this work we show that, for systems of electronic spin $S\geq1$ possessing an intrinsic zero-field splitting, a separate class of stronger hyperfine interactions based on lab-frame cross relaxation may be utilised to improve DNP efficiency and yield, whilst operating at moderate fields. We analytically review existing methods, and determine that this approach increases the rate of polarisation transfer to the nuclear ensemble by up to an order of magnitude over existing techniques. This result is demonstrated experimentally at room temperature using the optically polarisable $S=1$ electron spin system of the nitrogen vacancy (NV) defect in diamond as the source of electron spin polarisation. Finally we assess the utility of these NV-based approaches for the polarisation of macroscopic quantities of molecular spins external to the diamond for NMR and MRI applications.