  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.
