Polarization transfer from optically-pumped NV center ensembles to multinuclear spin baths
NV-diamonds have attracted keen interest for nanoscale sensing and spin manipulation. In particular, the non-equilibrium electron spin polarization after optical excitation of single NV centers has successfully been transferred to nuclear spin baths in the surrounding of the defects. However, these experiments need to be extended to NV-ensembles which have promising practical applications in the hyperpolarization of bulk sample volumes for NMR signal enhancement. Here, we use a dense, shallow ensemble of NV-centers to demonstrate polarization transfer to nuclear spins in a well-defined composite diamond sample system. This allows us to address three different types of nuclear spins in different positions with respect to the NV polarization source: from the close proximity of $^{13}$C inside the diamond lattice to the self-assembled molecular system consisting of $^{1}$H and $^{19}$F spins outside the diamond and over multiple interfaces. We show that ensemble NV experiments face problems different from single NV experiments. In particular, using spinlock pulses, the inhomogeneously broadened ESR line of the NV ensemble limits the minimal resonance linewidth with which the transfer protocol can occur. Furthermore, we compare the NV spin-polarization losses and polarization transfer rates to the different nuclear baths and discuss the role of spin-diffusion as detrimentally affecting the direct observation of nuclear polarization build-up within the detection volume of nanoscale NV-NMR experiments.