Polarization Transfer from Optically Pumped Ensembles of N- v Centers to Multinuclear Spin Baths

Nitrogen-vacancy (N-V) diamonds have attracted keen interest for nanoscale sensing and spin manipulation. In particular, the nonequilibrium electron spin polarization after optical excitation of single N-V centers has successfully been transferred to nuclear spin baths in the surrounding of defects. However, these experiments need to be extended to N-V ensembles that have promising practical applications in the hyperpolarization of bulk sample volumes for NMR signal enhancement. Here, we use a dense, shallow ensemble of N-V 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 N-V polarization source: from the close proximity of 13C inside the diamond lattice to the self-assembled molecular system consisting of 1H and 19F spins outside the diamond and over multiple interfaces. We show that ensemble N-V experiments face problems different from single N-V experiments. In particular, using spinlock pulses, the inhomogeneously broadened electron spin resonance line of the N-V ensemble limits the minimal resonance linewidth with which the transfer protocol can occur. Furthermore, we compare the N-V 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 buildup within the detection volume of nanoscale N-V-NMR experiments.