  Optically-pumped color centers in semiconductor powders can potentially
induce high levels of nuclear spin polarization in surrounding solids or fluids
at or near ambient conditions, but complications stemming from the random
orientation of the particles and the presence of unpolarized paramagnetic
defects hinder the flow of polarization beyond the defect's host material.
Here, we theoretically study the spin dynamics of interacting nitrogen-vacancy
(NV) and substitutional nitrogen (P1) centers in diamond to show that outside
protons spin-polarize efficiently upon a magnetic field sweep across the NV-P1
level anti-crossing. The process can be interpreted in terms of an NV-P1 spin
ratchet, whose handedness - and hence the sign of the resulting nuclear
polarization - depends on the relative timing of the optical excitation pulse.
Further, we find that the polarization transfer mechanism is robust to NV
misalignment relative to the external magnetic field, and efficient over a
broad range of electron-electron and electron-nuclear spin couplings, even if
proxy spins feature short coherence or spin-lattice relaxation times.
Therefore, these results pave the route towards the dynamic nuclear
polarization of arbitrary spin targets brought in proximity with a diamond
powder under ambient conditions.
