  We introduce an alternate route to dynamically polarize the nuclear spin host
of nitrogen-vacancy (NV) centers in diamond. Our approach articulates optical,
microwave and radio-frequency pulses to recursively transfer spin polarization
from the NV electronic spin. Using two complementary variants of the same
underlying principle, we demonstrate nitrogen nuclear spin initialization
approaching 80% at room temperature both in ensemble and single NV centers.
Unlike existing schemes, our approach does not rely on level anti-crossings and
is thus applicable at arbitrary magnetic fields. This versatility should prove
useful in applications ranging from nanoscale metrology to sensitivity-enhanced
NMR.
