  We studied the dynamic nuclear spin polarization of nitrogen in negatively
charged nitrogen-vacancy (NV) centers in diamond both experimentally and
theoretically over a wide range of magnetic fields from 0 to 1100 G covering
both the excited-state level anti-crossing and the ground-state level
anti-crossing magnetic field regions. Special attention was paid to the less
studied ground-state level anti-crossing region. The nuclear spin polarization
was inferred from measurements of the optically detected magnetic resonance
signal. These measurements show that a very large (up to $96 \pm 2\%$) nuclear
spin polarization of nitrogen can be achieved over a very broad range of
magnetic field starting from around 400 G up to magnetic field values
substantially exceeding the ground-state level anti-crossing at 1024 G. We
measured the influence of angular deviations of the magnetic field from the NV
axis on the nuclear spin polarization efficiency and found that, in the
vicinity of the ground-state level anti-crossing, the nuclear spin polarization
is more sensitive to this angle than in the vicinity of the excited-state level
anti-crossing. Indeed, an angle as small as a tenth of a degree of arc can
destroy almost completely the spin polarization of a nitrogen nucleus. In
addition, we investigated theoretically the influence of strain and optical
excitation power on the nuclear spin polarization.
