  Nitrogen-vacancy (NV) centers in diamond have shown promise as inherently
localized electric-field sensors, capable of detecting individual charges with
nanometer resolution. Working with NV ensembles, we demonstrate that a detailed
understanding of the internal electric field environment enables enhanced
sensitivity in the detection of external electric fields. We follow this logic
along two complementary paths. First, using excitation tuned near the NV's
zero-phonon line, we perform optically detected magnetic resonance (ODMR)
spectroscopy at cryogenic temperatures in order to precisely measure the NV
center's excited-state susceptibility to electric fields. In doing so, we
demonstrate that the characteristically observed contrast inversion arises from
an interplay between spin-selective optical pumping and the NV centers' local
charge distribution. Second, motivated by this understanding, we propose and
analyze a novel scheme for optically-enhanced electric-field sensing using NV
ensembles; we estimate that our approach should enable order of magnitude
improvements in the DC electric-field sensitivity.
