  We report measurements of optically detected magnetic resonance spectra of
ensembles of negatively charged nitrogen-vacancy (NV) centers in diamonds in
the presence of strain and DC external electric fields. The Stark shift of the
spectral lines is stronger when the axial magnetic field along the NV centers
quantization axis is minimized. The shift is also enhanced at avoided crossings
between the hyperfine levels at an axial field of 77 uT. Since the intrinsic
strain in the diamond also induces a Stark shift, we are able to calculate the
magnitude and direction of the strain within the crystal. We also use the Stark
effect to map the electric field in the diamond volume between patterned
electrodes.
