Nitrogen-vacancy (NV) centers in diamonds are a powerful tool for accurate
magnetic field measurements. The key is precisely estimating the
field-dependent splitting width of the optically detected magnetic resonance
(ODMR) spectra of the NV centers. In this study, we investigate the optical
power dependence of the ODMR spectra using NV ensemble in nanodiamonds (NDs)
and a single-crystal bulk diamond. We find that the splitting width
exponentially decays and is saturated as the optical power increases.
Comparison between NDs and a bulk sample shows that while the decay amplitude
is sample-dependent, the optical power at which the decay saturates is almost
sample-independent. We propose that this unexpected phenomenon is an intrinsic
property of the NV center due to non-axisymmetry deformation or impurities. Our
finding indicates that diamonds with less deformation are advantageous for
accurate magnetic field measurements.Comment: 9 pages, 7 figure