Anticrossing Spin Dynamics of Diamond Nitrogen-Vacancy Centers and All-Optical Low-Frequency Magnetometry

We investigate the photoinduced spin dynamics of single nitrogen-vacancy (N-V) centers in diamond near the electronic ground-state level anticrossing (GSLAC), which occurs at an axial magnetic field around 1024 G. Using optically detected magnetic resonance spectroscopy, we first find that the electron-spin transition frequency can be tuned down to 100 kHz for the N14-V center, while, for the N15-V center, the transition strength vanishes for frequencies below about 2 MHz owing to the GSLAC structure. Using optical pulses to prepare and read out the spin state, we observe coherent spin oscillations at 1024 G for the N14-V center which originate from spin mixing induced by residual transverse magnetic fields. This effect is responsible for limiting the smallest observable transition frequency, which can span 2 orders of magnitude ranging from 100 kHz to tens of megahertz, depending on the local magnetic noise. A similar feature is observed for the N15-V center at 1024 G. As an application of these findings, we demonstrate all-optical detection and spectroscopy of externally generated fluctuating magnetic fields at frequencies ranging from 8 MHz down to 500 kHz using a N14-V center. Since the Larmor frequency of most nuclear-spin species lies within this frequency range near the GSLAC, these results pave the way towards all-optical, nanoscale nuclear magnetic resonance spectroscopy, using longitudinal spin cross-relaxation.