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 electronspin transition frequency can be tuned down to 100 kHz for the N-14-V center, while, for the N-15-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 N-14-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 N-15-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 N-14-V center. Since the Larmor frequency of most nuclearspin 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.