Optically detected magnetic resonance of nitrogen-vacancy centers in diamond under weak laser excitation

As promising quantum sensors, nitrogen-vacancy (N-$V$) centers in diamond have been widely used in frontier studies in condensed-matter physics, material science, and life sciences. In practical applications, weak laser excitation is favorable as it reduces the side effects of laser irradiation, such as phototoxicity and heating. Here we report a combined theoretical and experimental study of (near) zero-field optically detected magnetic resonance (ODMR) of N-$V$-center ensembles under weak 532-nm laser excitation. In this region, both the width and splitting of the ODMR spectra decrease with increasing laser power. This power dependence is reproduced with a model that accounts for the laser-induced charge neutralization of $\mathrm{N}\text{−}{V}^{-}$–${\mathrm{N}}^{+}$ pairs, which alters the local electric field environment. These results are useful for the understanding and development of N-$V$-based quantum sensing in light-sensitive applications.

Figure 1

Laser-induced charge neutralization of $\mathrm{N}\text{−}{V}^{-}$–${\mathrm{N}}^{+}$ pairs in diamond. (a) Schematic representation of the $\mathrm{N}\text{−}{V}^{-}$–${\mathrm{N}}^{+}$ model. After the absorption of photons, $\mathrm{N}\text{−}{V}^{-}$–${\mathrm{N}}^{+}$ pairs could change into $\mathrm{N}\text{−}{V}^0$–${\mathrm{N}}^0$ pairs, which leads to a more dilute charge distribution in the diamond lattice and thus smaller internal electric fields acting on the remaining N-$V$ centers. (b) PL spectra of a nanodiamond measured at 8.4 K. The curves are vertically offset for better visibility. (c) Zero-field ODMR spectra of the same nanodiamond, measured at different laser powers. (d) Laser power dependence of the $\mathrm{N}\text{−}{V}^{-}$ ratio and photon count (upper pane), the splitting, and width of the ODMR spectra (lower pane).

Figure 2

(a)–(c) ODMR spectra of N-$V$ centers in microdiamonds (MDs), and bulk diamonds (S5 and E6-2) under weak laser excitation, see Table 1 for detailed sample information. The spectra of MD and sample S5 are measured under Earth’s magnetic field. The spectra of sample E6-2 are measured under zero magnetic field. (d)–(f) Laser power dependence of the ODMR splitting and width for the (d) MD sample, (e) S5 sample, and (f) E6-2 sample.

Figure 3

Numerical results on nitrogen concentration. (a) The probability distribution of N-$V$ resonant frequency under different nitrogen concentrations ($[\mathrm{N}\text{−}{V}^{-}]=3$ ppm). (b) Splitting and width of the numerical spectra as a function of the nitrogen concentration. The dashed lines are fitting results with Eq. (1). Note that spin relaxation, spin dephasing, and other technical broadening factors (e.g., microwave power broadening) are not considered in this simulation.

Figure 4

Comparison between numerical and experimental results. (a),(b) Experimentally measured (gray dots) and numerically simulated (red lines) ODMR spectra of an ND at various laser powers. For the numerical simulation, each laser power is converted into the corresponding $\mathrm{N}\text{−}{V}^{-}$ ratio for the simulation. The parameters for the numerical simulation are $[\mathrm{N}]=200$ ppm, $[\mathrm{N}\text{−}{V}^{-}]=3$ ppm, $1/T_2^{\star }=2$ MHz and ${\mathrm{\Omega }}_R=2\pi \times 4$ MHz. (c) Splitting of the ODMR spectra. (d) Width of the ODMR spectra. The red line are the fitting result from the simulated ODMR spectra (without light narrowing), while the blue line also considers the light narrowing effect [19].