Spin-dependent charge state interconversion of nitrogen vacancy centers in nanodiamonds

The nitrogen vacancy (NV) center in diamond is the most widely studied color center in diamond for its wide range of applications in both physics and biology. The negatively charged state ${\mathrm{NV}}^{-}$ is often favored for its better spin-optical properties, whereas the presence of the neutral charge state ${\mathrm{NV}}^0$ is often minimized or neglected by using a single optimized excitation wavelength. In many cases, the use of additional laser wavelengths has an important impact on the NV optical properties and can lead to a dramatically quenched fluorescence. Through measurements of the photophysics of the NV center we show in this work that this quenching mechanism is mediated by charge state interconversion providing additional channels for nonradiative decay. Our results show that this mechanism is also sensitive to the spin state of the ${\mathrm{NV}}^{-}$. A better understanding of this physical mechanism will help mitigate the spin and charge state depolarization effects that are detrimental for quantum technology applications.

Figure 1

Experimental approach. (a) Schematic of the experimental setup where the nanodiamond, deposited on a coverslip, is positioned using a piezo stage. The two incident lasers (532 and $785\mathrm{nm}$) are combined using two dichroic mirrors (${\mathrm{DM}}_1$ and ${\mathrm{DM}}_2$) and focused by the 1.4 numerical aperture objective. The small portion of the incident power that is transmitted through the dichroic mirrors is used to monitor the power of each laser using photodiodes (PDs). The emission from the nanodiamond is collected by the same objective and sent through the dichroic mirrors and a filter to an avalanche photodiode (APD). (b) Spectrum recorded for $0.75\mathrm{mW}$ of 532 nm for four different 785-nm powers: 0, 2.3, 9.7, and $21\mathrm{mW}$ in order from dark- to light-colored curves. (c) Saturation curve for a given nanodiamond without and with a magnet in close proximity in light and dark colors, respectively. The open symbols are the experimental measurement, while the curves show the result from the best model obtained by fitting both the saturation curve and the fluorescence quenching presented in Fig. 2.

Figure 2

Experimental data. (a) Fluorescence quenching for a given nanodiamond as a function of the incident near-infrared power. The different curves are obtained for different green powers: 0.03, 0.07, 0.22, 0.5, 0.74, and $0.72\mathrm{mW}$ from dark- to light-colored curves. (b) Same as (a), with a magnet in close proximity to the nanodiamond. (c) Comparison of the average relative error between the fit and the experimental fluorescence quenching when accounting or not for a spin-dependent ionization in the model.

Figure 3

Energy levels of both charge states of the NV center.

Figure 4

Predicted fluorescence, charge polarization, and spin polarization. (a) Fluorescence count, (b) charge polarization (1 is completely in the ${\mathrm{NV}}^{-}$ state), and (c) spin polarization (1 is completely in the $m_s=0$ state) obtained using our model with the average parameters obtained from fitting the experimental measurements from five distinct nanodiamonds. While for very low 532-nm laser power the spin state should thermalize, the lowest power used here is sufficient to overcome this spin relaxation effect.

Figure 5

Ionization and recombination processes. Schematic of the internal electron orbital structure for the NV charge conversion process between the negative and neutral charge states of the NV center. The NV center is in the ${\mathrm{NV}}^0$ charge state when there are five electrons in the structure and in the ${\mathrm{NV}}^{-}$ center when there is an extra sixth electron in the structure. The ${\mathrm{NV}}^{-}$ to ${\mathrm{NV}}^0$ conversion involves two-photon excitation followed by an Auger process that releases enough energy to ionize an electron from the structure, leaving the defect in the ground state of the ${\mathrm{NV}}^0$. The recombination process from ${\mathrm{NV}}^0$ to ${\mathrm{NV}}^{-}$ is also a two-step process. First, the neutral defect is excited, as shown by the movement of an electron from the $a_1$ orbital to the $e$ orbital. Before decay can occur an additional electron is transferred from the $a_1$ orbital in the valence band to the vacant place on the $a_1$ orbital in the band gap. The vacant place in the deep-lying ${\mathrm{a}}_1$ orbital is then replaced by a new electron from the valance band leaving the center in the ground state of the ${\mathrm{NV}}^{-}$. The positions of orbital levels change with different electron configurations throughout both processes. The energy level labels in this diagram arise from a group theory representation of the internal orbital structure and are different from the typical energy level diagrams that are shown for the NV center.

Figure 6

Energy levels of both charge states of the NV center showing all the stimulated, spontaneous, and nonradiative processes. The black arrows indicate dominantly nonradiative processes, the orange arrows indicate spontaneous emission processes, and the red (785 nm) and green (532 nm) arrows indicate processes that can be stimulated by the corresponding laser wavelength.