Figure 1

Simplified energy diagram of an NV center. The straight lines represent radiative transitions (absorption and emission of a photon). Wavy lines symbolize nonradiative transitions. Solid lines sketch the conventional path for transferring a population from $m$ $=$ $\pm$1 states to $m$ $=$ 0 states, and the dashed lines represent transitions proposed in this paper to explain our experimental observations (they transfer a part of the population from $m$ $=$ 0 back to $m$ $=$ $\pm$1). All singlet states are shown as a gray box and labeled S. Label C stands for conduction band and V for the vibronic states. The absorption rates from the ground and the excited electronic states are $\sigma I$ and ${\sigma }^{\prime }I$, respectively, with $I$ being the laser intensity and $\sigma$, ${\sigma }^{\prime }$ the corresponding absorption cross sections.Reuse & Permissions

Figure 2

Photoluminescence of ${\mathrm{NV}}^{-}$ centers at different energies of the exciting laser pulse (pulse repetition rate is 1 MHz). The smooth solid curve is obtained by fitting Eq. (1) to the first eight low-energy points. The saturation energy is 0.02 μJ. The dashed line is a fit to a model as explained later in the paper.Reuse & Permissions

Figure 3

Spectra of an ensemble of NV centers in a nanocrystal at two different pulse energies. The intensity units are the same for both spectra. The spectrum with higher intensity is measured at 0.08 μJ, and the lower spectrum is measured at 1 μJ of the excitation-pulse energy. These energies are significantly above the saturation energy of 0.02 μJ. In this saturation regime, there is no significant change in NV${}^0$ emission, but the decrease in NV${}^{-}$ emission is clearly visible.Reuse & Permissions

Figure 4

Spin polarization can be recovered using low-energy pulses after the initial high-energy depolarizing pulse. The pulse energy in the figure is the energy of the first pulse in the sequence (periodically repeated). The energy of the following pulses was always $E_{\mathrm{m}}$. The dots are experimental points, and the solid line is the best fit to the model (see Table  for fitted parameters). The inset shows schematically the pulse sequences used to obtain each of the curves.Reuse & Permissions

Figure 5

Left panel: saturation curves gathered with external fields between 0 and 290 mT (the field values are shown on the right panel). Right panel: theoretical fits of the data shown on the left. Fits were derived from the model described above. From the data, it is clear that externally depolarized centers show a marked decrease in anomalous saturation behavior. The insert shows the four possible orientations of the NV centers. For the best fit, the magnetic field was oriented approximately along one of these directions. The angles indicated are 109.4°.Reuse & Permissions

Figure 6

Luminescence decay curves gathered at three different pulse energies: 0.013 μJ (curve 1), 0.085 μJ (curve 2), and 1.1 μJ (curve 3). In the presence of an external magnetic field of 290 mT, curves were measured at pulse energies of 0.013 μJ (curve 4) and 1.1 μJ (curve 5). We found that all decay curves can be fitted with a two-exponential decay. The decay rates and the amplitudes of the fast/slow components for curve 1 are 57(7)/18(1) MHz and 0.059(8)/0.115(9), curve 2, 61.4(3)/17.9(3) MHz and 0.126(6)/0.216(6); curve 3, 85(5) /18.9(6) MHz and 0.130(5)/0.117(5); curve 4, 70(3)/20(2) MHz and 0.074(4)/0.029(4); curve 5, 70(2)/17(1) MHz and 0.195(5)/0.050(5), where the numbers in brackets show the standard deviation of the last digit (e.g., the amplitude of the fast component of curve 5 is 0.195 and the standard deviation is 0.005). The increased significance of the faster decaying component at higher energies of the pulse is clearly visible when comparing curves 1 and 3. The presence of the magnetic field also increases the relative contribution of the faster decaying exponent, as can be seen from comparison of curves 3 and 5.Reuse & Permissions