Figure 1

(a) Energy levels of the NV center in $C_{3v}$ symmetry. The excited state is split by spin-orbit interaction whereas the ground state is split by spin-spin interaction (not shown to scale). The figure gives the symmetry adapted wave-functions. The solid arrows indicate the spin-allowed optical transitions. The dashed arrows indicated weak transitions which are allowed through the mixing of the $\left({}^{3}E\right)E$ and $\left({}^{3}E\right)E^{\prime }$ basis states by transverse spin-orbit interaction. The diagonal arrows give intersystem crossing allowed by spin-orbit interaction. (b) Energy levels and wave functions of the NV center in the presence of a strain field. The wave functions are appropriate for a strain field perpendicular to a reflection plane. The transitions are derived from those allowed in (a).Reuse & Permissions

Figure 2

Energy levels for a perturbed NV center. The significant energy levels of Fig. 1b are redrawn to highlight the inter-system crossing. All the radiative and nonradiative transitions are shown. The allowed transitions considered in the text are shown by solid arrows. The dashed vertical arrows on the right are allowed but can be taken to be zero for present experiments. Also the intersystem crossing indicated by the dashed arrows are zero in first order.Reuse & Permissions

Figure 3

Spectral characteristics of the NV center. (a) Emission spectra of a diamond containing a high concentration of NV centers measured near room temperature using low excitation densities of $1\mathrm{W}∕{\mathrm{cm}}^3$. Inset: the variation of the NV emission at $666\mathrm{nm}$ as a function of sample temperature (b) Low temperature emission spectra of the NV center measured with various excitation densities (from Ref. 36). Inset: variation of NV emission intensity as a function of excitation energy density (units of $1\times {10}^5\mathrm{W}∕{\mathrm{cm}}^3$). Emission normalized to excitation intensity.Reuse & Permissions

Figure 4

Emission obtained when gating on excitation at time 0. The excitation is at $532\mathrm{nm}$ and is gated with a mechanical shutter. (a) Response for various laser powers; $10\mathrm{mW}$ corresponds to an intensity of $1\mathrm{W}∕{\mathrm{cm}}^2$. The emission increases to an initial value A, 86% of its final value. The rate of increase of the emission to its final CW value $B$ depends on intensity of excitation. The sample has a spin-lattice relaxation time of $100\mathrm{ms}$ and sample held in dark for $500\mathrm{ms}$ prior to excitation. (b) Responses for various periods in the dark. For these traces the sample is cooled to $100\mathrm{K}$ and the spin-lattice relaxation time is increased to $500\mathrm{ms}$.Reuse & Permissions

Figure 5

Emission response to a double light pulse each $2\mu \mathrm{s}$ separated by a delay which is varied. (a) The emission is shown with various delays between the two pulses. The responses for the first pulses all overlap whereas the second pulse is delayed by the dark period. The peak associated with the second pulse recovers with the dark period and the height of this pulse is plotted in (b) as a function of the duration in the dark. The dashed curve illustrates the response for a $0.3\mu \mathrm{s}$ recovery rate. The repetition rate of the pulse pair was $10\mathrm{kHz}$. The intensity of the excitation was $3\times {10}^6\mathrm{W}∕{\mathrm{cm}}^2$.Reuse & Permissions

Figure 6

Emission of the NV center for a pair of square wave excitation pulse of light at $532\mathrm{nm}$. The light is focused with a microscope objective and for $100\mathrm{mW}$ the intensity is $2\times {10}^6\mathrm{W}∕{\mathrm{cm}}^2$ The back scattered emission is detected using a (a) $700\mathrm{nm}$ long pass filter, (b) $100\mathrm{nm}$ band pass filter at $590\mathrm{nm}$.Reuse & Permissions

Figure 7

Emission of the NV center for a pair of square wave excitation pulses of light at $532\mathrm{nm}$ with laser intensity of $100\mathrm{mW}$. No magnetic field is applied in the case of the upper trace and corresponds to the $100\mathrm{mW}$ trace in Fig. 6. For the lower trace a field of $500\mathrm{G}$ is applied in a random direction (not aligned with an axis of any center).Reuse & Permissions

Figure 8

Emission calculated from solution of the rate equation for energy scheme in Fig. 2. The intensities $k$ are given in units of $1∕\tau$. The value of the parameters in units of ${10}^6{\mathrm{s}}^{-1}$ introduced in the text are $k_{z{z}^{\prime }}=k_{x{x}^{\prime }}=k_{y{y}^{\prime }}=77$; $k_{sz}=3.3$, $k_{sx}=k_{sy}=0$; $k_{z^{\prime }s}=0$; $k_{x^{\prime }s}=k_{y^{\prime }s}=30$; $k_{z{x}^{\prime }}=k_{z{y}^{\prime }}=k_{x{z}^{\prime }}=k_{y{z}^{\prime }}=1.5$.Reuse & Permissions

Figure 9

(a) Emission predicted from the solution of the rate equations for a pair of excitation pulses. The emission is shown for various excitation intensities $k$ shown on the right in units of $1∕\tau$. The value of the parameters are given in the text and summarized in the caption of Fig. 8.Reuse & Permissions

Figure 10

Emission predicted from solutions of rate equations illustrating effect of an applied magnetic field. The solid line in the upper trace shows the emission of system determined from solution of rate equations for energy scheme as shown in Fig. 2. The dashed line indicates the variation in population of singlet level. For the lower trace the three ground states are mixed, effectively maintaining equal population in the three spin projections. The excitation rate is $k=1$ in units of $1∕\tau$. The value of the parameters are given in the text and summarized in the caption of Fig. 8.Reuse & Permissions