Electron spin contrast of Purcell-enhanced nitrogen-vacancy ensembles in nanodiamonds

Nitrogen-vacancy centers in diamond allow for coherent spin-state manipulation at room temperature, which could bring dramatic advances to nanoscale sensing and quantum information technology. We introduce a method for the optical measurement of the spin contrast in dense nitrogen-vacancy (NV) ensembles. This method brings insight into the interplay between the spin contrast and fluorescence lifetime. We show that for improving the spin readout sensitivity in NV ensembles, one should aim at modifying the far-field radiation pattern rather than enhancing the emission rate.

Figure 1

(a) Sample layout: islands of plasmonic TiN (200 nm thick and 500 $\mu \mathrm{m}$ in diameter) and dispersed nanodiamonds with NV center ensembles (NVEs) on a $c$-sapphire substrate. Blowups show a fluorescence map of NVEs and an SEM image of a typical nanodiamond used in the study. (b) Ground-state spin-level diagram showing the processes of optical initialization and subsequent thermal relaxation. (c) Spin-contrast measurement scheme. The number of photons registered during the detection window of duration $t_{det}$ depends on the degree of spin relaxation occurring during the time $\mathrm{\Delta }t$.

Figure 2

(a) Typical spin-relaxation and (b) fluorescence decay curves for NVEs found on sapphire (blue) and TiN (orange). The spin contrast is measured as a normalized difference between the fluorescence signals of partially thermalized spins and optically initialized spins.

Figure 3

Spin decay contrast $C_{T1}$ values for NVEs with different lifetimes measured for an optical excitation rate of approximately 1.5 MHz. The trend agrees well with the results of the simulation based on a kinetic model of the NV.

Figure 4

Dependences of $C_{T1}$ on optical power for NVEs on sapphire and TiN show a decrease at strong excitation rates. Solid lines are obtained from the five-level kinetic model of the NV center.

Figure 5

A simplified representation of the NV center's energy levels and transition rates. The two levels on the left (right) are the excited and ground triplet states of the $m_{\mathrm{s}}=0(\pm 1)$ subsystem. The level $s$ in the middle is the metastable spin-singlet level.

Figure 6

Calculated electron spin readout signal-to-noise ratio for a single NV, under weak and strong optical excitation rates, assuming optimal durations of detection window $t_{det}$. Dashed line: single-shot signal-to-noise ratio as a function of the total fluorescence lifetime, assuming unity quantum yield and total collection of fluorescence, $k_{\mathrm{opt}}=1500\mathrm{MHz}$ (deep saturation). Solid line: single-shot signal-to-noise ratio assuming optical excitation rate of $k_{\mathrm{opt}}=1.5\mathrm{MHz}$, used in our experiment.