Narrow-Linewidth Homogeneous Optical Emitters in Diamond Nanostructures via Silicon Ion Implantation

The negatively charged silicon-vacancy (${\mathrm{SiV}}^{-}$) center in diamond is a bright source of indistinguishable single photons and a useful resource in quantum-information protocols. Until now, ${\mathrm{SiV}}^{-}$ centers with narrow optical linewidths and small inhomogeneous distributions of ${\mathrm{SiV}}^{-}$ transition frequencies have only been reported in samples doped with silicon during diamond growth. We present a technique for producing implanted ${\mathrm{SiV}}^{-}$ centers with nearly lifetime-limited optical linewidths and a small inhomogeneous distribution. These properties persist after nanofabrication, paving the way for the incorporation of high-quality ${\mathrm{SiV}}^{-}$ centers into nanophotonic devices.

Figure 1

Properties of the ${\mathrm{SiV}}^{-}$ center. (a) Atomic structure of the SiV center. The V-Si-V axis lies along the $⟨111⟩$ lattice direction. The ${\mathrm{SiV}}^{-}$ has $D_{3d}$ symmetry. (b) Level structure of the ${\mathrm{SiV}}^{-}$ center. The ${\mathrm{SiV}}^{-}$ is a single-hole system with double orbital and spin degeneracy. This degeneracy is partially lifted by spin-orbit coupling (${\lambda }_g^{\mathrm{SO}}=47\mathrm{GHz}$ and ${\lambda }_u^{\mathrm{SO}}=260\mathrm{GHz}$ [11, 13]). Lattice strain increases the splitting between these spin-orbit levels, shifting the transition frequencies. (c) Fluorescence spectra of the ZPLs of single ${\mathrm{SiV}}^{-}$ centers in high-strain (blue, dashed) and low-strain (red) environments at 9–15 K. Transitions $B$ and $C$ are less sensitive to strain compared with transitions $A$ and $D$ because the ground and excited states shift in the same (opposite) directions for transitions $B$ and $C$ ($A$ and $D$) [12]. Unstrained spectrum offset and scaled vertically for clarity. (d) Linewidth (FWHM) of representative implanted ${\mathrm{SiV}}^{-}$ in bulk (unstructured) diamond measured by PLE spectroscopy (blue points, data; red line, Lorentzian fit). Inset: histogram of emitter linewidths in bulk diamond. Almost all emitters have a linewidth within a factor of 3 of the lifetime limit (94 MHz).

Figure 2

Inhomogeneous distribution of fluorescence wavelengths of implanted ${\mathrm{SiV}}^{-}$ transitions. (a) Kernel density estimation of distribution of bulk ${\mathrm{SiV}}^{-}$ wavelengths after $800°\mathrm{C}$ ($N=19$, red dashed curve) and $1100°\mathrm{C}$ ($N=13$, blue solid curve) annealing. The distribution narrows from 3–4 nm ($800°\mathrm{C}$ anneal) to 0.03 nm (15 GHz, $1100°\mathrm{C}$ anneal). (b) Enlarged distribution (transition $C$) after $1100°\mathrm{C}$ annealing. Note the smaller wavelength range on the horizontal axis. (c) Sum of spectra for different ${\mathrm{SiV}}^{-}$ centers after $1100°\mathrm{C}$ annealing. The ${\mathrm{SiV}}^{-}$ fine structure is clearly present, demonstrating that the inhomogeneous distribution is small. (d),(e) Spatial map of collected fluorescence (thousands of counts per second) over a region of bulk diamond exciting off (d) and on (e) resonance. By comparing the densities of emitters, we estimate that $(30\pm 15)\%$ of the emitters are nearly resonant. These measurements are taken at 9–15 K.

Figure 3

${\mathrm{SiV}}^{-}$ centers in nanostructures. (a) Scanning electron micrograph of six nanobeam waveguides. Inset: schematic of a triangular diamond nanobeam containing an ${\mathrm{SiV}}^{-}$ center. (b) Spatial map of ZPL fluorescence collected by scanning confocal microscopy with off-resonant excitation. Multiple ${\mathrm{SiV}}^{-}$ centers are visible in each waveguide. (c) Linewidth of representative implanted ${\mathrm{SiV}}^{-}$ inside a nanowaveguide measured by PLE spectroscopy (blue points, data; red line, Lorentzian fit). Inset: histogram of emitter linewidths in nanostructures. Most emitters have linewidths within a factor of 4 of the lifetime limit. (d) Spectral diffusion of the emitter measured in part (c). The total spectral diffusion is under 400 MHz even after more than an hour of continuous measurement. This diffusion is quantified by measuring the drift of the fitted center frequency of resonance fluorescence scans as a function of time. Error bars are statistical error on the fitted center position. The lighter outline is the FWHM of the fitted Lorentzian at each time point.

Figure 4

Confocal microscope design. The 520-nm and 700-nm lasers are used to excite the ${\mathrm{SiV}}^{-}$ off resonantly. The 737-nm external-cavity diode laser is used to excite the ${\mathrm{SiV}}^{-}$ resonantly. Optical fibers are used to collect the ${\mathrm{SiV}}^{-}$ fluorescence and transmit it to avalanche photodiodes (APDs) for detection. Collection can be performed either on the ZPL (if the excitation is off resonance) or the PSB (in either excitation scheme).

Figure 5

Fluorescence autocorrelation measurement of a ${\mathrm{SiV}}^{-}$ center inside a diamond nanobeam as described in the text. Error bars are estimated assuming the noise on the number of detected photons follows a Poisson distribution (shot noise). The extent of the dip at $\tau =0$ is limited by finite detector bandwidth: we measure $g^{(2)}(0)=0.45$; deconvolving the detector response yields $g^{(2)}(0)=0.15$.