Enhanced single-photon emission from a diamond-silver aperture

Solid-state quantum emitters, such as the nitrogen-vacancy centre in diamond1, are robust systems for practical realizations of various quantum information processing protocols2–5 and nanoscale magnetometry schemes6,7 at room temperature. Such applications benefit from the high emission efficiency and flux of single photons, which can be achieved by engineering the electromagnetic environment of the emitter. One attractive approach is based on plasmonic resonators8–13, in which sub-wavelength confinement of optical fields can strongly modify the spontaneous emission of a suitably embedded dipole despite having only modest quality factors. Meanwhile, the scalability of solid-state quantum systems critically depends on the ability to control such emitter–cavity interaction in a number of devices arranged in parallel. Here, we demonstrate a method to enhance the radiative emission rate of single nitrogen-vacancy centres in ordered arrays of plasmonic apertures that promises greater scalability over the previously demonstrated bottom-up approaches for the realization of on-chip quantum networks. Efficient single-photon generation and extraction from solid-state quantum emitters is an important problem to be overcome in quantum photonic devices and systems. In the case of devices based on the nitrogen-vacancy (NV) centre in diamond, the photon generation rate and out-coupling efficiency are limited by the relatively long radiative lifetime and total internal reflection (TIR) at the diamond/air interface. Approaches to overcome this issue have therefore been focused on two fronts: enhancement of the radiative decay rate and improvement of the overall collection efficiency. The latter has been realized by direct fabrication of nanowires14 and solid-immersion lenses15–17 in bulk diamond crystals, allowing the collected single-photon count rates to be increased by roughly an order of magnitude. The spontaneous emission rate and intrinsic radiance of the colour centre can be improved by means of the Purcell effect, which can be achieved by coupling diamond nanocrystals18 or nanopillars to evanescent or confined optical fields produced by dielectric19–24 or metallic25–28 nanostructures. Given the maturity of diamond nanofabrication techniques, it has also recently become possible to thin bulk diamond membranes to optically thick slabs on which planar resonators with embedded colour centres could be fabricated29,30. A bottleneck in many of these techniques is the deterministic coupling of single quantum emitters to photonic elements, which is typically challenging and incompatible with large-scale production of devices. For instance, bottom-up approaches involving diamond nanocrystals have predominantly relied on random (drop/spin-casting) or alignment-sensitive (pick-and-place by an atomic force microscope tip) techniques of positioning diamond nanocrystals to achieve emitter–optical field coupling, resulting in one-of-a-kind devices. Although top-down nanofabrication has been shown to provide a reliable and high-throughput means to generate large arrays of devices14,31,32, the demonstrated structures have thus far mainly focused on improving the out-coupling efficiency without modification of the radiative lifetime. In this Letter, we present a high-yield approach to directly embed single NV centres into metallic nanostructures, leading to a reduction in the spontaneous emission lifetime of the enclosed NV centres. Specifically, we consider plasmonic apertures (Fig. 1a) consisting of cylindrical diamond nanoposts (radius, r≈ 50 nm; height, h≈ 180 nm) surrounded by silver. These structures support modes with mode volumes as small as 0.07(l/n) and can provide good spatial overlap between the highly localized optical fields and enclosed dipole due to nearly uniform field distributions in the transverse direction (Fig. 1b,c). This results in enhancement of the spontaneous emission rate of the dipole. The spontaneous emission rate enhancements for our structures were calculated with a three-dimensional finite-difference timedomain (3D FDTD) solver using a classical approach13 by comparing the total power emitted from a dipole when it is placed inside the aperture to the total power emitted in a homogeneous medium. The simulations were performed with measured parameters from ref. 33 and take into account material losses in the silver. The theoretical spontaneous emission rate enhancement spectrum, plotted in Fig. 1d for aperture radii of 50, 55 and 65 nm, exhibits a broad resonance that redshifts with increasing radius and can therefore be tailored to overlap with the NV emission while keeping the height of the structure constant13. Based on our simulations, enhancements of the spontaneous emission rate on the order of 30 can be expected for a radially polarized NV centre placed at the maximum field intensity in an optimized structure. The hybrid diamond–metal device depicted in Fig. 1a was realized using a combination of blanket ion implantation and topdown nanofabrication techniques14,31,32 (Fig. 2a). Two ultrapure bulk diamond crystals (type IIa, Element 6) were used. The samples were implanted with nitrogen ions and subsequently annealed to generate a layer of NV centres roughly 20 nm and 90 nm below the diamond surface. Arrays of diamond nanoposts with radii from 50 nm to 70 nm and height of 180 nm were then fabricated using electron-beam lithography followed by inductively coupled plasma reactive ion etching (ICP RIE) (Fig. 2b). We have previously shown32 that such a procedure can result in a high yield (.10%) of single-centre devices. The nanoposts were finally embedded in a 500-nm-thick silver film that was deposited by electron-beam evaporation. The thickness of the silver layer was chosen to ensure that the diamond nanoposts were fully covered and to minimize oxidation of silver at the device layer.