Color centers in diamond are promising spin qubits for quantum computing and quantum networking. In photon-mediated entanglement distribution schemes, the efficiency of the optical interface ultimately determines the scalability of such systems. Nano-scale optical cavities coupled to emitters constitute a robust spin-photon interface that can increase spontaneous emission rates and photon extraction efficiencies. In this work, we introduce the fabrication of 2D photonic crystal slab nanocavities with high quality factors and cubic wavelength mode volumes—directly in bulk diamond. This planar platform offers scalability and considerably expands the toolkit for classical and quantum nanophotonics in diamond.

Defect centers in diamond have emerged as leading atom-like systems for quantum information processing.1,2 Recent demonstrations of spin-photon entanglement with the diamond nitrogen vacancy (NV) center have enabled important steps towards quantum networks, including remote NV-NV spin entanglement3 and entanglement purification of two quantum nodes.4 However, these processes remain inefficient because of poor photon collection and largely incoherent NV optical transitions, as the zero-phonon line (ZPL) constitutes less than 4% of the NV's spontaneous emission naturally. A variety of methods have been investigated to improve the NV's ZPL collection efficiency. Solid immersion lenses,5–7 dielectric antennas,8,9 waveguides10–13 and parabolic reflectors14 address the problem of collection efficiency but do not significantly improve the spontaneous emission fraction into the ZPL. Recently, the negatively charged silicon-vacancy (SiV) center has gained attention as another promising spin qubit system15 with a more favorable ZPL emission fraction (Debye-Waller factor) of 0.7 and long-term spectral stability.16 However, the overall radiative quantum efficiency of SiV is rather low at 10%–30%,17,18 which also reduces the entanglement rate.

Photonic nanocavities can improve the efficiency of spin-photon interfaces through spontaneous emission rate modification of cavity-resonant transitions.19 In particular, the overall emission rate and fraction into the ZPL increase with the cavity Purcell factor in proportion to the quality factor (Q) to mode volume ( V m ) ratio. A variety of approaches have resulted in high- Q / V m diamond-based 1D photonic crystal (PhC) nanocavities20–23 coupled to isolated NV (Ref. 22) and SiV (Ref. 18) centers. Compared to 1D PhC cavities, 2D PhC slab cavities can offer further improvements in the overall Purcell factor or cooperativity, as the 2D bandgap can inhibit unwanted SE channels out of the cavity through the reduction in the optical density of states.24–27 However, due to the absence of high quality single-crystal diamond films, current fabrication methods of PhC slabs require laborious thinning of bulk diamond films to wavelength-thickness films.17,28–30 This fabrication process involves extensive reactive ion etching (RIE) processing along with complex micromanipulation, resulting in low yield of high quality devices.30 Ion-slicing approaches31 present another alternative but entail crystal damage that can be mitigated only via overgrowth techniques.32–34 

Here, we demonstrate a process for fabricating 2D PhC slabs directly from bulk diamond. This technique, which extends our recently demonstrated fabrication of diamond 1D PhC nanocavities based on plasma quasi-isotropic undercutting of diamond,23 enables the complete release and suspension of PhC slabs on the bulk diamond surface. As with the 1D PhC cavity fabrication, which achieved record-high Q factors near the NV ZPL, this process also produces high-Q cavities and fabrication consistent across a standard, bulk diamond chip.

As summarized in Fig. 1, the fabrication process combines standard lithography with sidewall masking by atomic layer deposition (ALD) and quasi-isotropic dry etching. The process begins with RIE of bulk single-crystal diamond [3 mm × 3 mm × 0.3 mm, (100) surface Type IIa CVD diamond with (N) <1 ppm from Element Six] using an inductively coupled oxygen plasma (ICP = 500 W, RF = 240 W, T = 32 °C, and P = 0.15 Pa). This step uses a hard mask composed of a 230 nm-thick silicon nitride (SiN) layer, produced by plasma-enhanced chemical vapor deposition, patterned by electron-beam lithography (using ZEP-520A resist coated with the Espacer conductive polymer, developed at 0 °C), and etched with tetrafluoromethane (CF4) plasma (RF = 200 W). The optimized ICP-RIE parameters provide a 30:1 etch selectivity of diamond and near-vertical sidewalls. As sketched in Fig. 1(a), this directional etch is ∼7× deeper than the desired PC slab thickness to facilitate release and precise tuning of the slab thickness in later steps.

FIG. 1.

(a) Electron-beam lithography and oxygen plasma reactive-ion etching (RIE) of diamond (grey) using a SiN hard mask (blue). (b) Atomic layer deposition (ALD) of alumina (black) for conformal protection. (c) Break-through etch of alumina using tetrafluoromethane. (d) RIE of diamond. (e) Quasi-isotropic undercut of diamond using oxygen plasma. (f) Mask removal using hydrofluoric acid. (g)–(i) Scanning electron micrograph (SEM) of 2D photonic crystals suspended in air.

FIG. 1.

(a) Electron-beam lithography and oxygen plasma reactive-ion etching (RIE) of diamond (grey) using a SiN hard mask (blue). (b) Atomic layer deposition (ALD) of alumina (black) for conformal protection. (c) Break-through etch of alumina using tetrafluoromethane. (d) RIE of diamond. (e) Quasi-isotropic undercut of diamond using oxygen plasma. (f) Mask removal using hydrofluoric acid. (g)–(i) Scanning electron micrograph (SEM) of 2D photonic crystals suspended in air.

Close modal

Next, ALD of aluminum oxide (Al2O3) provides conformal coverage of the chip, including the sidewalls and PC holes [Fig. 1(b)]. A CF4 RIE step then selectively removes the top layer of Al2O3, which exposes the top facets of the chip [Fig. 1(c)]. At this stage, the PhC region remains protected by the SiN mask and the sidewall by Al2O3 coating. Diamond is exposed only in the bottom of etched patterns.

The diamond undercut relies on a quasi-isotropic oxygen plasma in an ICP-RIE chamber (ICP = 900 W, RF = 0 W, T = 200 °C, and P = 3 Pa), as illustrated in Fig. 1(e). It is possible to image the diamond through the thin Al2O3 sidewall coatings to periodically check and control the extent of the undercut with high precision. We find that an optional directional ICP-RIE [Fig. 1(e)] prior to the isotropic step reduces the total time to ∼8 hours to fully suspend ∼4 μm-wide, ∼200 nm-thick planar PhC slabs. Finally, a hydrofluoric acid wet etch removes the residual SiN and Al2O3 to reveal the air-clad diamond PhC devices [Fig. 1(f)]. Figures 1(g)–1(i) show various scanning electron micrographs of resulting diamond PhCs.

We fabricated two types of PhC cavity designs: L3-defect cavities and photonic heterostructure (HS) waveguide cavities35,36 optimized for a fundamental mode at λ = 637 nm. The former design consists of a line of three missing holes in an otherwise regular triangular-lattice PhC slab. The three adjacent holes on either side of the cavity region are slightly displaced outwards by { D 1 , D 2 , D 3 } = { 0.219 a , 0.025 a , 0.2 a } , where a = 214 nm is the lattice constant, to minimize out-of-plane scattering. The PhC radius and the slab height are { r , H } = { 0.285 a , a } . Figure 2(a) shows the electric field density of the fundamental TE-like cavity mode with the quality factor Q = 8560 and mode volume V m = 0.76 ( λ / n ) 3 , calculated using the Finite-Difference Time-Domain method (Lumerical Inc). The second design, shown in Fig. 2(b), is a line-defect PhC waveguide cavity with parameters { W , r , H } = { 1.69 a , 0.275 a , 0.96 a } with increasing lattice constants from a = 210 nm to a 1 = 1.025 a and a 2 = 1.05 a at the center of the cavity. The HS cavity forms over four periods of a1 and a single period of a2, which maximizes the Q/V ratio,36 where Q = 250000 and V m = 1.28 ( λ / n ) 3 in simulation. Figures 2(c)–2(f) show the corresponding scanning electron micrographs of the fabricated cavities.

FIG. 2.

Normalized electric field density of (a) a modified L3-defect cavity with a lattice constant of a = 214 nm and (b) a photonic heterostructure (HS) cavity with lattice constants of a = 210 nm, a 1 = 1.025 a , and a 2 = 1.05 a . (c) and (e) SEM images of fabricated L3 cavities. (d) and (f) SEM images of fabricated HS cavities.

FIG. 2.

Normalized electric field density of (a) a modified L3-defect cavity with a lattice constant of a = 214 nm and (b) a photonic heterostructure (HS) cavity with lattice constants of a = 210 nm, a 1 = 1.025 a , and a 2 = 1.05 a . (c) and (e) SEM images of fabricated L3 cavities. (d) and (f) SEM images of fabricated HS cavities.

Close modal

The fabricated cavities were characterized by photoluminescence (PL) confocal microscopy. Ensembles of native NV centers present in the diamond crystal serve as internal light sources for the cavities upon 532 nm laser illumination. A spectrometer records the cavity-modified PL, from which we deduce the quality factors. These measurements reveal high-Q resonances near the NV zero-phonon line for both nanocavity designs, with the highest Q L 3 = 6080 and Q HS = 2670 , shown in Figs. 3(a) and 3(b), respectively. The experimental Q L 3 is close to the simulated Q = 8560, which indicates that intrinsic cavity radiative losses limit the measured Q. However, the HS nanocavity Q factor is considerably lower than the simulated Q due to fabrication imperfections. By directly modeling the device with FDTD simulations based on SEM images, we find a reduction to Q 7500 . We attribute the remaining discrepancy to two possible sources. First, the slab cross-section may not be perfectly rectangular but may be wedged. FDTD simulations confirm that a wedge angle of 3° to 5° would further reduce Q to ∼3000. Second, non-vertical holes are known to reduce the Q factors as well.37 We believe that with continued improvement in fabrication, the Q factor will increase.

FIG. 3.

(a) L3 cavity resonance with Q = 6080 in the photoluminescence spectrum of NV centers. Photonic HS cavity resonance with Q = 2670. (c) Relationship between the PhC device size (s), the undercut trench size (w), and cavity resonance.

FIG. 3.

(a) L3 cavity resonance with Q = 6080 in the photoluminescence spectrum of NV centers. Photonic HS cavity resonance with Q = 2670. (c) Relationship between the PhC device size (s), the undercut trench size (w), and cavity resonance.

Close modal

As this fabrication technology opens opportunities to various planar photonic architectures in diamond, an important question is the compatibility of this technique with the fabrication of multiple photonic components with dissimilar lateral dimensions. Towards this end, we realized PhC devices with different lateral sizes on the same chip in one fabrication run. As summarized in Fig. 3(c), the simultaneous fabrication of HS PhCs with different lateral sizes is possible by tuning their undercut trench size. The trench width, w, controls the area of exposed diamond surfaces, thereby providing a knob to locally tune the undercut etch rates and hence the slab thickness, for each device. A larger trench around the PhC blueshifts the cavity resonance, as shown in Fig. 3(c). Further, we adjusted w to accommodate narrow (s = 3.3 μm) and broader (s = 4.1 μm) devices in the same fabrication run, as seen in the figure. Notably, the Q factors are highest when the cavity resonance is nearest the targeted wavelength of 637 nm.

The fabrication of high quality PhC slab nanocavities directly from bulk diamond significantly expands the potential for controlling light-matter interactions in diamond. Immediate experiments include NV-cavity resonant coupling, which is important to enhance the rates in photon-mediated quantum entanglement schemes. In the 2D PhC cavity modes, the electric field energy density is maximized in the dielectric (as these modes are produced from the ‘air-band’ of the Bloch vectors), which allows for excellent overlap between the emitter and the cavity mode. In addition, the PhC slab design allows the emitter to be relatively far from the etched surfaces, which improves the emitters' spectral stability.38,39 The PhC slab architecture greatly expands the types of devices that can be fabricated, including photonic crystal waveguides27,40,41 which are of interest for investigating waveguide quantum electrodynamics and many-body physics with color centers. Other prospects that are now possible also include phononic and optomechanical crystals to extend the spin coherence of the SiV center.16 Finally, the ability to fabricate planar PhC architectures on bulk dielectrics provides an excellent platform for compact nanophotonic device integration, and it should open up new possibilities in other fields, such as spectroscopy or classical light sources and modulators.

N.H.W. was supported in part by the Army Research Laboratory Center for Distributed Quantum Information (CDQI) and Master Dynamic Limited. S.M. was supported by the NSF program ACQUIRE: “Scalable Quantum Communications with Error-Corrected Semiconductor Qubits.” Fabrication was supported in part by the STC Center for Integrated Quantum Materials (CIQM), NSF Grant No. DMR-1231319. We thank James Daley, Mark Mondol, and Tim Savas at the Nanostructures Laboratory at MIT for invaluable discussions and support during the development of this fabrication technique.

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