# Fileset

[s41467-018-05117-4.pdf](https://mdr.nims.go.jp/filesets/b08965cb-333d-49ab-88a9-b6b2dfe84a2b/download)

## Creator

Sejeong Kim, Johannes E. Fröch, Joe Christian, Marcus Straw, James Bishop, Daniel Totonjian, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Milos Toth, Igor Aharonovich

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Photonic crystal cavities from hexagonal boron nitride](https://mdr.nims.go.jp/datasets/757d6df3-dd92-4d42-a182-09f058fd47e0)

## Fulltext

Photonic crystal cavities from hexagonal boron nitrideARTICLEPhotonic crystal cavities from hexagonal boronnitrideSejeong Kim1, Johannes E. Fröch1, Joe Christian2, Marcus Straw2, James Bishop1, Daniel Totonjian 1,Kenji Watanabe 3, Takashi Taniguchi3, Milos Toth 1 & Igor Aharonovich1Development of scalable quantum photonic technologies requires on-chip integration ofphotonic components. Recently, hexagonal boron nitride (hBN) has emerged as a promisingplatform, following reports of hyperbolic phonon-polaritons and optically stable, ultra-brightquantum emitters. However, exploitation of hBN in scalable, on-chip nanophotonic circuitsand cavity quantum electrodynamics (QED) experiments requires robust techniques for thefabrication of high-quality optical resonators. In this letter, we design and engineer suspendedphotonic crystal cavities from hBN and demonstrate quality (Q) factors in excess of 2000.Subsequently, we show deterministic, iterative tuning of individual cavities by direct-writeEBIE without significant degradation of the Q-factor. The demonstration of tunable cavitiesmade from hBN is an unprecedented advance in nanophotonics based on van der Waalsmaterials. Our results and hBN processing methods open up promising avenues for solid-state systems with applications in integrated quantum photonics, polaritonics and cavity QEDexperiments.DOI: 10.1038/s41467-018-05117-4 OPEN1 Faculty of Science, Institute of Biomedical Materials and Devices (IBMD), University of Technology Sydney, Ultimo, NSW 2007, Australia. 2 Thermo FisherScientific, 5350 NE Dawson Creek Drive, Hillsboro, OR 97214-5793, USA. 3 National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044,Japan. These authors contributed equally: Sejeong Kim, Johannes E. Fröch. Correspondence and requests for materials should be addressed toS.K. (email: Sejeong.Kim-1@uts.edu.au) or to M.T. (email: milos.toth@uts.edu.au) or to I.A. (email: igor.aharonovich@uts.edu.au)NATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunications 11234567890():,;http://orcid.org/0000-0002-9412-9416http://orcid.org/0000-0002-9412-9416http://orcid.org/0000-0002-9412-9416http://orcid.org/0000-0002-9412-9416http://orcid.org/0000-0002-9412-9416http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-1564-4899http://orcid.org/0000-0003-1564-4899http://orcid.org/0000-0003-1564-4899http://orcid.org/0000-0003-1564-4899http://orcid.org/0000-0003-1564-4899mailto:a4.3dmailto:a4.3dmailto:a4.3dwww.nature.com/naturecommunicationswww.nature.com/naturecommunicationsControlling and manipulating light at the nanoscale isimportant for a vast variety of applications that includesensing, quantum information processing, secure com-munications and cavity QED experiments1–11. Key componentsfor most of these applications include optical resonators such asphotonic crystal cavities, and non-classical light sources that canemit single photons on demand such as color centers in solids12,defects in carbon nanotubes (CNTs)13, single molecules14 andquantum dots15. Remarkable progress has been achieved overrecent years to realize on-chip integrated quantum photoniccircuits that employ various combinations of these systems. Forexample, in a monolithic approach where nanophotonic elementsand a quantum light source are embedded in the same material,coupled systems have been implemented using diamond5,11, rareearth crystals4, and gallium arsenide16. Alternatively, hybridsystems, where an external source is positioned in close proximityto the photonic element made from a foreign material, have beenassembled from a broad range of materials such as CNTs17 andInAsP quantum dots (QDs)18,19 integrated with silicon nitridecomponents.In recent years, layered van der Waals materials have emergedas promising hosts of ultra-bright quantum emitters20. Integra-tion of these light sources with dielectric and metallic waveguideshas been achieved by placing flakes of the van der Waals hosts ontop of the waveguides21–23. However, in such a hybrid approach,the emitter couples only to the evanescent field of the cavitymode. Hence, spatial matching between the emitter and theelectric field maximum is limited, and scattering losses areincreased. A monolithic system, in which the photonic resonatorhosts the quantum emitter is required for ideal on-chip devices.Here, we design and fabricate optical cavities from hBN – awide bandgap, hyperbolic van der Waals material24 that hasrecently attracted considerable attention as a promising host ofultra-bright, room-temperature quantum emitters20,25–29. Inaddition, hBN is naturally hyperbolic and exhibits volume-confined phonon polariton modes30 that open up opportunitiesto study light–matter interaction in the deep subwavelengthregime and advance infrared and terahertz nanophotonics30,31. Itis therefore important to engineer nanostructures that enablecontrol over a broad spectral range that accommodates bothvisible on-chip nanophotonics and integrated polaritonics.Finally, the layered nature of hBN allows flakes to be easilyrelocated and combined with other platforms. This, together witha high degree of inherent chemical inertness, makes hBN resilientto a broad range of environments such as liquids, and henceapplications in sensing where the relatively low refractive index ofhBN is beneficial as it leads to large evanescent fields that enhancesensing efficiency.ResultsFabrication hBN photonic crystals. As a first step towardapplications, we demonstrate nanofabrication of two-dimensional(2D) and one-dimensional (1D) photonic crystal cavities (PCCs)with optical Q-factors of up to 2100. Such fabrication protocolsare needed because, in contrast to mature semiconductors,techniques for making high Q-factor optical devices from van derWaals layered materials that operate in the visible wavelengthrange are not yet fully developed. For example, it was unknownwhether 2D stacked layers are sufficiently robust to endurenanofabrication processes without being destroyed. Also, under-cutting techniques that are used to achieve suspended structuresand bulk angle etch processes have not been applied successfullyto these materials. This is particularly significant because of thesmall refractive index of hBN of ~ 1.8 (at λ= 600 nm) makes ithard to achieve a high refractive index contrast that is needed forefficient light confinement in the visible spectral range. In thiswork, we resolve these challenges by demonstrating the fabrica-tion and iterative editing/tuning of suspended photonic cavitiesfrom the van der Waals material hBN using a combination ofhBN exfoliation onto a trenched substrate, reactive ion etching(RIE) and single-step, direct-write electron beam induced che-mical etching32.As a first step toward the nanofabrication of photonic cavities,we prepared hBN flakes by scotch tape exfoliation from bulkcrystals, which were synthesized in a high pressure–hightemperature process that yields carbon and oxygen impurityconcentrations below 1018 cm−3 33. Examples of exfoliated hBNfilms on a bulk substrate, as shown in Fig. 1a, reveal a sufficientlateral size for fabrication of devices on the micrometer scale, asfilms appear smooth over several microns with low amounts ofgrain boundaries, while the different thicknesses of hBN stacksare apparent by their different colors. Due to the low refractiveindex of hBN, substrates cause severe optical losses. We thereforeemployed a substrate with pre-patterned trenches so that opticalcavities can be fabricated in the suspended regions, asschematically depicted for the case of a 2D photonic crystal inFig. 1b. Figure 1c is a scanning electron microscope (SEM) imageof the suspended hBN showing the layered nature of hBN. Layersare held together by van der Waals forces and each 2D monolayerconsists of alternating boron and nitrogen atoms arranged in ahexagonal pattern. The thicknesses of hBN flakes vary from amonolayer to a few hundred nanometer and these flakes appearsmooth and flat, as is shown for several examples in theSupplementary Note 1. From exfoliated hBN flakes, only thosewith a thickness in the range of 200–500 nm were considered forfabrication. The lower limit was chosen to provide a sufficient Q-factor, as thinner films will not confine light sufficiently, whereasthe upper limit is selected to allow for only a single guided modein the out-of-plane direction, as generally preferred for photoniccavities.The first resonators that we show are 2D PCCs fabricated usinga nanofabrication procedure that combines RIE with EBIE.Briefly, a tungsten layer that serves as a hard mask is deposited ontop of hBN, followed by a thin layer of e-beam resist (polymethylmethacrylate (PMMA)). Using electron beam lithography, the 2DPCC pattern is written in the PMMA film followed by an RIE stepin SF6 gas, which transfers the 2D PCC pattern into the tungstenmask. The underlying hBN is then etched by material-selectiveEBIE using water vapor as a precursor gas34, which results innearly-straight sidewalls. Moreover, EBIE is a chemical processwhich does not rely on physical removal of atoms through knock-on processes that commonly occur in RIE and FIB techniques andcause substantial damage in the host crystal32. As a result, nopostprocessing (via annealing or wet-chemical treatment) isnecessary after EBIE in order to observe and subsequently tunecavity modes, as is shown below. The EBIE process does notinvolve heavy ions (e.g., Ga) hence re-sputtering of ions and re-deposition of the etched material are absent. The processing stepsand a comparison with other etching methods are explainedfurther in the Supplementary Notes 2 and 3, respectively.Figure 1d shows a side view, false-color SEM image of the 2DPCC fabricated in a suspended flake of hBN with nine holesmissing in the center (L9 cavity). A top-view SEM image of thesame cavity is shown in Fig. 1e. The geometries of thePCCs presented here were designed to have resonances in thevisible spectral range35, where hBN quantum emitters aretypically observed. To achieve the desired resonances, we used alattice constant (a) in the range of 240–300 nm, with anair hole radius in the photonic mirror region of 0.33a and twoair holes at the end of the line defects with 0.22a, shifted outwardsby 0.22a.ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-42 NATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsThe fabricated cavities were analyzed optically using a confocalmicroscope setup at room temperature. A broadband hBNbackground emission is excited by a 532 nm continuous wavelaser, and an objective lens with a numerical aperture of 0.9 isused for excitation and collection. A comparison of roomtemperature photoluminescence (PL) spectra measured byexcitation of the cavity center with line defects (red) comparedto excitation of an adjacent periodically patterned PCC area(gray) is shown in Fig. 1f. Excitation of the cavity yields an opticalmode at 586.6 nm, whilst only a broad PL emission is observedwhen the laser spot is off the cavity. The electric field intensityprofile of the measured mode is depicted in the inset, calculatedusing the 3D finite difference time domain (FDTD) method. Byusing a Lorentzian fit, a Q-factor of 160 is obtained. The observedQ-factor is relatively low, yet there are no other studies on 2DPCCs with similar or lower refractive index materials in thevisible wavelength range. We note that, in practice, it is verychallenging to open a photonic bandgap using a low refractiveindex material and a 2D cavity geometry.1D Photonic crystal nanobeam. To achieve high Q cavities fromlow refractive index materials, it is more favorable to fabricate 1Dladder-type PCCs. Figure 2a shows representative SEM images ofa 1D ladder PCC (beam width w= 750 nm) fabricated using thesame combination of RIE and EBIE that was used to fabricate thecavity in Fig. 1. The ladder contains 25 uniform-sized, rectangularair holes (hx= 150 nm, hy= 550 nm) with a lattice constant a of250 nm (shown in Fig. 2b). The lattice constants of 19 air holesare modulated to form a photonic well at the center of the ladderstructure with decreased lattice constants in the cavity regioncompared with the mirror region.Detailed information on the cavity design and the formation ofthe depicted modes from the photonic band is given inSupplementary Note 4. The designs must accommodate a rangeof mode wavelengths in order to enable applications such assensing and integrated photonic devices. This is particularlyimportant in the case of hBN since quantum emitters in thismaterial possess a relatively wide range of emission wave-lengths36, thus necessitating the fabrication of 1D PCCs withmodes spanning ~576–762 nm. Figure 2c shows room tempera-ture PL spectra acquired from three 1D ladder PCCs with latticeconstants of 220 nm (black), 250 nm (red), and 280 nm (blue),and resonances that span 600–750 nm, and thus cover most ofthis range. The three cavities were fabricated in the same hBNflake with a thickness of 280 nm. The 1D ladder cavity design hasthree confined resonant modes from the lowest energy photonicband as is observed in the PL spectrum obtained from the cavitywith a= 220 nm (Fig. 2c). These three modes are in goodagreement with a spectrum obtained from FDTD simulation (seeSupplementary Note 4). The zeroth order mode, also called thefundamental mode, from the first dielectric band (F0) is observedat 612.8 nm, while first order (F1) and second order (F2) modesare observed at 633.9 and 655.5 nm, respectively. Here, “F”designates that the modes originate from the “First” dielectricband, and the numbers 0, 1, and 2 represent the number of nodesalong the x-axis. All of these modes are both TE modes and thedielectric modes, for which the electric field maximum is in thehigher dielectric region (i.e., within the suspended hBN beams).Cavities with larger lattice constants have optical modes at longerwavelengths. For example, the measured F0 mode is at 678.4 nmfor a= 250 and 756.1 nm for a= 280 nm (Fig. 2c). Higher energymodes, in this case the zeroth order mode from the seconddielectric band (S0), which are of interest for sensing applicationsdue to their extended evanescent fields37, are also observed forcavities with a= 250 and 280 nm. The electric field profiles of allobserved modes, determined by 3D FDTD simulations, aredepicted in Fig. 2d.We fabricated and analyzed a number of cavities with a rangeof lattice constants. The Q-factor as a function of wavelength forboth the first dielectric (F0, red circle) and the second dielectric(S0, black triangle) modes are summarized in Fig. 2e, includingthe cavity modes shown in Fig. 2c. Here among F0, F1, and F2, we570 580 590hBNWavelength (nm)PL intensity (a.u.)600 610 620MaxhBNSiMinOff cavityza b cfedxyOn cavityFig. 1 Free-standing hexagonal boron nitride 2D photonic crystal cavities. a Optical microscope image of exfoliated hBN crystals on a silicon substrate. Thescale bar corresponds to 50 µm. b Schematic of a free-standing hBN cavity on a trenched silicon substrate. c SEM image of the hBN showing the layeredstructure. The scale bar corresponds to 500 nm. d false color SEM image (45°) of a free-standing hBN photonic crystal cavity fabricated using acombination of RIE and EBIE. The scale bar corresponds to 2 µm. e Top view of a 2D photonic crystal cavity. The scale bar corresponds to 2 µm. fPhotoluminescence spectra with a laser exciting the cavity mode (red) compared with an off-cavity excitation (gray). The inset depicts the electric fieldintensity profile of the fundamental mode for the cavity calculated using 3D FDTDNATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-4 ARTICLENATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunications 3www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsonly included F0 because modes with a lower number of nodeshave higher Q-factors. The highest Q-factor measured from thesecavities is 1700. Theoretical Q for a 1D photonic crystal cavityobtained using the 3D FDTD method for the measuredparameters a= 250 nm, w= 750 nm, hx= 150 nm and hy= 550nm corresponds to 9000 with a resonant wavelength of 662.8 nm.The experimental Q is lower than the theoretical value mainly dueto the roughness of the sidewalls seen in Fig. 2a. We also note thathigher Q-factors are expected from cavities with resonances atlonger wavelengths because scattering caused by surface rough-ness is reduced. It is important to note that this is the first reportof such experimental Q values in a resonator made entirely from alayered van der Waals material.To further investigate potential techniques for the fabricationof cavities from suspended hBN, we employed a focused ionbeam. While FIB is an attractive option for direct milling of somematerials, it causes damage within the crystal that must berecovered by annealing. However, the effectiveness of annealingtreatments varies substantially for different materials. Forexample, diamond cavities fabricated using a FIB have very lowQ (<800)38 while yttrium orthosilicate (YSO) crystals show higherQ-factors of several thousands4. Figure 2f shows the PL spectrumof a cavity that was fabricated by FIB milling, and annealed (900°C, Vacuum, 2 h) to recover FIB-induced damage. Additionaldetails on fabrication of cavities by FIB milling is given in theMethods section. Modes are observed at 634.2 nm (F0), 659.6 nm(F1), and 682.0 nm (F2), with a Q-factor as high as 2100 for theF0 mode. We note that no modes were observed immediatelyafter FIB fabrication, and the annealing step is required to removeresidual crystal damage. The inset is an SEM image of the cavity.A detailed comparison of cavities fabricated by both methods isgiven in the Supplementary Note 5 and 6, which shows that forEBIE-fabricated cavities sidewalls within the air holes appearmore vertical but rougher, while FIB-fabricated cavities showsmooth sidewalls that are slanted. However, Raman analysisshows no evidence of damage directly after fabrication by EBIE,whilst residual damage is evident after FIB milling even after theannealing treatment that is required to observe optical modes.Hence, at present, the limitations and variations in Q-factor ofcavities fabricated by EBIE and FIB are determined by surfaceroughness and crystal damage, respectively. The fact that EBIEprocessing does not require annealing is highly favorable forsubsequent editing and iterative tuning of the cavities, as isdiscussed below. Moreover, this method can be potentiallyexpanded to be totally mask-free to directly fabricate devices asshown in Supplementary Note 7.The obtained cavities from hBN are on par, yet lower thanother bulk dielectric semiconductors that have much higherrefractive indices in the visible wavelength range, as issummarized in the Supplementary Note 8. Here, we focus on180015001200900600300Q-factorPL intensity (a.u.)PL intensity (a.u.)600 650 700Wavelength (nm) Wavelength (nm)750 800600wyxahxhyF0cabe fdF0F0|E |2< F0 >< F1 >< F2 >< S0 >S0S0F1F1F2650 700Wavelength (nm)750Q~2100S0F0�=220 nm�=250 nm�=280 nmMin Max600 620 640 660 680 700Fig. 2 Optical analysis of one dimensional (1D) hBN photonic crystal cavities. a SEM image of a 1D ladder PCC with 25 rectangular air holes. The scale bar is1 µm. b magnified view showing the geometrical parameters; width (w), lattice constant (a), air hole width (hx) and air hole height (hy). The cavity wasfabricated using a combination of RIE and EBIE. The scale bar corresponds to 200 nm. c Photoluminescence spectra of different 1D ladder PCCs in the samehBN crystal with varying lattice constants of 220 nm (black), 250 nm (red), and 280 nm (blue), respectively. F0, F1, and F2 mark the position of the zeroth,first, and second order mode from the first dielectric band. S0 marks the position of the zeroth order from the second dielectric band mode. d 3D FDTDsimulation result showing field profiles of the measured optical modes. e Experimentally obtained Q-factors of various 1D PCCs fabricated from hBN. f PLspectrum of a 1D cavity fabricated by focused ion beam milling, showing a high Q (~ 2100) mode in the visible spectral range. The inset is an SEM image ofthe cavity and the scale bar corresponds to 1 µmARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-44 NATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunicationswww.nature.com/naturecommunicationscomparison with materials that are known to host quantumemitters in the visible range and have the potential for monolithicintegration of emitter – cavity systems. These attributes makethem particularly appealing to quantum photonic applications.Cavities fabricated using such materials have been shown to havethe following Q-factors: YSO – Q ~3000 at λ= 596 nm4, GaN – Q~5200 at λ= 461 nm39, 4H SiC – 6700 at λ= 700 nm40 anddiamond – Q ~11,000 at λ= 734 nm41.The Q-factors obtained from our hBN cavities are indeedpromising for quantum photonic applications. Purcell enhance-ment of quantum emitters that are located optimally in the cavityfield maximum is given by Eq. 1, where λ is the cavity resonancewavelength in free space, n is the refractive index at field antinode, and V is the mode volume. Using the ladder cavity, with Q~2100, a Purcell enhancement of ~110 can be achieved.Fp ¼34π2λn� �3QVð1ÞTunable photonic cavity. Next, we demonstrate that the opticalmodes of the fabricated cavities can be tuned deterministically byEBIE. Tuning is essential to match the resonances to the emissionwavelengths of quantum emitters, particularly in the case of hBNwhere the emission wavelength varies substantially and controlledfabrication of emitters with a given wavelength is yet to bedemonstrated. In dielectric cavities, the resonant mode stronglydepends on cavity parameters such as the width and thickness.Therefore, numerous methods exist to tune cavity modes, such asthinning via oxidation or RIE, deposition of thin dielectric layers,or gas condensation. In this study, we demonstrate the use ofEBIE to controllably tune individual cavities to shorter wave-lengths without significant degradation of the Q-factor. As isshown schematically in Fig. 3a, the focused electron beam isguided along the outer sidewalls of a cavity, selectively etchingthese regions and reducing the width of the beam. In Fig. 3b thePL spectrum of a cavity is shown before tuning (black) withmodes F0, F1, and F2 at wavelengths of 745.2, 771.7, and 797.8nm respectively. A reduced width of the 1D cavity by ~10 nmresults in a lower effective refractive index for the optical modes,which resulted in a blue-shift of the cavity resonance by 2.4 nm(red) after the first tuning step. The same process was repeatedagain to demonstrate further tuning of an additional 4.2 nm(blue). It can be seen (Fig. 3c) that overall the Q-factor of thefundamental mode did not degrade significantly (a change of lessthan 10%) while that of the first-order mode increased in bothsteps, and the second-order mode showed a mixed behavior aftertuning. We attribute the different trends in Q-factor change todifferent positions of the field intensity maxima along the beam,and the fact that the EBIE process used for tuning alters theroughness profile of the sidewall (i.e., the roughness of locallysmooth regions and those containing asperities can decrease/increase, respectively, during tuning). However, the repeatabletuning by EBIE can be seen as a reliable, robust method fortuning of PCC optical modes over a relatively wide spectral range.It does not cause substantial damage to the cavity and does notrequire subsequent RIE or deposition steps. The EBIE approach istherefore attractive for two reasons. First, being a localized, direct-write technique, it can be applied to individual cavities withoutaffecting the modes or Q-factors of neighboring structures on thesame chip. This is very challenging using deposition or RIEthinning techniques as they require additional masking steps andthe low refractive index of hBN necessitates the use of suspendedstructures to realize high Q factors. Second, EBIE does not requireany postprocessing such as annealing that is needed to removeFIB-induced damage, or wet-etching used to etch regions pro-cessed by laser oxidation. As a result, the EBIE process can beapplied iteratively to individual cavities on a single chip until thedesired tuning is achieved.Creation of quantum emitters. Finally, we discuss the potentialof integrating quantum emitters hosted by hBN with the fabri-cated PCCs. Coupling of an emitter to an optical mode requiresboth spatial and spectral overlap between the emitter and thecavity mode. The spatial overlap probability can be increased bydeterministically creating single photon emitters in a non-destructive way with high precision. However, unlike the casesof other materials such as diamond and SiC, ion implantation andelectron irradiation have not, to date, been shown to be reliablemethods for the generation of quantum emitters in hBN. Fur-thermore, methods relying on local strain field engineering orlaser irradiation are not applicable in this case, as they will causestrong deformation of the suspended cavity and severe damage inthe material, resulting in reduction of Q-factors. Hence, to createemitters in hBN, we employed an additional annealing step at850 °C after all the fabrication steps, as this method is known togenerate single photon emitters20 (albeit in random locations). Todemonstrate the efficiency of this process, we compare the tworegions shown in Fig. 4a – an unprocessed area on the left and aprocessed area that contains several nanobeams on the right. Thecorresponding PL map is shown in Fig. 4b. In the analyzedregions, we found a total of 13 single photon emitters, whoseposition are marked with yellow circles, all of which are located2.4Electron beama b cH2ONormalized PL intensityQ-factor2.0No tuning1st2ndF0F1 F21.61.20.80.40.0720 740 760Wavelength (nm)780 800No tuning 1st 2nd020040060080010001200Fig. 3 Tuning of a 1D nanobeam cavity using direct-write, maskless EBIE. a Schematic of the etch process in which a focused electron beam (blue) isscanned along the outer sidewalls of the nanobeam cavity to induce the etch reaction in the presence of water molecules. b Photoluminescence (PL)spectra of a 1D photonic cavity before tuning (black), and after the first (red) and second (blue) tuning steps were performed by EBIE. c Q-factor of the F0(circles) and F1 (triangles) modes measured immediately after fabrication (black), and after the first (red), and second (blue) tuning stepsNATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-4 ARTICLENATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunications 5www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsdirectly in the fabricated cavities compared with zero emittersfound in the unpatterned area. The creation of extra emitters inthe cavity-containing regions can be attributed to an increasedsurface area, creation of dangling bonds, activation of passivateddefects, or chemical modification of sites that undergo partialchemical reactions during etching (we note that stable brightemitters were found consistently on the nanobeams only insamples processed by both RIE and EBIE, but not in samplesprocessed only by RIE or only by FIB, irrespective of whether ornot the samples were annealed). Bright spots in the confocal mapwere analyzed spectrally and show clear narrowband zero phononlines (ZPLs). In several cavities, we clearly observed spatialoverlap between cavity modes and emitters, as is shown in Fig. 4c.The top plot (red) includes the ZPL at 710 nm from a color centerand the optical mode of the cavity at 647.7 nm. The emitter iswithin the laser spot, which has a diameter of ~400 nm. Once thelaser is spatially detuned from the emitter (bottom, blue curve),only the cavity mode remains, as expected.To confirm that the peak is indeed a single photon emitter, theemission lines are isolated by spectral filtering and the quantumnature is demonstrated using a Hunbury Brown and Twissinterferometer. The corresponding g2(τ) curve with a zero delaytime of g2(0)=0.26 (measured without background subtraction) isshown in Fig. 4d, thus classifying it as a single photon source. Afurther discussion on the characteristics of the observed emittersis given in the Supplementary Note 9.We note that to demonstrate coupling between emitter and theoptical mode, both spatial and spectral matching are required.However, despite the fact that the probability of spatial matchingwas increased by the processing steps used to fabricate the cavities(Fig. 4b), spectral matching was not observed because the emittersdisappeared after an EBIE tuning step. This most likely happenedbecause of the removal of material from the cavity edges duringtuning, where the emitter was physically located. At this stage ithas been difficult to find an emitter located in the middle of thedielectric mode of the cavity. Nevertheless, our method of tuningthe cavity resonances, along with recent progress in fabrication ofthe emitters on demand27,42,43 is promising for eventualrealization of coupled monolithic emitter-cavity systems madefrom hBN.For hBN to mature into a reliable platform for integratednanophotonics, an effort into fabrication of membranes with pre-defined thickness is required, with an emphasis on controlledgrowth of large-area multilayers. This need is analogous to thatwhich existed in the field of diamond photonics, which has beenreinvigorated dramatically when growth of high quality singlecrystal material became available. We envision such progress willbe achieved with BN as well. In addition to the efforts being madetoward deterministic growth of BN, further developments intechniques for both deterministic fabrication of quantum emittersin hBN and resonant wavelength tuning will make weak andstrong coupling experiments feasible. To realize coupling betweenemitters and cavities, tuning into resonance will be required. Gascondensation can be used, in principle, for emitters that are in aclose proximity (spectrally) to the cavity mode and overlapspatially with the mode. Alternatively, the hBN cavities can bemounted on stretchable substrates and strain tuning may beemployed. This would change cavity dimensions homogeneouslyin one direction without introducing additional scattering centers.Our present demonstration of hBN PCCs is already highlypromising for applications in integrated on-chip quantumnanophotonics, optomechanics, cavity QED and quantumsensing experiments. Moreover, integration of hBN cavities withother 2D materials, may yield hybrid heterostructures for studiesof light confinement at the nanoscale, and thus position 2D vander Waals systems as a unique platform in the field of integratednanophotonics. Finally, the demonstrated ability to determinis-tically fabricate and tune hBN resonators will further expand its2500Optical modeNon-patterned areaa bdcPatterned area EmittersEmitterOn emittersOff emitters1.41.21.00.80.60.40.20.020001500PL intensity (a.u.)1000640 660 680 700 720 –60 –40 –20 0g2(0) = 0.2620 40 60Delay time (ns)Wavelength (nm)Fig. 4 Generation of single photon emitters within the hBN cavities. a optical microscope image of the analyzed area comparing an unprocessed site (left),with a processed site (right) that contains several 1D nanobeam cavities. The scale bar corresponds to 1 µm. b corresponding Photoluminescence map ofthis region. Positions of quantum emitters are indicated by yellow circles c PL spectra from two regions of the same cavity showing an optical mode only(blue) and the combination of an optical mode and an emitter (red). d Measured g2(τ) curve obtained from this emitterARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-46 NATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsapplicability as a hyperbolic material for explorations of lightconfinement and polaritonics in the mid and near infraredspectral ranges.DiscussionIn summary, we have demonstrated the fabrication of photoniccrystal cavities with Q-factors in excess of 2000 from hBN, apromising van der Waals crystal. We employed both EBIEetching and FIB milling in protocols used to fabricate 1D and 2DPCCs. We also demonstrated a technique to spectrally tune theoptical mode of individual cavities using a maskless EBIE method.Finally, we showed that the cavity-patterned regions host a highdensity of quantum emitters, marking a first step towardmonolithic integration of a hBN emitter–cavity system.MethodsPreparation of suspended hBN. We used trenched silicon substrates in order tomaximize the refractive index contrast in z-direction (normal to the plane of 2DhBN layers). The fabrication process from exfoliation to final cavity for the fab-rication route via EBL-RIE-EBIE is schematically depicted in SupplementaryNote 2. First, high crystalline hBN flakes were mechanically exfoliated from stickytape onto trenched silicon substrates. Tape residuals were removed by calcinationin air for 2 h at 450 °C on a hot plate. Further desorption of contaminants and anincrease of adhesion of flakes to the substrate was achieved by annealing in a tubefurnace (Lindberg/Blue M) with Argon at 850 °C for 30 min.EBIE etching method. A 15 nm tungsten film was deposited in a homebuilt plasmasputter deposition chamber using a growth rate of ~0.8 Å/s. On top of the tungstenlayer, 950 PMMA A3 was spun on to a thickness of ~100 nm. Cavity designs werepatterned via conventional EBL at 20 kV, 40 pA, on a commercial Zeiss Supra55VP SEM with a RAITH E-beam Lithography System. The cavity patterns weretransferred into the tungsten film via RIE (homebuilt) using SF6 flown at a rate of60 sccm and a pressure of 10 mTorr (RF power= 100 watt, self-bias= 200 V).Under these conditions the tungsten film was fully etched in the exposed areaswithin 20 s. The PMMA was removed in an Acetone bath followed by ashing ofresidual cross-linked PMMA in Oxygen plasma. Further cleaning was done inAcetone and IPA. EBIE of hBN was done at 1 kV, 15 nA in a water atmosphere of150 mTorr in a Zeiss Evo LS15. Finally, the tungsten mask was removed usingH2O2 (30%). Although we observe optical modes without further processing steps,the chance of activating/ creating emitters is highly increased after annealing.Therefore, after fabrication the samples were annealed in a tube furnace in Argon,at pressure of 1 Torr, and Argon flow rate of 50 SCCM at 850 °C for 30 min.FIB milling. Cavities fabricated by focused ion beam (FIB) were milled using a Ga+ion beam at 30 kV, using beam current of 32 nA. Water vapor was delivered to theprocessing site through a gas injection nozzle in order to suppress charging arte-facts during FIB milling. In order to remove damaged areas of hBN and removeimplanted Ga ions, the samples were annealed for 2 h in vacuum (1mTorr) in atube furnace (Lindberg/ Blue Minimite) at 900 °C.Optical characterization. Optical measurements were performed in a setup asshown in Supplementary Note 10. Broad band emission in hBN and single emitterswere excited by a continuous-wave 532 nm laser (Gem 532, Laser Quantum Ltd.).The laser beam passed a polarizer and a half waveplate, and was focused using ahigh numerical aperture objective lens (NA= 0.9, Nikon). The acquired signalfrom the sample was collected using the same objective lens, then separated fromexcitation by a dichroic mirror (BrightLine, Semrock). After the dichroic mirror,further spectral filtering was achieved using a tuneable 20 nm bandpass filter. A flipmirror guided the light either to a Hanburry-Brown and Twiss (HBT) Inter-ferometer or a Spectrometer (Acton SpectraPro, Princeton Instrument Inc.). TheHBT setup consists of two avalanche photodiodes (APD, Excelitas Technologies),with a 100 ns delay time induced in one of the APDs connected to a time-correlation card (PicoHarp 300).FDTD simulation. Numerical modeling was performed by using the 3D finite-difference time-domain (FDTD) method (Lumerical Inc). The refractive indices forhBN are nx= ny= 1.84, nz= 1.72. Two dimensional cavities were designed byintroducing line defects in a periodic 2D photonic crystal lattice. One dimensionalcavities were designed by modulating the lattice constant.Data availability. The data that support the findings of this study are availablefrom the corresponding author upon request.Received: 22 February 2018 Accepted: 11 June 2018References1. Majumdar, A., Rundquist, A., Bajcsy, M. & Vučković, J. Cavity quantumelectrodynamics with a single quantum dot coupled to a photonic molecule.Phys. Rev. B 86, 045315 (2012).2. Sekoguchi, H., Takahashi, Y., Asano, T. & Noda, S. Photonic crystalnanocavity with a Q-factor of ~9 million. Opt. Express 22, 916–924(2014).3. Shen, Y. et al. Deep learning with coherent nanophotonic circuits. Nat.Photonics 11, 441 (2017).4. Zhong, T., Kindem, J. M., Miyazono, E. & Faraon, A. Nanophotonic coherentlight–matter interfaces based on rare-earth-doped crystals. Nat. Commun. 6,8206 (2015).5. Faraon, A., Barclay, P. E., Santori, C., Fu, K. M. C. & Beausoleil, R. G.Resonant enhancement of the zero-phonon emission from a colour centre in adiamond cavity. Nat. Photonics 5, 301–305 (2011).6. Mauranyapin, N. P., Madsen, L. S., Taylor, M. A., Waleed, M. & Bowen, W. P.Evanescent single-molecule biosensing with quantum-limited precision. Nat.Photonics 11, 477 (2017).7. Wu, S. et al. Monolayer semiconductor nanocavity lasers with ultralowthresholds. Nature 520, 69–72 (2015).8. Lodahl, P., Mahmoodian, S. & Stobbe, S. Interfacing single photons and singlequantum dots with photonic nanostructures. Rev. Mod. Phys. 87, 347–400(2015).9. O’Brien, J. L., Furusawa, A. & Vučković, J. Photonic quantum technologies.Nat. Photonics 3, 687 (2009).10. Kim, H., Bose, R., Shen, T. C., Solomon, G. S. & Waks, E. A quantum logicgate between a solid-state quantum bit and a photon. Nat. Photonics 7, 373(2013).11. Sipahigil, A. et al. An integrated diamond nanophotonics platform forquantum optical networks. Science 354, 847–850 (2016).12. Aharonovich, I., Englund, D. & Toth, M. Solid-state single-photon emitters.Nat. Photonics 10, 631–641 (2016).13. He, X. et al. Tunable room-temperature single-photon emission at telecomwavelengths from sp3 defects in carbon nanotubes. Nat. Photonics 11, 577(2017).14. Chu, X.-L., Götzinger, S. & Sandoghdar, V. A single molecule as a high-fidelityphoton gun for producing intensity-squeezed light. Nat. Photonics 11, 58(2016).15. Senellart, P., Solomon, G. & White, A. High-performance semiconductorquantum-dot single-photon sources. Nat. Nanotechnol. 12, 1026 (2017).16. Englund, D. et al. Controlling the spontaneous emission rate of singlequantum dots in a two-dimensional photonic crystal. Phys. Rev. Lett. 95,013904 (2005).17. Khasminskaya, S. et al. Fully integrated quantum photonic circuit with anelectrically driven light source. Nat. Photonics 10, 727 (2016).18. Elshaari, A. W. et al. On-chip single photon filtering and multiplexing inhybrid quantum photonic circuits. Nat. Commun. 8, 379 (2017).19. Davanco, M. et al. Heterogeneous integration for on-chip quantumphotonic circuits with single quantum dot devices. Nat. Commun. 8, 889(2017).20. Tran, T. T., Bray, K., Ford, M. J., Toth, M. & Aharonovich, I. Quantumemission from hexagonal boron nitride monolayers. Nat. Nanotechnol. 11,37–41 (2016).21. Tonndorf, P. et al. On-chip waveguide coupling of a layered semiconductorsingle-photon source. Nano Lett. 17, 5446–5451 (2017).22. Cai, T. et al. Coupling emission from single localized defects in two-dimensional semiconductor to surface plasmon polaritons. Nano Lett. 17,6564–6568 (2017).23. Schell, A. W., Takashima, H., Tran, T. T., Aharonovich, I. & Takeuchi, S.Coupling quantum emitters in 2D materials with tapered fibers. ACSPhotonics 4, 761–767 (2017).24. Cassabois, G., Valvin, P. & Gil, B. Hexagonal boron nitride is an indirectbandgap semiconductor. Nat. Photonics 10, 262–266 (2016).25. Shotan, Z. et al. Photoinduced modification of single-photon emitters inhexagonal boron nitride. ACS Photonics 3, 2490–2496 (2016).26. Jungwirth, N. R. et al. Temperature dependence of wavelength selectable zero-phonon emission from single defects in hexagonal boron nitride. Nano Lett.16, 6052–6057 (2016).27. Chejanovsky, N. et al. Structural attributes and photodynamics of visiblespectrum quantum emitters in hexagonal boron nitride. Nano Lett. 16,7037–7045 (2016).28. Bourrellier, R. et al. Bright UV single photon emission at point defects in h-BN. Nano Lett. 16, 4317–4321 (2016).NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-4 ARTICLENATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunications 7www.nature.com/naturecommunicationswww.nature.com/naturecommunications29. Exarhos, A. L., Hopper, D. A., Grote, R. R., Alkauskas, A. & Bassett, L. C.Optical signatures of quantum emitters in suspended hexagonal boron nitride.ACS Nano 11, 3328–3336 (2017).30. Giles, A. J. et al. Ultralow-loss polaritons in isotopically pure boron nitride.Nat. Mater. 17, 134 (2017).31. Caldwell, J. D. et al. Sub-diffractional volume-confined polaritons in thenatural hyperbolic material hexagonal boron nitride. Nat. Commun. 5, 5221(2014).32. Utke, I., Moshkalev, S. & Russell, P. Nanofabrication using focused ion andelectron beams (Oxford University Press, New York, NY, 2012).33. Taniguchi, T. & Watanabe, K. Synthesis of high-purity boron nitride singlecrystals under high pressure by using Ba–BN solvent. J. Cryst. Growth 303,525–529 (2007).34. Elbadawi, C. et al. Electron beam directed etching of hexagonal boron nitride.Nanoscale 8, 16182–16186 (2016).35. Kim, S., Toth, M. & Aharonovich, I. Design of photonic microcavities inhexagonal boron nitride. Beilstein J. Nanotech 9, 102–108 (2018).36. Tran, T. T. et al. Robust multicolor single photon emission from point defectsin hexagonal boron nitride. ACS Nano 10, 7331–7338 (2016).37. Kim, S., Kim, H.-M. & Lee, Y.-H. Single nanobeam optical sensor with a highQ-factor and high sensitivity. Opt. Lett. 40, 5351–5354 (2015).38. Riedrich-Moller, J. et al. One- and two-dimensional photonic crystalmicrocavities in single crystal diamond. Nat. Nanotechnol. 7, 69–74(2012).39. Rousseau, I. et al. Quantification of scattering loss of III-nitride photoniccrystal cavities in the blue spectral range. Phys. Rev. B 95, 125313 (2017).40. Bracher, D. O. & Hu, E. L. Fabrication of high-Q nanobeam photonic crystalsin epitaxially grown 4H-SiC. Nano Lett. 15, 6202–6207 (2015).41. Burek, M. J. et al. High quality-factor optical nanocavities in bulk single-crystal diamond. Nat. Commun. 5, 5718 (2014).42. Proscia, N. V. et al. Near-deterministic activation of room temperaturequantum emitters in hexagonal boron nitride. Preprint at https://arxiv.org/abs/1712.01352 (2017).43. Xu, Z.-Q. et al. Single photon emission from plasma treated 2D hexagonalboron nitride. Nanoscale 10, 7957–7965 (2018).AcknowledgementsThe authors thank Dr T. Tran for assistance with Raman measurements and Dr C.Elbadawi for the assistance with EBIE. K.W. and T.T. acknowledge support from theElemental Strategy Initiative conducted by the MEXT, Japan, and JSPS KAKENHI, GrantNumber JP15K21722. Financial support from the Australian Research council (viaDP140102721, DP180100077, LP170100150), the Asian Office of Aerospace Researchand Development grant FA2386-17-1-4064, the Office of Naval Research Global undergrant number N62909-18-1-2025 are gratefully acknowledged. I.A. acknowledges thegenerous support provided by the Alexander von Humboldt Foundation.Author contributionsS.K., I.A and M.T conceived the idea and designed experiments. J.E.F. developed thefabrication procedure for EBIE and tuning of cavities. S.K. performed the optical mea-surement and FDTD simulation. J.C. and M.S. provided cavities fabricated by FIB mil-ling. K.W. and T.T. grew the hBN samples. J.B. assisted with EBIE experiments. D.T.prepared trenched silicon substrate. S.K., J. E. F., I.A., and M.T. co-wrote the manuscript.Additional informationSupplementary Information accompanies this paper at https://doi.org/10.1038/s41467-018-05117-4.Competing interests: The authors declare no competing interests.Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, as long as you giveappropriate credit to the original author(s) and the source, provide a link to the CreativeCommons license, and indicate if changes were made. The images or other third partymaterial in this article are included in the article’s Creative Commons license, unlessindicated otherwise in a credit line to the material. If material is not included in thearticle’s Creative Commons license and your intended use is not permitted by statutoryregulation or exceeds the permitted use, you will need to obtain permission directly fromthe copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2018ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05117-48 NATURE COMMUNICATIONS |  (2018) 9:2623 | DOI: 10.1038/s41467-018-05117-4 | www.nature.com/naturecommunicationshttps://doi.org/10.1038/s41467-018-05117-4https://doi.org/10.1038/s41467-018-05117-4http://npg.nature.com/reprintsandpermissions/http://npg.nature.com/reprintsandpermissions/http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/www.nature.com/naturecommunications Photonic crystal cavities from hexagonal boron nitride Results Fabrication hBN photonic crystals 1D Photonic crystal nanobeam Tunable photonic cavity Creation of quantum emitters Discussion Methods Preparation of suspended hBN EBIE etching method FIB milling Optical characterization FDTD simulation Data availability References Acknowledgements Author contributions Competing interests ACKNOWLEDGEMENTS