Photoinduced Modification of Single-Photon Emitters in Hexagonal

Article pubs.acs.org/journal/apchd5

Photoinduced Modification of Single-Photon Emitters in Hexagonal Boron Nitride Zav Shotan,†,¶ Harishankar Jayakumar,†,¶ Christopher R. Considine,† Mažena Mackoit,§ Helmut Fedder,∥,⊥ Jörg Wrachtrup,∥,⊥ Audrius Alkauskas,§ Marcus W. Doherty,# Vinod M. Menon,*,†,‡ and Carlos A. Meriles*,†,‡ †

Department of Physics, CUNY-City College of New York, New York, New York 10031, United States CUNY-Graduate Center, New York, New York 10016, United States § Center for Physical Sciences and Technology, Vilnius LT-01108, Lithuania ∥ 3rd Physics Institute, University of Stuttgart, 70569 Stuttgart, Germany ⊥ Max Planck Institute for Solid State Research, 70174 Stuttgart, Germany # Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia ‡

S Supporting Information *

ABSTRACT: Fluorescent defects recently observed under ambient conditions in hexagonal boron nitride (h-BN) promise to open novel opportunities for the implementation of on-chip photonic devices that rely on identical photons from single emitters. Here we report on the room-temperature photoluminescence dynamics of individual emitters in multilayer h-BN flakes exposed to blue laser light. Comparison of optical spectra recorded at successive times reveals considerable spectral diffusion, possibly the result of slowly fluctuating, trappedcarrier-induced Stark shifts. Large spectral jumpsreaching up to 100 nmfollowed by bleaching are observed in most cases upon prolonged exposure to blue light, an indication of one-directional photochemical changes possibly taking place on the flake surface. Remarkably, only a fraction of the observed emitters also fluoresce on green illumination, suggesting a more complex optical excitation dynamics than previously anticipated and raising questions on the physical nature of the crystal defect at play. KEYWORDS: hexagonal boron nitride, single-photon emitters, optical spectroscopy, spectral jumps

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that operate at room temperature.11−15 Similar to graphite, hBN has a planar, layered structure with atoms in each layer forming a honeycomb lattice of covalent, in-plane bonds; outof-plane bonds are comparatively weaker, thus leading to the formation of thin, few-layer structures hereby referred to as “flakes”. Unlike prior observations in other van der Waals crystals,16,17 h-BN emitters show strong room-temperature photoluminescence because the comparatively larger band gap of the host material averts nonradiative decays. Therefore, given the fairly narrow lines in the visible region of the spectrum, these defects promise to serve as efficient sources of indistinguishable photons that can be readily integrated into on-chip photonic waveguides, optical microcavities, or electroluminescent devices. Further, large static shifts of the zerophonon line (ZPL) over hundreds of nanometers12 hint at the possibility of spectral tuning.

olid-state single-photon emitters are presently attracting broad interest for applications in quantum computing, quantum communication, encryption, and nanoscale sensing. Driving this trend is the notion of a photon as a tool for enhanced measurement1 and lithography2 or as a high-speed, low-noise quantum bus connecting distant nodes in a largescale, secure network.3 Thus far, single-photon emission has been realized in various systems including quantum dots,4 single molecules,5 and point defects in wide-band-gap semiconductors such as diamond6−8 and silicon carbide.9,10 Many of these photon sources, however, suffer from a poor roomtemperature quantum yield (i.e., are not sufficiently bright) or emit over a broad spectral range (which negatively impacts protocols demanding photon indistinguishability); some otherssuch as the silicon vacancy in diamond7reside in a material hard to process. The recent observation of single-photon emitters in commercial hexagonal boron nitride (h-BN) opens new practical opportunities for the design of photonic structures © XXXX American Chemical Society

Received: September 27, 2016

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DOI: 10.1021/acsphotonics.6b00736 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. Confocal microscopy of single emitters in hexagonal boron nitride. (a) Schematics of the experimental setup. The inset shows a fluorescence image exposing individual defects. (b) Optical spectroscopy (main) of different defects with various ZPL shifts. For some of the defects, antibunching plots displaying the measured photon autocorrelation g2(τ) as a function of the delay τ between two consecutive detection events are presented on the side plots. Optical spectradisplaced vertically for clarityare recorded with a laser power of 10 mW and an integration time of 1 s per spectra; the same laser power is used during the g2(τ) measurements.

Figure 2. Optical spectroscopy of individual defects. We record the optical response of two representative defects at successive times under continuous 405 nm illumination. While emitter A shows a stable response over an observation interval exceeding 90 s, emitter B exhibits strong spectral diffusion over a much shorter time window. Arrows indicate half-width at half-maximum (HWHM). The integration time per spectrum is 1 s, and the spectral resolution is 1 nm; the laser power at 405 nm is 10 mW. The dark traces in emitter A every 30 s are the result of periodic refocusing of the objective and can be ignored.

photon emitters in h-BN flakes exposed to blue (405 nm) laser excitation. We find that most fluorescing defects show sizable spectral diffusion at room temperature, with some of them exhibiting giant, reversible jumps between two (or more) welldefined wavelengths tens of nanometers apart. Further, we find that only a small fraction of the defects seen upon blue excitation also fluoresces under green (532 nm) illumination,

A necessary condition for any future practical application, however, is a fuller characterization of the emitter itselfwhose exact nature still remains elusiveas well as a better understanding of the role played by the host material, likely influenced by the incorporation of other types of defects or by proximity to the surface. Complementing recent studies,11−14 here we report on the room-temperature dynamics of single B


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Figure 3. Blue-light-induced emitter instability. (a) In this example the emitter exhibits a bistable behavior with the ZPL suddenly jumping between wavelengths centered at ∼580 and ∼600 nm. Spectral diffusion is present in either case although the effect is stronger at 600 nm. (b) The ZPL of this emitter undergoes comparable spectral jumps, but only the lower wavelength is reasonably well-defined. On occasion the emitter goes temporarily dark. The experimental conditions in (b) and (c) are identical to those in Figure 2.

which sets additional constraints on the allowed energy levels and hence on the physical nature of the photon source. Extended exposure to blue light introduces changes in the optical response and ultimately produces bleaching, suggesting that a drastic modification of the defect environment takes place, possibly through surface photochemistry. As a whole, these results unveil a complex but richer physics than previously assumed and hint at the prospect of defect engineering, for example, through the use of super-resolution or electron microscopy techniques.

therefore hint at site-specific (as opposed to emitter-specific) metastable states, possibly due to the modification of the defect’s charge state stability by local charge traps or the surface. Further observations consistent with site variation are presented in Figure 2, where we record a series of optical spectra at successive times for two sample emitters labeled A and B. Unlike emitter Adisplaying a stable, reproducible optical spectrum over a long time intervalthe data set for emitter B reveals strong spectral diffusion of the ZPL, in excess of 5 nm over a ∼20 s time window. This behaviorobserved in most defects but absent under green excitationcould be the result of blue-induced ionization of nearby charge traps, as seen at low temperatures in quantum dots,19 zinc oxide,20 or nitrogen-vacancy centers21 and other defects in diamond.22 The proximity of the surface in these flakes raises the possibility that the local charge environment may also be modified by permanent or transient molecular adsorbates (see below). To semiquantitatively consider local charge instability as a source of spectral diffusion, we use a crude model to calculate the electric field produced by a localized charge in a thin (i.e., few-layer-thick) flake of h-BN as a function of the emitter distance r from the charge relative to the flake thickness d. We find that the large charge screening present in the bulk crystalwhere the relative dielectric constant reaches an average value ε ≈ 5.7rapidly vanishes in the limit r ≫ d, a feature inherent to the two-dimensional (2D) geometry of the flake (see Supplementary Figure S1). We calculate an energy change of ∼0.01 eV (∼0.04 eV) for a charge located 4 nm (2 nm) away from the emitter, indicating that large spectral shifts are conceivable if one or more charges are trapped sufficiently close. Incidentally, defect ionization upon 405 nm excitation and carrier migration in h-BN heterostructures have been reported recently.23 While the ionization and charging of neighboring defects is likely to play a role in the observed time dependence of the spectra, several clues hint at a photochemical reaction as the dominant driving mechanism for the observed instabilities. For example, we find that a variable exposure intervalranging from seconds to minutesis needed to induce fluctuations in the emission properties, which can be interpreted in terms of a slowly progressing reaction. Along these lines, the response of emitter Bwhose ZPL gradually transitions from a 1.8 nm

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RESULTS AND DISCUSSION For the present experiments we use small (50−200 nm diameter) h-BN flakes in solution, which we first drop-cast on a SiO2 substrate and then anneal at high temperatures to generate sparse emitters on an otherwise nonfluorescing background (see Methods). To probe the h-BN sample, we use a custommade, two-color confocal microscope;18 Figure 1a shows a system schematic along with an example image under ambient conditions. In agreement with previous observations,11,12 we find that most emitters exhibit a narrow zero-phonon line and a characteristic ∼44 nm red shifted (∼160 meV), two-peak phonon sideband; each spectrum seems to be subject to a broad static wavelength shift, with observed ZPLs ranging from a maximum of 680 nm to a minimum of 500 nm, limited by our dichroic mirror. The similarity between band shapes, however, suggests that all emitters are of the same type though subject to strongly varying environments (e.g., near the surface, near grain boundaries, or experiencing different local densities of dark defects). The size of these static shifts appears to be too large to be explained by local strain, as noted previously.11,12 Hanburry−Brown−Twiss (HBT) “antibunching” experiments were carried out to identify single-photon sources. Some representative second-order correlation results upon blue excitation are presented in Figure 1b, where we consistently detect pronounced dips as expected for individual emitters. On occasion we find prominent “bunching” shoulders exposing the presence of metastable states. Although similar observations are common in emitters undergoing intersystem crossing to intrinsic, long-lived states, the case at hand seems to be different, in part because the response is seen to change substantially from one emitter to another. These findings C


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Figure 4. Fluorescence bursts in PMMA-coated h-BN flakes. (Center) Time trace of the emitted fluorescence for emitters E and F under 405 nm excitation in a sample PMMA-coated, single-emitter-hosting h-BN flakes. Green (red) arrows indicate the times when the blue laser is turned on (off) by the autorefocusing system. The dashed blue/purple lines are just guides to the eye. (Top insets) HBT photon antibunching plots before (left) and after (right) the fluorescence burst in emitter E. (Bottom insets) Optical spectroscopy of the emitted fluorescence before (left), during (center), and after (right) the fluorescence burst in emitter F. The emitter contribution to the observed spectra remains virtually unchanged throughout the process. The experimental conditions for optical spectroscopy are those of Figure 3.

Taken as a whole, these findings support the notion of a change in the charge state of the emitter and/or the local crystal symmetry by some form of photochemical reaction that varies from site to site due to local densities of dark defects or the proximity to the surface. This hypothesis is supported by previous observations of photochemical reactions at the surface of h-BN in the presence of molecular oxygen.24−28 For example, studies on h-BN exposed to below-band-gap, ultraviolet (UV) excitation under ambient conditions show that oxygen can be fixated into the surface to produce large changes in the emission of select crystal defects; this process, however, is reversible if the sample is exposed to UV light while in a vacuum for sufficiently long.24,25 Further, high-resolution electron microscopy has directly revealed the presence of substitutional oxygen impurities in the h-BN lattice29 (forming during crystal growth or from exposure to the probe electron beam). It is presently unclear as to whether the processes we observe are accompanied by a permanent transformation (e.g., damage or chemical change) of the h-BN lattice. Interestingly, however, we note that the activation energy required to break a boron−nitrogen bond in h-BN is 2.57 eV26,27 (corresponding to a photon wavelength of 482 nm). Given the stable response of emitters exposed to 532 nm future work will seek to explore whether this or a related photochemical reaction can be associated with the present observations. To further test our hypothesis that surface photochemical reactions are at play, we spin-coat a solution of poly(methyl methacrylate) (PMMA) onto the sample to produce a thin (∼100 nm) polymeric film (see Methods); the goal is to insulate the flakes from the environment and hopefully alter the photochemistry at the h-BN surface. Consistent with the notion of an ambient-mediated reaction, this time we find that the emitters are considerably more stable (e.g., no spectral jumps similar to those in Figure 3 were seen in any of the many emitters we probed). Prolonged exposure to blue light, however, is complicated by the observation of strong (but short-lived) fluorescence bursts, temporarily enhancing the

width at 20 s to about twice that much at later timesprovides a representative illustration of a behavior observed in most emitters experimenting spectral drifts. Prolonged excitation with blue light can lead to more dramatic effects, difficult to explain in a model of charge-trapgoverned Stark shifts: Example data setsobtained after continued excitation at 405 nm for about 1 minare presented in Figure 3 for two emitters labeled C and D. Besides spectral diffusion, we observe in both cases large spectral jumps of the ZPL, although each emitter displays a distinct behavior: Emitter C shows a bistable dynamics, with the ZPL alternating between ∼581 nm the characteristic wavelength observed at earlier times, not shownand ∼607 nm. The ZPL of emitter D, on the other hand, cycles between ∼570 nmthe wavelength intrinsic to the “pristine” emitterand a range of more loosely defined values red-shifted from the former by up to ∼100 nm. Remarkably, the ZPL broadens substantially at the longer wavelengths, which we attribute to enhanced spectral diffusion on a time scale faster than the sampling time (approximately 1 s per trace, see Methods). A clear indication is seen in the left plots of Figure 3b (see, for example, spectra at 93, 102, and 103 s), where the emitter spectrum changes from “pristine” to “shifted” within the collection time window (∼1 s per spectrum), thus leading to coexisting emission peaks at different wavelengths. The conditions driving this change are themselves dynamically evolving, thus leading to heterogeneously broadened, fainter spectral features. This latter time behavior proved more common among the emitters we probed; an expanded set of examples is presented in Supplementary Figure S2. On occasion, both emitters C and D undergo brief periods of darkness where no ZPL can be observed (our detection window goes from 500 to 850 nm). Prolonging the exposure to 405 nm light for more than a few minutes invariably leads to fluorescence bleaching. This response is in stark contrast with that observed under green excitation, nearly perfectly stable in nearly all cases (mild spectral diffusion is observed only on rare occasions). D


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Figure 5. Comparison of blue- and green-induced fluorescence. (a) Confocal image of a section of the sample recorded under 405 nm (left) or 532 nm (right) excitation. Most emitters are visible only under blue illumination; a small fraction of these show strong absorption of green light (circled). The blue (green) laser power is 10 mW (5 mW), the number of points is 200, and the integration time per pixel is 1 ms. (b) Experimental protocol. We park the 405 nm beam at a fixed location in the sample and illuminate for a time interval t, after which we record the optical spectra resulting from 405 or 532 nm laser pulses. (c) Spectra from the protocol in (b). The system corresponds to two emitters, G and H, contained within the laser focal spot (∼250 nm diameter). The laser power during 405 nm (532 nm) excitation is 10 mW (1 mW); the blue laser park time t is indicated in the lower right corner of each plot. The integration time per spectrum is 1 s except at t = 90 s, where we use a 15 s integration time in the case of green excitation (plot in the lower right corner).

differences with changes in the emission spectra. As a matter of fact, we find that both types of emitters display similar phonon replicas, suggesting that even if the emitting defects are distinct, they may share the same local crystal symmetry. As shown in Figure 5b,c, however, separating emitters into two distinct classes may not be necessarily correct: Here we park the 405 nm laser beam at a fixed location and sporadically probe the fluorescence under blue or green illumination. Figure 5c focuses on the optical response of a pair of emitters dubbed G and H at 570 and 675 nm, respectivelyboth contained within the focal spot of the laser. At early times (i.e., immediately after the first exposure to 405 nm light) both emitters fluoresce under blue and green excitation; upon continued blue illumination, the emission becomes unstable, with the amplitude of both peaks randomly growing or shrinking without exhibiting, however, major shifts (a pair of intermediate spectra for blue and green excitation is presented in the middle insets of Figure 5c). Ultimately, prolonged exposure to blue light permanently bleaches emitter G (i.e., no fluorescence is observed under blue or green light), which can be understood along the lines of Figures 2 and 3 as a transformation of the emitter charge state or the local environment. Emitter H, on the other hand, behaves differently: No change is seen in the emission wavelength although the ZPL amplitude becomes considerably more intense; the emission under green excitation, however, becomes nearly negligible. These findings suggest, therefore, an alternate scenario where the charge state of the emitter or its immediate vicinity (e.g., ≲1 nm away) remains nearly unchangedhence preserving the ZPL wavelengthbut the emission quantum efficiency at 532 nm is modified. The latter amounts to introducing an alternate relaxation channel, e.g., due to the photogeneration or change in charge state of a dark defect 2−10 nm distant that couples resonantly to green light but decays nonradiatively. An attractive feature of this picture is that one can think of all

photon count from 1 to 3 orders of magnitude. An example is presented in Figure 4, showing the observed fluorescence time trace upon blue excitation of two different h-BN flakes each hosting a single emitter: After a few seconds of illumination, we witness a steep increase of the photon emission followed by a fast decay that brings the fluorescence nearly back to the initial level. Spectrally resolved observations before, during, and after the burst (lower insets in Figure 4) show that the photon source causing this response is unrelated to the emitter (the broad emission bandwidth during the burst contrasts with the emitter’s narrow spectrum, virtually unchanged throughout the process). Blue light is needed to trigger the photon flare, but green excitation during the burst also yields an enhanced photon count. Further, scanning imaging shows that the process is limited to the flake; that is, bare PMMA is inert to blue excitation. Having discarded blue-light-induced lensing30,31 as a possible mechanism, our findings hint at yet another, PMMA-specific photochemical process at the interface with the h-BN surface. These and related additional observations are discussed at greater length in the Supporting Information (see Supplementary Figure S2). In the absence of external coatings the impact of blue excitation is not limited to the time response of the emitters but seems to be more fundamental. One first indication is presented in Figure 5a, where we scan a blue or green beam to image a section of the uncoated sample (i.e., no PMMA is present). Surprisingly, we find that the majority of the defects we observe under 405 nm excitation do not fluoresce when illuminated with 532 nm. This simplethough intriguing finding can be interpreted in different ways. One first possibility is that these two responses originate from emitters associated with inherently different types of point defects, with only one of them displaying a non-negligible absorption cross-section at 532 nm. Different types of emitters have been hypothesized before, albeit based on subtle differences in the ZPL shape.12 In the present case, we could not correlate the fluorescence E


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emitters as belonging to the same type; their ability to fluoresce under green excitation is dictated by the presence or absence of a nonradiative channel, in turn, defined by the crystal vicinity. In particular, the observation of darker spots in areas otherwise bright under blue excitation supports the notion of efficient absorption at 532 nm even for nonfluorescing emitters (see circled areas in the images of Figure 5a and b).

Photoluminescence experiments were performed with a home-built confocal laser-scanning microscope. The microscope consists of a 0.7 NA objective (Mitutoyo BD Plan Apo 100X) used in a collinear excitation and collection geometry. The primary excitation source is a 405 nm diode laser combined with a fiber coupled 532 nm DPSS laser. A dichroic mirror with a 550 nm cutoff along with a 532 nm notch filer is used to separate collected photons from the excitation laser. The collected photons are coupled into a 8 μm core fiber, which also acts as the confocal pinhole, and detected with a single-photon counting module from Excelitas Technologies. A multimode fiber beam splitter is used for the photon correlation experiments and for parallel spectroscopy and count rate measurements. Emission spectra were acquired using a Horiba iHR320 spectrometer (grating: 300 grooves/mm) with a liquid nitrogen cooled Symphony II line detector. A PicoHarp 300 from PicoQuant is used for autocorrelation measurements. Autocorrelation and lifetime measurements were also performed with a pulsed 430 nm laser (76 MHz) derived from a Ti:sapphire femtosecond laser pumped frequency doubler from Coherent Inc. All experiments are carried out under ambient conditions.

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CONCLUSIONS In summary, as the research community seeks to understand and make use of single-photon emitters in h-BN, the results herein expose a complex dynamics largely impacted by the system photochemistry. Blue excitation of single emitters in hBN flakes leads to pronounced fluorescence instability taking the form of large ZPL spectral diffusion and discrete jumps of up to 100 nm under ambient conditions. Since the emitters are mostly stable under illumination at 532 nm, our observations suggest a photochemical reaction with activation energy between 2.3 and 3.0 eV, possibly similar to previously observed photo-oxidation processes on the h-BN surface.26−28 Whether the local crystal composition is in part responsible for the large “static” ZPL shifts in Figure 1tentatively assigned to strain11is an interesting related question. Initial attempts to “freeze” the emission wavelength after a photoinduced spectral jump (by rapidly switching from 405 to 532 nm excitation) were accompanied by a near-complete darkening of the greeninduced emission; it is presently unclear, however, as to whether this same response is general to all emitters. Coating the h-BN flakes with PMMA renders the emitter spectra comparatively more stable but also introduces spontaneous fluorescence bursts temporarily increasing the photon count by up to 3 orders of magnitude. The large peak intensity as well as the structure of the spectra recorded during the flares suggests some form of photoinduced transformation, which, nonetheless, has no direct impact on the emitter. Additional experimental and theoretical work is needed to shed light on the charge dynamics at play both under a controlled atmosphere or when embedded in a solid-state matrix. Similarly, numerical and ab initio studies will be key to understanding the emitter-selective response to green excitation, a problem directly connected to lingering questions on the nature of the host defect11−13 and the influence other neighboring lattice imperfections have on its fluorescence.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b00736. Calculation of the emission shift produced by an isolated charge in h-BN and additional data sets on the generation of fluorescence bursts from PMMA-coated h-BN flakes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (V. M. Menon): [email protected]. *E-mail (C. A. Meriles): [email protected]. ORCID

Carlos A. Meriles: 0000-0003-2197-1474 Author Contributions

Z.S., H.J., and C.R.C. conducted the experiments. A.A., M.W.D., and M.M. provided the theoretical framework for interpreting the observations. H.F. and J.W. provided key technical insight. V.M. and C.A.M. supervised the work. All authors contributed to analyzing the data and writing the article.

METHODS

All samples are prepared from a prefixed solution of suspended h-BN flakes in ethanol/water purchased from Graphene Supermarket. The lateral size specified from the manufacturer is from 50 to 200 nm, and the thickness is 1 to 5 atomic layers; the solution is stable under ambient conditions and is rated to have a purity in the dry phase better than 99%. To disperse the flakes on a substrate, we drop-cast a small amount of the solution onto a bare SiO2 wafer and let it dry in air overnight. Emitters are subsequently activated through thermal annealing. For this purpose we use a Limberg Blue M tube furnace with an argon flow at 1 Torr for 30 min at 850 °C. After the 30 min annealing process the furnace is immediately opened for rapid cooling and samples are removed for testing. One sample was spin-coated with 950 PMMA A4 at 3000 rpm for 1 min with a target thickness of 230 nm. After spin coating, the sample was baked on a hot plate for 90 s at 100 °C. Unless explicitly noted, all our experiments are carried out on the uncoated samples.

Author Contributions ¶

Z. Shotan and H. Jayakumar contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.J. and C.A.M. acknowledge support from Research Corporation via a FRED Award and from the National Science Foundation through award NSF-1545649. A.A. and M.M. acknowledge the support of the Research Council of Lithuania via Grant No. M-ERA.NET-1/2015, as well as computational resources at the High Performance Computing Center “HPC Sauletekis” (Physics Department, Vilnius University). Z.S., C.R.C., and V.M.M. acknowldege support from NSF through the EFRI-2-DARE program (EFMA - 1542863). J.W. and H.F. F


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(18) Jayakumar, H.; Henshaw, J.; Dhomkar, S.; Pagliero, D.; Laraoui, A.; Manson, N. B.; Albu, R.; Doherty, M. W.; Meriles, C. A. Optical patterning of trapped charge in nitrogen-doped diamond. Nat. Commun. 2016, 7, 12660. (19) Ha, N.; Mano, T.; Chou, Y.-L.; Wu, Y.-N.; Cheng, S.-J.; Bocquel, J.; Koenraad, P. M.; Ohtake, A.; Sakuma, Y.; Sakoda, K.; Kuroda, T. Size-dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charge on spectral diffusion. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 075306. (20) Neitzke, O.; Morfa, A.; Wolters, J.; Schell, A. W.; Kewes, G.; Benson, O. Investigation of line width narrowing and spectral jumps of single stable defect centers in ZnO at cryogenic temperature. Nano Lett. 2015, 15, 3024−3029. (21) Acosta, V. M.; Santori, C.; Faraon, A.; Huang, Z.; Fu, K-M.C.; Stacey, A.; Simpson, D. A.; Ganesan, K.; Tomljenovic-Hanic, S.; Greentree, A. D.; Prawer, S.; Beausoleil, R. G. Dynamic stabilization of the optical resonances of single nitrogen-vacancy centers in diamond. Phys. Rev. Lett. 2012, 108, 206401. (22) Müller, T.; Aharonovich, I.; Lombez, L.; Alaverdyan, Y.; Vamivakas, A. N.; Castelletto, S.; Jelezko, F.; Wrachtrup, J.; Prawer, S.; Atatüre, M. Wide-range electrical tunability of single-photon emission from chromium-based colour centres in diamond. New J. Phys. 2011, 13, 075001. (23) Velasco, J., Jr.; Ju, L.; Wong, D.; Kahn, S.; Lee, J.; Tsai, H.-Z.; Germany, C.; Wickenburg, S.; Lu, J.; Taniguchi, T.; Watanabe, K.; Zettl, A.; Wang, F.; Crommie, M. F. Nanoscale control of rewriteable doping patterns in pristine graphene/boron nitride heterostructures. Nano Lett. 2016, 16, 1620−1625. (24) Museur, L.; Kanaev, A. Near band-gap photoluminescence properties of hexagonal boron nitride. J. Appl. Phys. 2008, 103, 103520. (25) Museur, L.; Anglos, D.; Petitetc, J.-P.; Michelc, J.-P.; Kanaev, A. V. Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation. J. Lumin. 2007, 127, 595−600. (26) Kanaev, A. V.; Petitet, J. P.; Museur, L.; Marine, V.; Solozhenko, V. L.; Zafiropulos, V. Femtosecond and ultraviolet laser irradiation of graphite-like hexagonal boron nitride. J. Appl. Phys. 2004, 96, 4483− 4489. (27) Hirayama, Y.; Obara, M. Ablation characteristics of cubic-boron nitride ceramic with femtosecond and picosecond laser pulses. J. Appl. Phys. 2001, 90, 6447−6450. (28) Berzina, B.; Korsaks, V.; Trinkler, L.; Sarakovskis, A.; Grube, J.; Bellucci, S. Defect-induced blue luminescence of hexagonal boron nitride. Diamond Relat. Mater. 2016, 68, 131−137. (29) Krivanek, O. L.; Chisholm, M. F.; Nicolosi, V.; Pennycook, T. J.; Corbin, G. J.; Dellby, N.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Oxley, M. P.; Pantelides, S. T.; Pennycook, S. J. Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy. Nature 2010, 464, 571−574. (30) Baset, F.; Popov, K.; Villafranca, A.; Guay, J. M.; Al-Rekabi, Z.; Pelling, A. E.; Ramunno, L.; Bhardwaj, R. Femtosecond laser induced surface swelling in poly-methyl methacrylate. Opt. Express 2013, 21, 12527−12538. (31) Zheng, C.; Hu, A.; Li, R.; Bridges, D.; Chen, T. Fabrication of embedded microball lens in PMMA with high repetition rate femtosecond fiber laser. Opt. Express 2015, 23, 17584−17598.

thank the German Science Foundation (DFG) and Max Planck Society. V.M.M. and C.A.M. acknowledge partial support from the National Science Foundation via the CREST-IDEALS Grant NSF-1547830. We thank Prof. A. Matulis for his kind assistance with the electrostatics problem and Prof. S. O’Brian for his help with the sample annealing. We also acknowledge helpful discussions with N. Chejanovsky and F. Fávaro de Oliveira.

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