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Xiliang Yang, Dong Hoon Shin, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Peter G. Steeneken, Sabina Caneva

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[Microsphere-assisted generation of localized optical emitters in 2D hexagonal boron nitride](https://mdr.nims.go.jp/datasets/d24aa2ac-27d7-4da8-8e2d-5ea4d3b14b17)

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Microsphere-assisted generation of localized optical emitters in 2D hexagonal boron nitrideNanophotonics 2025; 14(14): 2419–2430Research ArticleXiliang Yang, Dong Hoon Shin, Kenji Watanabe, Takashi Taniguchi, Peter G. Steeneken andSabina Caneva*Microsphere-assisted generation of localizedoptical emitters in 2D hexagonal boron nitridehttps://doi.org/10.1515/nanoph-2024-0625Received November 11, 2024; accepted May 11, 2025;published online June 5, 2025Abstract: Crystal defects in hexagonal boron nitride (hBN)are emerging as versatile nanoscale optical probes with awide application profile, spanning the fields of nanopho-tonics, biosensing, bioimaging, and quantum informationprocessing. However, generating these crystal defects asreliable optical emitters remains challenging due to theneed for deterministic defect placement and precise controlof the emission area. Here, we demonstrate an approachthat integrates microspheres with hBN crystal lattices toenhance both hBN defect generation and optical signalreadout. This technique harnesses microspheres to amplifylight–matter interactions at the nanoscale through twomechanisms: focused femtosecond (fs) laser irradiation intoa photonic nanojet (PNJ) for highly localized defect gen-eration and enhanced light collection via the whisperinggallerymode (WGM) effect. Ourmicrosphere-assisted defectgeneration method reduces the emission area by a factorof 5 and increases the fluorescence collection efficiency byapproximately 10 times compared tomicrosphere-free sam-ples. These advancements in defect generation precisionand signal collection efficiency open new possibilities for*Corresponding author: Sabina Caneva, Department of Precision andMicrosystems Engineering, Delft University of Technology, Mekelweg 2,2628 CD, Delft, The Netherlands, E-mail: s.caneva@tudelft.nl.https://orcid.org/0000-0003-3457-7505Xiliang Yang and Peter G. Steeneken, Department of Precision andMicrosystems Engineering, Delft University of Technology, Mekelweg 2,2628 CD, Delft, The Netherlands, E-mail: X.Yang-3@tudelft.nl. (X. Yang),https://orcid.org/0009-0003-8055-079X (X. Yang).https://orcid.org/0000-0002-5764-1218 (P. G. Steeneken)Dong Hoon Shin, Department of Electronics and Information Engineer-ing, Korea University, Sejong 30019, Republic of Korea,https://orcid.org/0000-0002-0438-0835Kenji Watanabe and Takashi Taniguchi, National Institute for Materi-als Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.https://orcid.org/0000-0003-3701-8119 (K. Watanabe).https://orcid.org/0000-0002-1467-3105 (T. Taniguchi)optical emitter manipulation in hBN, with potential appli-cations in quantum technologies and nanoscale sensing.Keywords: hexagonal boron nitride; optical emitter; micro-sphere; laser fabrication1 IntroductionNanoscale optical emitters are a cornerstone of nanopho-tonics, bioimaging, and biosensing and an invaluable toolfor investigating dynamics, mechanics, and interactions ofbiological components in aqueous environments [1]. Whiletraditionally relying on fluorescent dyes for biophysicsresearch, novel probes are emerging that offer better com-binations of chemical and mechanical stability, brightness,and lifetime [2]. Among next-generation probes, 2D mate-rial optical emitters (OE) based on hexagonal boron nitride(hBN) crystal defects are particularly attractive [3]. Thesenanoscale probes exhibit exceptional brightness (4,000kcts/s) [4], long lifetimes (∼3 ns) [5], high quantumefficiency(87 %) [6], emission in the visible [7], stability in liquid envi-ronments [8], and biocompatibility [9], making them idealfor a wide range of applications. Unlike other 2D material-based emitters, such as transition metal dichalcogenides[10], hBN defects are fully operational at room temperature[11], [12], significantly broadening their potential in bothfundamental research and applications. However, the useof hBN emitters also face several challenges: (1) Fabrica-tion can be complex, costly, or low throughput requiringadvanced techniques like ion/electron beam irradiation [13],[14], fs laser writing with high numerical aperture objectivelenses, or indentation with sharp atomic force microscopy(AFM) tips [15]. (2) Controlling the nature and location ofdefects within the crystal structure is difficult withoutmodi-fying the substrate through the addition of, e.g., nanopillars,wrinkles, or microsphere arrays, impacting reproducibil-ity and performance [16], [17]. Addressing these challengesis crucial to integrate hBN emitters into high-resolutionimaging and nanophotonics applications, necessitatingOpen Access. © 2025 the author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License.https://doi.org/10.1515/nanoph-2024-0625mailto:s.caneva@tudelft.nlhttps://orcid.org/0000-0003-3457-7505mailto:X.Yang-3@tudelft.nlhttps://orcid.org/0009-0003-8055-079Xhttps://orcid.org/0000-0002-5764-1218https://orcid.org/0000-0002-0438-0835https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-1467-31052420 — X. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBNfurther advances in fabrication and emission collectionmethods.To generate OE in hBN, various fabrication methodshave been developed and are broadly categorized into twogroups: methods with random spatial distributions and site-specific techniques [17], [18]. The former methods includethermal annealing [19], [20], chemical [21] and plasma etch-ing [22], and bottom-up growth [23]. These techniques typ-ically create defects over larger areas with little or no spa-tial control or require nonplanar surfaces features. In con-trast, spatially controlled techniques, such as AFM inden-tation, substrate strain-induced fabrication, ion/electronbeam irradiation, and direct laser writing, offer more pre-cise control over emitter placement [24], [25]. However, theyface restrictions in terms of scalability, high-end, expensiveinstrumentation or require non-planar substrate engineer-ing [24], [26]–[28]. Irradiation with ions also presents therisk of introducing foreign atomic species/implantation ofions [29]–[31] Direct laser writing, especially using femto-to pico-second lasers, achieves precision and scalability [32],[33], yet high laser powers can lead to considerable surfaceroughening, caused by thermal damage over the irradiatedarea [34]. Additionally, the spot size cannot be reducedbelow the diffraction limit determined by the numericalaperture (NA) of the objective lens, which restricts the spa-tial precision of the fabrication process.Here, we report an approach that utilizes a micro-sphere membrane to reduce the size of the irradiated areaand concurrently control the position of optically generatedhBN emitters. Leveraging the PNJ focusing capability of themicrosphere, previously demonstrated for SiO2/Si and goldsurfaces [35], this method achieves a fivefold reduction inthe size of the area in which emitters are generated com-pared to fs-laser writing without microspheres, resulting inmore localized hBN emitters. This improvement arises fromthe effective NA enhancement by the microsphere, whichresults in a focal spot size of <1 μm. In addition, due to theprinciple of reversible optical paths, microsphere-assistedfabrication significantly boosts fluorescence collection effi-ciency by approximately 10 times. The enhancement of theoptical absorption by the defects and reduced backgroundnoise [36], [37] is further accompanied by the enhanced cap-ture of refracted light throughWGMandPNJ. In this process,light traveling around themicrosphere’s surface is confinedby total internal reflection, creating resonant modes thatamplify and direct the light. This effect optimizes the extrac-tion of photons into the far field, improving the efficiencyof light collection and emission from the optically activehBN defect regions [38]. Practically, the microsphere trapsthe emitted light and directs it efficiently into a preferredout-coupling direction, further amplifying the fluorescencesignal strength.Therefore, microsphere integration not only enhancesthe spatial accuracy of emitter fabrication but also max-imizes fluorescence signal collection. This platform canbe readily integrated into single-molecule fluorescencemicroscopy systems, where hBN surfaces are findingincreasing use as biocompatible substrates for wide-fieldimaging of biomolecules and their dynamics [39]. Addition-ally, by leveraging existing microsphere array technology[40], and the simple integration in microfluidic devices, thismicrosphere implementation has the potential to achievehighly parallel signal collection in optofluidic sensing forhealth and environmental monitoring.2 Results and discussion2.1 Microsphere-assisted laser fabricationFigure 1(a) shows the laser incident on the microsphere,forming a PNJ upon emerging at the bottom. The fs laserbeam (515 nm, pulse duration 290 fs and linear polariza-tion) was focused through a Theta lens. To precisely con-trol microsphere positioning, we utilize high-refractive-index (n = 1.9) barium titanate (BaTiO3) glass micro-spheres (50 μm diameter) embedded in a polydimethyl-siloxane (PDMS) membrane (n = 1.41). This design ensuresthree key objectives: (1) Noncontact optical focusing: lever-aging the microspheres’ high refractive index and spher-ical geometry, the focal plane is engineered to resideexternally to the microsphere, as confirmed by finite-difference time-domain (FDTD) computational modelingas shown in Figure S1 of the Supporting Information (SI).This ensures intense near-field confinement (𝜆/2.5 res-olution) on hBN samples while circumventing interfa-cial Fresnel reflections and multiple scattering artifactsinherent to direct sphere-sample contact. (2) Deterministicalignment: the 50 μm microsphere diameter – selected toexceed twice the positional uncertainty (∼20 μm) of thefs laser translation stage – enables submicron registra-tion accuracy during laser-induced photonic array fabri-cation [41], [42]. (3) Index-engineered material compatibil-ity: the PDMS substrate simultaneously immobilizes micro-spheres via viscoelastic adhesion and minimizes parasiticscattering through a refractive index contrast (Δn = 0.49).This contrast suppresses Rayleigh backscattering at thePDMS-BaTiO3 interface [43]. The composite system providesX. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBN — 2421Figure 1: Schematic diagram of microsphere-assisted fs-laser fabrication of hBN emitters and simulations. (a) Experimental design of the MPM withspacer over the hBN flake surface on the SiO2/Si substrate. Bottom: Side view of microsphere enhancement of the fs-laser focus. (b)–(e) FDTDsimulation of the light field distribution of the MPM focus: (b) light distribution in the xz-plane and intensity distribution along the z-axis with directcontact between the MPM and the substrate. (c) Light distribution in the xy-plane and intensity distribution along the y-axis at the maximum intensityposition with direct contact. (d) Light distribution in the xz-plane and intensity distribution along the z-axis with a 6 μm distance between hBN andmicrosphere. (e) Light distribution in the xy-plane and intensity distribution along the y-axis at the maximum intensity position with a 6 μm distance.reconfigurable optical functionality while maintainingthermo-mechanical stability (PDMS glass transition tem-perature ≈−125 ◦C), ensuring reliable performance dur-ing fabrication and imaging processes. The microsphere-PDMS membrane (MPM) fabrication process is detailedin Section S2 of the Supporting Information. We note thatwhile we focus here on single microsphere-based hBN emit-ter fabrication, self-assembly of microsphere during mem-brane embedding can generate tightly packed arrays thatcan increase the fabrication throughput. The MPM wasplaced on a SiO2/Si substrate on which mechanically exfo-liated, high-quality hBN flakes were transferred. To ensurethe focus point could reach the hBN surface, we inserteda spacer consisting of double-sided tape between the MPMand the substrate. To determine the changes in optical fielddistribution caused by the microsphere, finite-differencetime-domain (FDTD) simulation designs were conducted.The results show that when the MPM directly touched thesubstrate, it produced ring-shaped patterns due to focusinginside the microsphere, as shown in Figure 1(b) and (c).Increasing the distance between the hBN and the micro-sphere improved PNJ focusing, as shown in Figure 1(d) and(e), with a full-width at half-maximum (FWHM) of 480 nmata distance of∼6 μm from themicrosphere, which is smallerthan the focal spot size of most high-NA objective lenses.Additionally, the PNJ length was approximately 5 μm, pro-viding a large working distance tolerance for fabrication.Based on the stack design and simulations in Figure 1,fs-laser fabrication was used to generate defects in hBNusing a 6-μm spacer, the profile of which is depicted inFigure S3. The irradiation spots on the xy-planewere spacedapproximately 30 μm apart to avoid crosstalk. Figure 2(a)compares the results of fs laser processing without andwithmicrospheres. Area I shows that during direct fs-laser writ-ing, the hBN layer (yellow) is fractured, producing an irreg-ular opening of ~15 μm that exposes the SiO2/Si substrate(blue) underneath it.When the laser pulse is applied directlyto the SiO2/Si, holes are formed in the surface (black). . Strik-ingly, on the same flake, laser writing with the MPM layerreduces the affected area (area II) to a diameter of approx-imately 3.6 μm. This reduction in diameter corresponds toa decrease in the affected area by a factor of ∼5. The hBNforms a bubble that exhibits a circular shape, with a radiusof 1.8 μm and a height of 45 nm (h/R = 0.025), as shownby the AFM analysis (Figure 2(b) and (c)). This observationsuggests that a tightly focused PNJ causes deformation ofthe top layers of the hBN flake (thbn ∼ 50 nm), caused bylocalized heating from high-intensity laser pulses, whichleads to thermal expansion and blistering [35]. Bubble for-mation in 2D materials has been previously attributed tomechanical stress from rapid heating and cooling, whichcan trap air as well as ambient or surface absorbents [44],[45]. The bubble’s ratio h/R = 0.025 here is lower than thatof monolayer hBN bubble formation (h/R = 0.11) [46] but2422 — X. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBNFigure 2: Characterization of fs-laser induced hBN defects. (a) Optical image of the hBN surface after fs-laser irradiation with (area II) and without(area I) MPM (inset: zoomed view of the MPM irradiated area, scale bar: 1 μm). (b) 2D AFM image of the hBN bubble. (c) 3D AFM image of the bubblecross section. (d) Raman spectra along the diameter of the bubble (highlighted in (b) by a purple dashed line). (e) PL mapping of the hBN bubble.(f) Emission spectra of two representative emitters and background, labeled as circles 1 and 2 in (e), and 3 in (a), respectively.is in good agreement with the multilayer hBN bubble for-mation seen in both experimental and theoretical results[45].Figure 2(e) shows the photoluminescence (PL) of thehBN bubble, with most emitters located at the edges ofbubble, where the strain is most pronounced. This suggeststhat the emission is largely dependent on lattice strain,as evidenced by a Raman hBN peak shift from 1,365 to1,363 cm−1 across the bubble as shown in Figure 2(d), indi-cating tension in the hBN flake [45]. Similar peak shiftsdue to maximized strain at the bubble’s center have previ-ously been reported [45]. Figure 3(f) displays the PL spec-tra from these bright spots (circles 1 and 2 in Figure 2(e)),showing the typical zero-phonon lines (ZPL) of hBNemitters.Figure 3 demonstrates different sizes of defects pro-duced with MPM by varying the laser irradiation power.When the power was below the surface modificationthreshold (∼0.07 W) in our setup, slight color changes arevisible in the hBN, but no clear defect areas could beobserved. However, at higher power levels, clear visiblestructural changes appeared in the material. Details ofthe laser-induced defects are shown in Figures 3(a) andS4.When the laser power exceeded the surface modifi-cation threshold but remained below 0.3 W, bubble pat-terns were observed. The diameter of these bubble struc-tures ranged from 3 to 4 μm, which was smaller than thefocal spot size of the f-theta lens (9 μm at 1/e2 intensity),as shown in Figures 2(a), 3(a), and S4. This size reductioncan be attributed to the PNJ generated by the microsphere,which effectively reduces the focal spot size. Pulses withpower above 0.3 W caused ablation at the center of thePNJ, resulting in the outward redeposition of hBN materialaround the hole. With 0.3 W and 0.45 W laser power, thedefect patterns generated by MPM enlarged to 4.5 μm and5.6 μm, respectively, breaking up into holes. In compari-son, direct laser fabrication patterns were about 5 timeslarger than those produced by theMPMmethod as shown inFigure S4(e). The defect patterns exhibited torn edges, likelyformed by the high pressure and shock waves generated bythe high-energy laser pulses, accompanied by a breakdownof the surrounding material, similar to laser-induced microexplosions confined within the bulk of sapphire [35], [47].X. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBN — 2423Figure 3: Optical and spectral characterization of MPM fs-laser-generated emitters in hBN. (a) Optical images of the defects of different sizesgenerated by MPM fs-laser with powers of 0.15, 0.3, 0.45 W. Scale bar: 3 μm. (b) The corresponding PL maps of the irradiated areas in (a).Scale bar: 3 μm. (c) Emission spectra of six representative emitters, as labeled in (b). The 550 nm peak in all panels arises from the hBN Ramancontribution. (d) Histograms of the emitters ZPLs, categorized by different color ranges. Bin size: 3 nm.Additionally, we confirm that the SiO2/Si substrate exhibitsno intrinsic fluorescence, ensuring that hBN defect emis-sions are free from background interference as shown inFigure S5. The average area of the defect patterns, based onover 15 samples, increased linearly with the pulse powerwith a scaling factor of 13 μm/W, as shown in Figure S4(e).We note that the observed variation in the size of thedefect patterns, even with the same pulse energies, can beattributed to factors such as the variation in hBNflake thick-ness (thBN: 40 nm–70 nm), the concentration of impuritiesor defects in the bulk material of hBN, wrinkles formedduring exfoliation, and misalignment of the laser writingsetup and the microsphere as shown in Figure S6 [15], [48].These factors also prevent the formation of smaller defectsas predicted by simulations.To activate and stabilize the photoemission from thehBN defects and remove the contamination induced duringthe fabrication process, the hBN samples were annealedin 1,000 ◦C at 10−7 bar for 2 h after laser writing with theMPM. As previously reported, annealing plays an importantrole in restructuring and forming new optically active colorcenters in hBN [33]. Before annealing, few PL signals wereobserved from the laser-processed sample. After annealing,sharp, bright PL peaks appeared at laser-irradiated sites asshown in Figure 3(c), unlike emitters produced by conven-tional direct thermal annealing methods, which are locatedon randomly generated wrinkles [20]. PL maps were subse-quently obtained on these processed areas using a 0.9 NAobjective with a 514 nm wavelength continuous-wave laserexcitation (Figure 3(b)).The maps indicate that multiple emission centers aregenerated and distributed along the edge of the affectedareas. Moreover, the number and type of emitters changedwith the defect pattern size. Single and double emissionspots can be seen around the defect pattern in the bubblestructure, yet due to the diffraction limit of the optical sys-tem, we cannot identify and determine the number of indi-vidual nanoscale emitters. When the pattern size exceeded∼3 μm, the emission spots started to merge into clusters, asshown in the middle panel of Figure 3(b).With 0.15 W laser power, the formation of bubblestructures introduces localized strain in the hBN’s crys-tal lattice, distorting molecular orbitals and perturbingthe energy levels of defect states [11]. This strain-inducedmodulation enhances the possibility of emitters exhibitingoverlapping zero-phonon line (ZPL) wavelengths, as latticedistortions can stabilize defects with similar electronic con-figurations [49]. While our analysis of 12 representative2424 — X. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBNbubble-structured samples reveals ZPL emissions spanningall categorized ranges (green: 555 ± 15 nm, yellow: 580 ±10 nm, orange: 605 ± 15 nm, red: 650 ± 30 nm), a statisticalpreference for orange (605 ± 15 nm) and red (650 ± 30 nm)ZPLs is observed as shown in Figures 3(d) and S7. This trendsuggests that strainfields in bubble structures preferentiallystabilize defects associated with longer wavelengths, suchas NBVN defects [5], [11], [13]. Strain magnitudes and latticedistortions vary spatially throughout the bubble geometry.This spatial heterogeneity results in a nonuniform distribu-tion of defect patterns, where specific defect types dominatein regions that match their stable strain conditions. As aresult, the zero-phonon line (ZPL) energies of these defectsshow a broad spectral distribution, reflecting the structuraldiversity of defect configurations within the bubble. Thisphenomenon is consistentwith defect aggregation observedduring low-power laser fabrication, where localized energydeposition and rapid thermal gradients promote the aggre-gation of vacancies, displacements, and strain-stabilizedcomplexes.As the laser power increases from 0.15 W to 0.45 W, theZPL fullwidth at half-maximum (FWHM)narrows by 1.1 nm,indicating a transition from a diverse strain-tuned defectlandscape to amore uniform vacancy-dominated state. Thissuggests that higher laser power disrupts strain-stabilizeddefects, favoring the formation of vacancy-rich sites withwell-defined electronic states. For defect patterns obtainedwith 0.45 W irradiation, the ZPLs are narrow and sharp,comparable to other emitter fabrication methods (e.g., FIB,AFM indentation) [14], [24].At 0.45 W laser power, the accumulated thermal stressexceeds the structural tolerance of hBN, rupturing the ini-tial bubble-like defects and creating holes with fracturededges (Figure 3(b)). This transition is accompanied by sig-nificant surface damage, dominated by atomic-scale vacan-cies at rupture sites. These vacancy-type defects predom-inantly emit in the shorter-wavelength regime (∼560 nm,Figures 3(c) and S7), consistent with reports attributing550–590 nm ZPLs to nitrogen vacancies in hBN [5], [50]Statistical analysis (Figure S7) reveals a pronouncedspectral shift: while bubble structures (low-power regime)host emitters with ZPLs clustered at 600–640 nm, the high-power regime (0.45 W) produces a dominant 550–590 nmemission band, with fewer than 10 % of emitters exceed-ing 600 nm. This power-dependent transition aligns withprior studies demonstrating that subablation fs laser ener-gies generate strain-tuned bubbles, whereas near-ablationthresholds produce vacancy-rich holes [12]. This starkcontrast underscores the role of laser power in mod-ulating defect populations: low-power regimes stabilizestrain-introduced defects with longer ZPLs, while high-power ablation generates vacancy-rich sites with shorterZPLs.2.2 Microsphere-enhanced optical signalcollectionBeyond using microspheres to deterministically generatehBN optical emitters with a tightly focused PNJ and reducedirradiated area, we further demonstrate that the sameMPMlayer can be employed as a versatile component for enhanc-ing optical signals. As shown in Figure 4, the MPM signifi-cantly boosts the intensity of both excitation and emissionsignals. This is not solely due to the focusing effect, butresults from two key factors: the efficient coupling of bothnear-field and far-field energy from the hBN optical emit-ters, and the additional enhancement provided by WGM,which together improve the transmission to the subsequentoptical components. This enhancement allows optical emit-ters to be imaged with low NA objectives (NA = 0.6, 50×)without requiring an oil-immersion high NA lens (NA > 1),enabling high-resolution imaging with a simpler, lower-costoptical setup.The incorporation of the MPM modifies the transmis-sion and distribution of light emitted by the optical emit-ters (Figure 4(a) and (b)), resulting in notable enhancementsin signal intensity and imaging resolution. These improve-ments are driven by two key mechanisms:(1) Spatial localization of the excitation area: The MPMconfines the laser focus to the submicron scale(Figure 1(g)), concentrating energy on a smaller,well-defined three-dimensional region. This enhancedfocus increases absorption efficiency at defect siteswhile minimizing excitation in nontarget areas,thereby reducing background noise and preventingunwanted excitation of neighboring emitters.Consequently, the overall excitation efficiency of thedesired emitters is significantly improved [51], [52].(2) Enhancement of the signal extraction efficiency:Whenemitters are excited outside the plane of the substrateand directly coupled into the MPM (Figure 4(c)), thelight is refracted and an enhancement arises fromtwo complementary mechanisms: (1) WGM: these trapRayleigh-scattered light within the microsphere, res-onantly amplifying emission rates via high-Q cavityeffects [36]. This is evidencedbynarrow spectral peaks,free spectral range (FSR) periodicity, and the refractiveindex (n)-dependent FSR trend (Figures 5(c), S8, andS9); (2) PNJ, which not only enhances excitation effi-ciency by focusing incident light but also improves col-lection efficiency by redirecting scattered light towardX. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBN — 2425Figure 4: Schematic of microsphere-enhanced fluorescence collection. (a) Illustration of the spatial emission distribution of hBN emitters withoutmicrosphere. The orange area shows the defect emission range, and the red arrows indicate the extreme angle of defect emission (we note that it willbe in 3D). (b) Illustration of the spatial emission distribution of hBN emitters with a microsphere, where emitted photons are coupled to the micro-sphere via WGM and re-radiated at an engineered angle that falls within the collection range of the objective lens (light gray area). (c) Simulation ofoptical WGM in the MPM system, enhancing the fluorescence signal of the emitters. The inset shows the spatial emission distribution of an emitterwithout a microsphere.a low-NA objective. While WGMs dominate the spec-tral features, nanojets facilitate efficient near-field tofar-field light convergence. This dual mechanism con-tributes to overall signal enhancement, particularlyin broadband emission, thereby significantly increas-ing the far-field photon extraction rate [53]. COMSOLsimulations indicate that this enhancement exceeds4,000 times at the top of the microsphere, as shown inTable S1. This mechanism optimizes the propagationpath of photons, improving their output efficiency andenhancing the fluorescence signal’s intensity and sta-bility [26].Through the synergistic effect of these mechanisms, theMPM can be used to significantly enhance the optical detec-tion of hBN emitters.TheMPMwith a single microsphere was first employedto quantitatively estimate the enhancement ratio by enhan-cing the hBN Raman peak. This measurement requires lesslocalization since the Raman peak can be detected any-where on the hBN flake surface. A laser beam was focusedusing a 50× objective with an NA of 0.6 onto the centerof the microsphere to achieve maximum enhancement.Figure 5(a) shows the Raman spectra obtained using MPMwith different gaps: 1 μmspacerwithmicrosphere (1 μm-SP-MS), 0.5 μm spacer with microsphere (0.5 μm-SP-MS), and0 μm spacer with microsphere (0 μm-SP-MS). Two controlgroupswere also included: onewith only PDMS and anotheruncovered hBN.With two control groups, we can determinethe original intensity of the Raman peak on the flat hBNsurface, and the peak contributed by PDMS.For MPM without a gap, the hBN Raman peak showedapproximately a factor 20 enhancement compared to thecase without MPM, but the enhancement factor decreasedto ∼4 and ∼2 for 0.5 μm and 1 μm gaps, respectively. Thisdemonstrates that most of the photon energy is lost infree space without microsphere-enhanced collection withinthese short distances. This also clarifies why, despite sim-ulations predicting a 4000-fold enhancement, real experi-ments achieve lower values, largely due to the challengeof perfectly aligning the emitter at the exact center of themicrosphere’s focal region (Figure S8(b), points A and B). Asa result, some enhancement is lost. Thus, achieving closeattachment of the MPM to the surface is crucial for maxi-mizing emission collection efficiency.We note that while the sameMPM chips can be used forboth fabrication and collection at an intermediate distanceof 0 and 6 μm between the microsphere and the substrate,the performance will be slightly compromised due to this2426 — X. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBNFigure 5: Signal enhancement from hBN flakes covered with MPM. (a) Processed Raman spectra of an hBN flake enhanced by MPM with varyingspacer distances. Inset: Schematic of the Raman measurement setup for the hBN flake covered by MPM. (b) Processed PL spectra of hBN emittersfabricated via MPM fs-laser, comparing MPM-covered and uncovered cases. Inset: Schematic of the PL measurement setup for emitters at the holeedge. (c) Raw PL emission spectra of WGM-regulated hBN emitters. Left inset: Zoom in on PL spectra highlighting enhanced peaks and FSR. Rightinset: Experimental versus theoretical FSR evolution across wavelengths.gap size. As shown in this study, varying this gap can opti-mize performance depending on the desired application,providing flexibility in choosing the most suitable gap sizefor either extraction or collection.Next, the MPM was attached directly to an emitter atthe hBN flake edge, created via microsphere fs-laser writ-ing. The microsphere serves as a small lens, forming a vir-tual image of the sample surface [54], allowing us to pre-cisely localize the emitters at the edge (inset, Figure 5(b);optical image in Figure S10). This ensures the MPM over-laps with and enhances emission from the region ofinterest.The MPM is mainly composed of BaTiO3, which hasa wide bandgap energy (∼3.2 eV) and does not supportlow-energy excitons, resulting in negligible optical loss inthe visible wavelength band [55]. This property makesBaTiO3 an excellent optical dielectric, enabling efficient lighttransmission and manipulation. In Figure 5(b), the PL spec-tra of the hBN emitter collected with (dark green) and with-out (light green) the MPM show a significant enhancementat 598 nm,with theMPMamplifying the PL signal by approx-imately a factor of 10.As shown in Figure 5(c), the periodic narrow peaksin the raw PL spectra confirm WGM contributions tofluorescence enhancement, with spectral intervals FSRmatching the theoretical model for dielectric microspheres:FSR ≈ 𝜆2∕2𝜋Rneff , where 𝜆 is the emission wavelength(550–750 nm), R is the microsphere radius, and neff isX. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBN — 2427the effective refractive index, which is approximated as√(n2 + n2medium)∕2, where n and nmedium are the refractiveindices of the microsphere (BaTiO3 glass, n = 1.90) and sur-rounding medium (PDMS, n = 1.41). The correlation coef-ficient (>0.98) between experimental and theoretical FSRacross wavelengths (Figure 5(c), inset) confirms the WGMorigin of the spectral periodicity.Systematic studies (Figure S9) demonstrate direct mod-ulation of FSR by different refractive index microspheres:for a microsphere with a given size (R = 25 μm), switchingfrom SodeLime Glass microspheres (n= 1.5) to BaTiO3 glassmicrospheres (n = 1.9 and n = 2.2) reduces the FSR from1.85 nm to 1.39 nm at 61 nm, consistent with the inversedependence on n in the WGM mode. The raw spectraldata in Figure 5(c) and refractive-index-dependent trends inFigure S9 collectively validate the role of WGMs.However, the PL peak enhancement ratio is lower com-pared to the Raman peak. Since Raman intensity scaleswith the fourth power of the electric field (E4), while PLscales with E2, the Raman signal benefits more from theenhanced field, leading to a higher enhancement ratio [56],[57]. Additionally, slight spatial misalignment between theemitter and the BaTiO3 microsphere could create a gap asshown in Figure S5(b), allowing some surface waves, suchas evanescent waves, to be lost into space rather than beingcollected by the microsphere and subsequent optical sys-tems. These factors may contribute to the reduced enhance-ment. Despite these factors, the observed PL enhancementdemonstrates the effectiveness of the MPM in improvingsignal strength,making full use of BaTiO3’s favorable opticalproperties.3 ConclusionsIn summary, our study demonstrates a new route enablingthe combined fabrication of hBN emitters and fluorescenceemission enhancement by using a microsphere chip asan effective and low-cost focusing lens. By combining theMPM with a fs-laser writer setup, we achieve a fivefoldreduction in the irradiated area, leading to better local-ization and higher quality emitters (i.e., smaller FWHM ofthe ZPLs) in hBN, and suppressing the extensive damagefound in microsphere-free fabrication. This approach notonly enables better control over the hBN defect genera-tion process but also substantially improves optical signalcollection efficiency by approximately 10 times comparedto microsphere-free measurements methods. The enhance-ment in defect absorption, combined with optimized pho-ton extraction and efficient light directionality imposedby the microsphere geometry, results in significantlystronger optical signal detection. We emphasize that thisproof-of-concept emitter fabrication and emission collec-tion with microsphere can be highly parallelized throughthe use of self-assembled microsphere arrays over large-area hBN surfaces. The MPM can also be integratedin microfluidics, enhancing the detection of fluorescentbiomolecules in physiological conditions. Furthermore, ourfindings canbe readily applied to other 2Dmaterials exhibit-ing optically active defects, setting the stage for furtherdevelopments in nanophotonics and fluorescence imagingat 2D material surfaces.4 Methods4.1 Sample preparation and laser irradiationMultilayer hBN flakes were first mechanically exfoliatedfrom bulk hBN crystals produced by high temperature andhigh-pressure synthesis (NIMS Japan) and then transferredonto SiO2/Si surface cleaned by ultrasonic sonication inacetone and IPA for 3 min and oxygen plasma cleaningfor 5 min. Using atomic force microscopy (AFM, Cypher),large-area hBN thin flakes with thickness of ∼50 nm wereselected. We use high-refractive index (n = 1.9) bariumtitanate microspheres (diameter = 50 μm, Cospheric, USA)embedded in PDMS (SYLGARD 184 Silicone Elastomer Kit,Germany) to generate the MPM. We inserted a ∼6 μmspacer consisting of double-sided tape (Zhuanyi ElectronicElechnology Co., SuZhou, China) between the MPM and thesubstrate.Subsequently, a single fs-laser pulse (Pharos PH1-15,Light Conversion) of wavelength 515 wavelength, pulseduration 290 fs and linearly polarization was focusedthrough a f-theta lens (F = 100 mm, 510–550 nm Fused Sil-ica Telecentric, VONJAN). The objective lens was mountedon a piezo nano positioning stage (PI E665) with nanome-ter resolution to precisely control the focal position. Afterlaser irradiation, the hBN samples were annealed in atube furnace at 1,000 ◦C in vacuum (4.6E-07 mbar) for2 h.4.2 Optical measurementRaman and PL spectra were acquired in Renishaw Ramansetup. The hBN samples were excited by a 514 nm argonlaser (MODU-LASER) with 0.15 mW. The spectra were col-lected with an objective lense (Olympus 50×) with numeri-cal aperture (NA) of 0.6 and a 1800 l/mmgrating. PLmappingwas performedwith the same setup and excitation laser, but2428 — X. Yang et al.: Microsphere-assisted generation of localized optical emitters in hBNusing a higher-resolution objective (Olympus 100×, NA 0.9)and a scanning step of 0.5 μm.4.3 Photonics simulationNumerical simulations of the electromagnetic field wereconducted using a finite-difference time-domain (FDTD)method in Ansys Lumerical FDTD for focusing analysis, anda finite element method (FEM) in COMSOL Multiphysicsfor optical WGMs. The simulations were performed overan area of 60 μm × 30 μm with a mesh size of 𝜆/100. Per-fectly matched layers and periodic boundary conditionswere applied to ensure accuracy, and the boundary match-ing layers were set as perfect absorption layers. The refrac-tive indices of the PDMS film and microspheres (diameter= 50 μm) were set to 1.41 and 1.9, respectively, with theincident light as a plane wave at 514 nm wavelength. Thesimulation conditions were consistent with those of theexperiment.Acknowledgments: The authors would like to acknowl-edge K. Watanabe and T. Taniguchi from the National Insti-tute of Materials Science (NIMS) for providing the bulk hBNcrystals. We also sincerely thank Binbin Zhang for his assis-tance with the WGM simulations and Zhenyuan Lin for hisinsights on fs-laser fabrication.Research funding: XY acknowledges funding from the Chi-nese Scholarship Council (ScholarshipNo. 202108270002). SCacknowledges funding from the European Union’s Horizon2020 research and innovation program (ERC StG, SIMPHON-ICS, Project No. 101041486).Author contributions: All authors have accepted respon-sibility for the entire content of this manuscript and con-sented to its submission to the journal, reviewed all theresults, and approved the final version of the manuscript.XY conceived the research idea, designed the study, con-ducted the experiments, developed the model code, per-formed the simulations, and drafted the manuscript. DHSsupported the experimental design and processes andreviewed the draft. KW and TT provided essential resourcesand materials for the experiments. PGS provided exper-imental support and reviewed the manuscript draft. SCsupervised the study, contributing significantly to thedesign, experimental setup, simulation development, dis-cussion, and manuscript review.Conflict of interest: Authors state no conflict of interest.Data availability: The datasets generated and/or analyzedduring the current study are available from the correspond-ing author upon reasonable request.References[1] M. Fernandez-Suarez and A. Y. Ting, “Fluorescent probes forsuper-resolution imaging in living cells,” Nat. Rev. Mol. Cell Biol.,vol. 9, no. 12, pp. 929−943, 2008..[2] X. Yang, et al., “Molecule fluorescent probes for sensing andimaging analytes in plants: developments and challenges,” Coord.Chem. Rev., vol. 487, p. 215154, 2023..[3] P. Steinleitner, et al., “Direct observation of ultrafast excitonformation in a monolayer of WSe(2),” Nano Lett., vol. 17, no. 3,pp. 1455−1460, 2017..[4] L. J. Martínez, et al., “Efficient single photon emission from ahigh-purity hexagonal boron nitride crystal,” Phys. Rev. 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Hecht, Principles of Nano-Optics, Cambridge,Cambridge University Press, 2012.Supplementary Material: This article contains supplementary material(https://doi.org/10.1515/nanoph-2024-0625).https://doi.org/10.1515/nanoph-2024-0625 1 Introduction 2 Results and discussion 2.1 Microsphere-assisted laser fabrication 2.2 Microsphere-enhanced optical signal collection 3 Conclusions 4 Methods 4.1 Sample preparation and laser irradiation 4.2 Optical measurement 4.3 Photonics simulation<<  /ASCII85EncodePages false  /AllowTransparency false  /AutoPositionEPSFiles true  /AutoRotatePages /None  /Binding /Left  /CalGrayProfile (Dot Gain 20%)  /CalRGBProfile (sRGB IEC61966-2.1)  /CalCMYKProfile (Euroscale Coated v2)  /sRGBProfile (sRGB IEC61966-2.1)  /CannotEmbedFontPolicy /Warning  /CompatibilityLevel 1.7  /CompressObjects /Tags  /CompressPages true  /ConvertImagesToIndexed true  /PassThroughJPEGImages false  /CreateJobTicket false  /DefaultRenderingIntent /Default  /DetectBlends true  /DetectCurves 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