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Shun Fujii, Nan Fang, Daiki Yamashita, [Daichi Kozawa](https://orcid.org/0000-0002-0629-5589), Chee Fai Fong, Yuichiro K. Kato

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[van der Waals Decoration of Ultra-High-<i>Q</i> Silica Microcavities for χ<sup>(2)</sup>–χ<sup>(3)</sup> Hybrid Nonlinear Photonics](https://mdr.nims.go.jp/datasets/29f580d4-4d17-40ac-8b15-74551def7906)

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van der Waals Decoration of Ultra-High-Q Silica Microcavities for χ(2)–χ(3) Hybrid Nonlinear Photonicsvan der Waals Decoration of Ultra-High‑Q Silica Microcavities forχ(2)−χ(3) Hybrid Nonlinear PhotonicsShun Fujii,* Nan Fang, Daiki Yamashita, Daichi Kozawa, Chee Fai Fong, and Yuichiro K. Kato*Cite This: Nano Lett. 2024, 24, 4209−4216 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Optical nonlinear processes are indispensable in awide range of applications, including ultrafast lasers, microscopy,and quantum information technologies. Among the diversenonlinear processes, second-order effects usually overwhelm thehigher-order ones, except in centrosymmetric systems, where thesecond-order susceptibility vanishes to allow the use of the third-order nonlinearity. Here we demonstrate a hybrid photonicplatform whereby the balance between second- and third-ordersusceptibilities can be tuned flexibly. By decorating ultra-high-Qsilica microcavities with atomically thin tungsten diselenide, weobserve cavity-enhanced second-harmonic generation and sum-frequency generation with continuous-wave excitation at a powerlevel of only a few hundred microwatts. We show that the coexistence of second- and third-order nonlinearities in a single device canbe achieved by carefully choosing the size and location of the two-dimensional material. Our approach can be generalized to othertypes of cavities, unlocking the potential of hybrid systems with controlled nonlinear susceptibilities for novel applications.KEYWORDS: two-dimensional materials, ultra-high-Q microcavities, second-harmonic generation, nonlinear optics,transition metal dichalcogenidesSince the landmark discovery of second-harmonic gen-eration (SHG)1 enabled by the invention of lasers,2nonlinear optics have played a central role in the developmentof diverse photonics applications. Frequency conversionprocesses are particularly important, being extensivelyemployed in ultrafast optics,3 metrology,4,5 quantum stategeneration,6,7 and microscopy.8,9 To achieve these function-alities, both second- and third-order processes, such as SHG,third-harmonic generation (THG), sum-frequency generation(SFG), parametric downconversion, and four-wave mixing(FWM), are utilized.With such a variety of nonlinear effects, combinations offrequency conversion processes would allow for a more flexiblespectral synthesis. The efficiencies of nonlinear processesdepend directly on the nonlinear susceptibility of conversionmedia, but the origins are markedly different for second- andthird-order susceptibilities. An essential requirement forsecond-order nonlinear processes to occur is inversionsymmetry breaking, and typical materials include dielectriccrystals (for example, lithium niobate and β-barium borate),III−V semiconductors, and organic crystals.10 Althoughsecond- and third-order nonlinearities can coexist in nanoscalestructures such as dielectric nanoparticles,11 nanocrystals,12and layered nanomaterials,13,14 conventional nonlinear opticalmaterials with broken inversion symmetry exhibit strongsecond-order susceptibility that overwhelms other higher-order nonlinearities. Conversely, second-order nonlinearsusceptibility vanishes in centrosymmetric crystals andamorphous materials (e.g., liquids, gases, and amorphoussolids), and only third-order processes can be utilized in theseχ(3) materials.In this regard, one promising strategy is to establish a hybridsystem by combining a noncentrosymmetric nonlinear materialwith an ultra-high-Q microcavity fabricated from a χ(3)material.15 The strength of second-order processes can becontrolled through mode overlap with the noncentrosym-metric material, while exceptional enhancement of opticaldensity in the tiny mode space can be facilitated to boost thethird-order process to a practical level.As a candidate system, we propose ultra-high-Q silicamicrocavities decorated by transition metal dichalcogenides(TMDs). Silica whispering-gallery microcavities boast ultra-high Q values (>108) that ensure high-circulating opticalintensities essential for inducing various optical nonlinearprocesses16−23 with a moderate continuous-wave (CW)excitation. Meanwhile, monolayer TMDs possess a magnitudeReceived: January 18, 2024Revised: March 19, 2024Accepted: March 20, 2024Published: April 1, 2024Letterpubs.acs.org/NanoLett© 2024 The Authors. Published byAmerican Chemical Society4209https://doi.org/10.1021/acs.nanolett.4c00273Nano Lett. 2024, 24, 4209−4216This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on August 9, 2024 at 09:23:34 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shun+Fujii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Nan+Fang"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Daiki+Yamashita"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Daichi+Kozawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Chee+Fai+Fong"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuichiro+K.+Kato"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.4c00273&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/24/14?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/14?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/14?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/14?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c00273?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/of second-order nonlinearity comparable to that of commonlyused nonlinear crystals24−26 and are thus expected to be usedfor practical nonlinear applications.27−32 Their atomically thinnature gives them mechanical flexibility to conform to thesurface of the optical microcavities, and the van der Waalscharacter makes them compatible for the heterogeneousinterface.33−36Here, we demonstrate a novel nonlinear photonic platformby decorating ultra-high-Q silica microspheres with tungstendiselenide (WSe2). Atomically thin layers of the two-dimen-sional (2D) material are transferred onto the cavity with aminimal level of scattering loss. Cavity-enhanced second-harmonic (SH) generation is achieved by CW excitation withonly a few hundreds of microwatts because of the strong light−matter interactions between a resonant optical field andintegrated WSe2. We also observe efficient SFG with a two-color excitation scheme. In addition, the pump powerdependence shows self-locking of the SH output, revealingthe mechanism of the dynamic phase-matching process. It isconfirmed that the SH process occurs for only odd layernumbers, and the coexistence of second- and third-ordernonlinearities in a single device is achieved by controlling thesecond-order susceptibility of the device.Figure 1a shows a conceptual illustration of a 2D material-decorated silica microcavity, capable of serving as a second-order nonlinear photonic platform. Strong light−matterinteraction assisted by cavity resonance permits efficientnonlinear optical processes that originate from the atomicallythin layered material with low-power CW excitation. Thefrequency-converted light that resonates with another longi-tudinal resonance mode, in a situation termed a doublyresonant condition, allows the cavity-enhanced signals tocouple to the same waveguide coupler utilized for excitation.The normalized mode intensity of a microsphere cavity isshown in Figure 1b, where the inset shows the optical modeprofile (the relationship between the evanescent field ratio andthe cavity radius is further detailed in the SupportingInformation).We first decorate a silica microsphere cavity (diameter of∼80 μm) by transferring mechanically exfoliated monolayerWSe2 onto the cavity surface using the polydimethylsiloxane(PDMS)-assisted dry-transfer technique.37 The layer numbersof WSe2 flakes are identified either through photoluminescence(PL) measurement38 or by optical contrast in microscopeimages prior to the transfer.39 Figure 1c shows a false-colorimage of the WSe2-decorated silica microsphere cavity (detailsof sample fabrication and interaction length presented in theSupporting Information).To characterize the influence of the WSe2 flake on the Qfactor of a microcavity, we compare the transmission spectrabefore and after the transfer process. The experimental setup ispresented in Figure 1d. All resonances observed within therange of 1530−1570 nm are numerically fitted to a Lorentzianfunction. This allows for the statistical analysis of the loadedfull width at half-maximum line width (=ω/Q) as shown inpanels e and f of Figure 1. The median value in a pristine (i.e.,before transfer) microsphere is 2 MHz, which corresponds toan ultrahigh Q factor of 1 × 108. After the transfer of a WSe2flake, the most probable loaded line width broadens toapproximately 40 MHz, corresponding to a Q factor of 5 × 106even though the highest Q values are ∼107.Figure 1. van der Waals decoration of a high-Q silica microcavity by atomically thin 2D material. (a) Conceptual illustration of a monolayermaterial-integrated silica microcavity realizing strong light−matter interaction. (b) Simulated normalized intensity of the optical mode across theequator of a silica microsphere with a radius of 40 μm. The calculations are conducted by using finite element method (FEM) software (COMSOLMultiphysics). The cavity modes exhibit a slight difference in the profiles, and transverse-magnetic (TM) modes exhibit evanescent fields slightlyhigher than transverse-electric (TE) modes. The inset shows the optical mode profile, where the white line indicates the boundary between thesilica and surrounding air. (c) False-color scanning electron micrograph image of a WSe2-integrated high-Q microsphere. The image has beenretouched to show WSe2 in red and the microsphere in yellow. The scale bar is 50 μm. (d) Experimental setup. Abbreviations: FPC, fiberpolarization controller; DUT, device under test; PD, photodetector; DAQ, data acquisition; OSA, optical spectrum analyzer; CCD, charged-coupled device installed in a spectrometer. (e and f) Histograms of cavity line widths in pristine and decorated microcavities, respectively. Thedegradation of Q factors is mainly attributed to an increase in surface scattering loss.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c00273Nano Lett. 2024, 24, 4209−42164210https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c00273/suppl_file/nl4c00273_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c00273/suppl_file/nl4c00273_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c00273/suppl_file/nl4c00273_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c00273?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asThe degradation in the Q factor is likely due to an increasein scattering loss resulting from the decoration, which is alsoobserved in the integration of materials into other nano-photonic cavities.33,40,41 We anticipate a minimal effect on theQ factor from the absorption loss caused by the WSe2 flakebecause the telecom band photon energy is significantly lowerthan the bandgap of monolayer WSe2 (∼1.75 eV). For thesame reason, we do not expect significant damage to the 2Dmaterial. It should be noted that the uniformity of transferredflakes is the key to maintaining high Q factors as well as theflake size and the transferred position, and placing a small flakeaway from the equator of a microcavity would greatly reducethe scattering loss in high-Q modes. For most of this study,however, we place priority on using uniform and large WSe2flakes and transfer onto the equator of the device to maximizethe interaction length between the optical modes and the WSe2material.Figure 2 presents optical spectra in the visible andcorresponding pump wavelength bands. By carefully tuningthe pump laser wavelength to a cavity resonance with a pumppower of 500 μW, we clearly observe second-harmonic (SH)light (Figure 2a,b). The frequency of the SH light (773.1 nm)exactly matches twice the pump frequency (1545.5 nm) with awavelength error of only 0.045%, and this fact confirms theoccurrence of a frequency-doubling process via second-orderoptical nonlinearity. We stress that other third-order (Kerr)nonlinear processes, which could arise from bulk silicamicrocavities, are absent in this experiment, because thethreshold powers are far beyond our pump power level. Therequired pump powers for FWM and Raman oscillation are12.6 and 36.1 mW, respectively, in the case of a loaded Qfactor of 5 × 106, as threshold powers of these processes scaleas V/Q2.17,18Next, we pump the device by using two CW lasers withdifferent frequencies (i.e., two-color excitation) at submilliwattpump powers. This scheme allows us to observe SFG as shownin panels c and d of Figure 2. A two-color pump imposes atriply resonant condition on the sum-frequency process to bephase-matched, but it is easy to find the phase-matchingcondition by slowly tuning one laser while keeping thefrequency of the other laser within a high-Q resonance. Panelse and f of Figure 2 show a unique example, where two SH andFigure 2. Observation of second-order nonlinear processes in material integrated microresonators. (a and b) Optical spectra of pump wavelengthand generated SH light. The frequency of the SH light exactly matches twice the frequency of the pump light, indicating the nonlinear frequency-doubling process. The energy diagram of an SH process is shown in the inset. (c and d) Optical spectra of two different pump wavelengths andgenerated second-order sum-frequency (SF) light. The frequency of SF light corresponds to the sum of pump frequencies as depicted in the inset.(e and f) Measured spectra of pump wavelengths and corresponding visible light, where the two-color excitation scheme enables simultaneousgeneration of SFG and SHG. The difference in signal powers is due to the phase-matching condition for near-infrared and visible cavity modes. (g)Optical spectra of SHG-mediated photoluminescence (PL) emission in a WSe2-decorated cavity. The SH light at a wavelength of 715 nm excitesexcitonic PL in a monolayer WSe2, where the broad emission is optically coupled to numerous cavity modes. The inset shows the energy diagram ofthe process. Abbreviations: CB, conduction band; VB, valence band.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c00273Nano Lett. 2024, 24, 4209−42164211https://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c00273?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asone SF light are generated from two laser inputs because of 5-fold resonant triple-phase matching.In addition to second-order nonlinearities, we also observedexcitonic photoluminescence (PL) from the monolayer WSe2.Figure 2g shows a spectrum of SHG at a wavelength of 715 nmand the associated PL emission when the device is pumped at awavelength of 1530 nm. The multiple spikes seen in the PLspectrum indicate that broad excitonic PL couples to the high-Q cavity modes and the intensities are enhanced due to thePurcell effect or modulated by the differing collectionefficiencies. The energy diagram is depicted in the inset ofFigure 2g. We emphasize that the observation of this uniqueresonance energy transfer, i.e, SHG-mediated PL andsubsequent resonant enhancement, has become possible onlywith our WSe2-decorated high-Q devices. We note that there isa possibility of two-photon absorption PL simultaneouslyoccurring under the infrared excitation,42 while it is difficult todistinguish these processes from optical spectra. This resultalso proves the strong interaction between a monolayer WSe2and whispering-gallery modes via an evanescent field.The dynamic phase matching is highlighted in the pumppower dependence of the SH power, as shown in Figure 3a(see the Supporting Information for details of the dynamicphase-matching process). We measure the SH power for thesame cavity mode and carefully tune the pump wavelength sothat the SH light is maximized at each pump power. Thismeasurement scheme allows us to find the perfect phase-matching condition at a certain pump power, which can bedynamically altered by the nonlinear resonance shifts. Thedouble logarithm plot is presented in the inset of Figure 3a,where three distinct regimes can be recognized. Below a pumppower of ∼2 mW, the SH powers exhibit a linear slope of ∼2.2,which is very close to the anticipated slope of 2 for an SHGprocess. As the pump power is increased from 2 to 4.5 mW,the fitted slope drastically changes to ∼5.5, and a furtherincrease in the pump power (>4.5 mW) induces saturation ofthe SH power. Such a kink behavior of the SHG intensity hasnot been reported in conventional SHG measurements ofTMD flakes on substrates43,44 or photonic nanostruc-tures.31,41,45,46We therefore consider the influence of the dynamic phase-matching condition in a double-resonance system. Figure 3bshows the schematic for the mechanism under consideration,where the SH light is blue-detuned at low pump powers. Inthis scenario, SH light is considered to be almost in an off-resonance condition with a large detuning [state (i)], yieldinga moderate conversion efficiency with a slope of ∼2. As thepump power increases, thermal and Kerr nonlinearities inducea significant red-shift of the resonances.47 While the frequencyof the SH light is twice the pump frequency (i.e., ωSH = 2ωp),resonance mode ω2 for SH generally shows shifts smaller thanthose of SH light (Δω2 < ΔωSH) due to the imperfect modeFigure 3. SH power dependence on the pump power. (a) Maximum SH power as a function of pump power for the same cavity mode. The data arepresented in a log−log scaling with power-law fits in the inset. The SH powers exhibit a slope of ∼2.2 at the relatively low pump power regime (<2mW), but the slope increases to ∼5.5 in the intermediate region (2−4.5 mW). In the high-pump power region (>4.5 mW), the SH power starts tosaturate and remains stable. The error bars correspond to the standard deviation of 10 repeated measurements. (b) Schematic for the mechanism ofthe dynamic phase-matching process. (c−e) Spectral mapping for different pump powers. The SH power exhibits a significant dependence onpump powers of 0.5, 1, and 2 mW, respectively. The color maps are normalized to a common scale.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c00273Nano Lett. 2024, 24, 4209−42164212https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c00273/suppl_file/nl4c00273_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c00273?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asoverlap between the pump and SH modes.48 The detuning ofthe SH light therefore decreases at a higher pump power,leading to a rapid increase in conversion efficiency [state (ii)].Once the SH power reaches its maximum when both cavitymodes exactly match the on-resonance condition [state (iii)], afurther increase in intracavity power results in the red detuningof SH light, which would reduce the output [state (iv)]. Themaximum SH power for higher pump powers would then beobtained for the specific intracavity power, where the double-resonance condition is retained. Because the intracavity poweris almost constant, the SH power saturates despite a furtherincrease in pump power. Such a complex power dependency isclearly observed in a separate experiment, where we record SHsignals while continuously scanning the pump laser frequencyat a certain pump power. As shown in Figure 3c−e, the SHsignal becomes more and more frequent in the spectral map,and the intensity is drastically enhanced with an increase inpump power. Given the presence of numerous higher-ordermodes in silica microspheres, the existence of fundamental andSH modes that fulfill the phase-matching condition is plausiblefor different order modes.16,20 We note that no pumppolarization dependence is observed (extended data presentedin the Supporting Information).It is possible to calculate the conversion efficiency from thedata depicted in Figure 3a. When we define PSH as the detectedSH power, the calculated maximum conversion efficiency(PSH/Pp2) is 6.6 × 10−4 % W−1 with a pump power Pp of 4.5mW. It should be noted that the internal (intracavity)conversion efficiency is expected to be >1 order of magnitudehigher than the value presented above because the waist of thenanofiber waveguide is optimized to the pump wavelengthband in this experiment, thus resulting in the poor couplingefficiency of SH light due to the phase mismatch between thevisible band and the nanofiber coupler.49,50 We note that thecollection efficiency can be improved by employing anadditional nanofiber designed for SH wavelengths, i.e., add−drop configuration,22,48 or by exploiting a chaotic channel indeformed microcavities.50As mentioned above, symmetry plays an important role indetermining the nonlinear susceptibility, and therefore, thenumber of layers in the two-dimensional material is a crucialfactor. The WSe2 crystals used in this work possess the 2H-phase (semiconducting) structure, which is more stable thanother crystal phases. The 2H-phase TMD crystals belonging tospace group D3h exhibit substantial second-order nonlinearityfor only odd layer numbers, whereas the χ(2) nonlinearityvanishes in even layer numbers because the net nonlineardipoles are canceled out due to inversion symmetry.43,44Considering these selection rules, we performed a comparativeexperiment in four different devices.Figure 4 shows the mapping of SH spectra in the visiblewavelength region when the pump wavelength is scanned from1500 to 1600 nm with a pump power of 3 mW. As weanticipate, strong SH light appears only in the ML and 3L-WSe2 devices (Figure 4b,d), whereas there is no distinct signalin the pristine and 2L-WSe2 devices (Figure 4a,c). This is clearevidence that second-order nonlinearity originates from theintegrated WSe2, not from intrinsic surface symmetry breakingof the cavity material.48 In a pristine device, third-orderprocesses such as THG and third-order SFG associated withFigure 4. Layer dependence of SH light intensity. (a) Spectral mapping of the signal intensity in the visible wavelength region in a pristine silicamicrosphere. Third-order nonlinear processes (e.g., THG and TSFG) are observed in the wavelength range of 500−600 nm, whereas no strongsignal appears in the wavelength range of 740−810 nm due to the absence of the second-order nonlinearity. (b) Spectral mapping in a monolayerWSe2-decorated microcavity. The strong SH signals are observed over a wide range of pump wavelengths (1500−1600 nm). (c) Mapping in a 2L-WSe2-decorated sample. No clear SH light is measured in the map because the inversion symmetry exists in a 2H-stacked bilayer WSe2; thus, theχ(2) nonlinearity vanishes. (d) Mapping in a 3L-WSe2 device. The distinct SH light is observed again similar to the monolayer case because theinversion symmetry is broken for 3L-WSe2 crystal structures. The measurements are performed with a fixed pump power of 3 mW.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c00273Nano Lett. 2024, 24, 4209−42164213https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c00273/suppl_file/nl4c00273_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c00273?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aspump, FWM, and stimulated Raman scattering (SRS) areobserved in the range of 500−620 nm (Figure 4a, left) becauseof the unaltered ultrahigh Q factors (>5 × 107). We find thatthe number of SH signal peaks in the map is surprisingly highin both ML- and 3L-WSe2 devices even though the Q factors ofmost resonances are not as high as 107. It should be noted thatthe size of the flakes (i.e., interaction length) is approximatelythe same for each experiment. We attribute the efficient, highlypopulated SHG to giant second-order nonlinearity of TMDmaterials and relaxed resonant phase-matching condition dueto cavity line width broadening. If we could achieve muchhigher Q factors with a larger overlap between the cavity modeand the material, the conversion efficiency is expected tosubstantially increase; nevertheless, the resonant phase-matching condition would become stricter as a trade-off.We have shown thus far the results focused on theemergence of second-order nonlinearity, but one keyadvantage of this technique is its flexible controllability ofnonlinear susceptibility. By carefully controlling the transferredposition and the flake size of materials, we can tune the balancebetween second- and third-order nonlinearity. Here, weintentionally place a small flake (width of <10 μm) awayfrom the equator of a cavity to keep the Q factors high enough(>107) to simultaneously observe both second- and third-ordernonlinear processes in the same device. A flake position a fewmicrometers (corresponding to the scale of the cavity modeprofile) from the equator balances the Q factors and efficientinteraction with cavity modes.Panels a and b of Figure 5 show the observed optical spectrain the pump and the visible wavelength bands in this WSe2-decorated microcavity. In the pump wavelength band, FWMsidebands are observed in the vicinity of the pump light and afew Raman peaks can be recognized around 1630−1670 nm,which coincides with the Raman gain band of silica.18 For thevisible wavelength band, the peaks around 520−600 nm arisefrom THG and third-order SFG processes involving the peaksseen in the pump band. In particular, the pump and severalRaman peaks allow a variety of sum-frequency combinations,resulting in multiple emissions in this regime. The signalsaround 600 nm are believed to involve a cascaded Ramanprocess.20,22,23 While these signals originate from third-ordernonlinearity, the strong signal at a wavelength of 772 nmcorresponds to the SH light of the pump light via second-ordernonlinearity induced by monolayer WSe2. The spectral map isshown in Figure 5c, where the strong visible light is recognizedas a result of the simultaneous generation of second- and third-order processes. The signals around 780−800 nm come fromthe second-order SFG process of the pump and Ramancomponents, which are not observed in the previousexperiments (Figure 4).In conclusion, we have demonstrated a novel approach forintroducing second-order optical nonlinearity in ultra-high-Qsilica microcavities through decoration by a two-dimensionalmaterial. Via integration of atomically thin TMD layers withbroken crystal inversion symmetry onto the surface ofamorphous silica microspheres, cavity-enhanced SHG andSFG arise from strong light−matter interactions via evanescentfields. The cavity-enhanced PL emission mediated by the SHGprocess reveals the distinct optical coupling between SH lightand the excitonic resonance of the monolayer WSe2. Theconversion efficiency of SH light is strongly dependent on thepump power as a result of the dynamic phase-matchingprocess, leading to a drastic increase and saturation of the SHpower. A carefully coordinated clean-stamp transfer techniqueallows for investigation of the layer number dependence, aswell as manipulation of the relative strength of the second- andthird-order optical nonlinearity in the device.Practical levels of second-order nonlinearity in χ(3) materialshave long been strongly desired. Surface symmetry break-ing48,51 and photoinduced effects52,53 can introduce second-order nonlinear susceptibility but are limited in various aspects.In comparison, this study offers a powerful way to controllablyenhance optical nonlinearity in high-Q microcavities throughthe size and placement of the 2D material, which would causebreak throughs in nonlinear optics. The results presented inthis work lead to an anticipation that optical nonlinearity canbe artificially designed in hybrid systems, where variousnonlinear processes are combined to implement unconven-tional functionalities.In addition, we note that this approach can be extended toother centrosymmetric high-Q cavity devices, includingintegrated ring resonators made of silicon or silicon nitride(Si3N4), and thus paves the way to few-photon coherentnonlinear optics and quantum photon manipulation in variousplatforms. The combination of ultra-high-Q cavities withnanomaterials opens up a novel regime in the investigation ofoptical processes at high fields under CW excitation,potentially leading to intriguing physical phenomena as wellas nanophotonic applications.Figure 5. Coexistence of second- and third-order nonlinearities. (aand b) Optical spectra of telecom and visible wavelength regions,respectively. The frequency-converted light is generated via χ(3)nonlinearity in the pump band, resulting in the complex spectrumin the visible band. (c) Spectral mapping of signal intensities in thetwo different wavelength bands, from 740 to 820 nm and from 500 to560 nm. The distinct signals are observed in both bands, revealing thecoexistence of second- and third-order nonlinearities.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c00273Nano Lett. 2024, 24, 4209−42164214https://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273?fig=fig5&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c00273?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273.Simulation of mode profiles, sample fabrication, cavitytransmission spectrum, and polarization dependence ofthe SH intensity (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsShun Fujii − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,Japan; Department of Physics, Faculty of Science andTechnology, Keio University, Yokohama 223-8522, Japan;orcid.org/0000-0002-0998-366X; Email: shun.fujii@phys.keio.ac.jpYuichiro K. Kato − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,Japan; Nanoscale Quantum Photonics Laboratory, RIKENCluster for Pioneering Research, Saitama 351-0198, Japan;orcid.org/0000-0002-9942-1459; Email: yuichiro.kato@riken.jpAuthorsNan Fang − Nanoscale Quantum Photonics Laboratory,RIKEN Cluster for Pioneering Research, Saitama 351-0198,Japan; orcid.org/0000-0002-1053-1900Daiki Yamashita − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,Japan; Platform Photonics Research Center, NationalInstitute of Advanced Industrial Science and Technology(AIST), Ibaraki 305-8568, Japan; orcid.org/0000-0002-6970-4677Daichi Kozawa − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,Japan; Nanoscale Quantum Photonics Laboratory, RIKENCluster for Pioneering Research, Saitama 351-0198, Japan;Research Center for Materials Nanoarchitectonics, NationalInstitute for Materials Science, Ibaraki 305-0044, Japan;orcid.org/0000-0002-0629-5589Chee Fai Fong − Nanoscale Quantum Photonics Laboratory,RIKEN Cluster for Pioneering Research, Saitama 351-0198,Japan; orcid.org/0000-0003-1676-4665Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.4c00273Author ContributionsS.F. and Y.K.K. conceived and designed the experiments. S.F.carried out sample preparation, numerical simulation, andexperimental measurements. N.F. assisted in the transfer ofmaterials, and D.Y., D.K., and C.F.F. aided the construction ofthe measurement setup. S.F. and Y.K.K. wrote the manuscriptwith input from all authors. Y.K.K. supervised the project.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work is supported by JSPS (KAKENHI JP22H01893,JP22K14623, JP22K14624, JP22K14625, and JP23H00262).C.F.F. is supported by the RIKEN Special PostdoctoralResearcher Program. The authors thank the AdvancedManufacturing Support Team at RIKEN for technicalassistance and H. Kumazaki for preparing experimental setups.■ REFERENCES(1) Franken, P. A.; Hill, A. E.; Peters, C. W.; Weinreich, G.Generation of Optical Harmonics. Phys. Rev. Lett. 1961, 7, 118−119.(2) Maiman, T. H. Stimulated Optical Radiation in Ruby. Nature1960, 187, 493−494.(3) Armstrong, J. A. Measurement of picosecond laser pulse widths.Appl. Phys. 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