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Lettervan der Waals Decoration of Ultra-High‑Q Silica Microcavities forχ(2)−χ(3) Hybrid Nonlinear Photonics
Shun Fujii,* Nan Fang, Daiki Yamashita, Daichi Kozawa, Chee Fai Fong, and Yuichiro K. Kato*
Cite This: Nano Lett. 2024, 24, 4209−4216

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sı Supporting Information*

ABSTRACT: Optical nonlinear processes are indispensable in awide range of applications, including ultrafast lasers, microscopy,
and quantum information technologies. Among the diverse
nonlinear processes, second-order effects usually overwhelm thehigher-order ones, except in centrosymmetric systems, where thesecond-order susceptibility vanishes to allow the use of the thirdorder nonlinearity. Here we demonstrate a hybrid photonic
platform whereby the balance between second- and third-ordersusceptibilities can be tuned flexibly. By decorating ultra-high-Q
silica microcavities with atomically thin tungsten diselenide, we
observe cavity-enhanced second-harmonic generation and sumfrequency generation with continuous-wave excitation at a power
level 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 dichalcogenides

S

ince the landmark discovery of second-harmonic generation (SHG)1 enabled by the invention of lasers,2nonlinear optics have played a central role in the development
of diverse photonics applications. Frequency conversion
processes are particularly important, being extensively
employed in ultrafast optics,3 metrology,4,5 quantum state
generation,6,7 and microscopy.8,9 To achieve these functionalities, 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 flexible
spectral synthesis. The efficiencies of nonlinear processes
depend directly on the nonlinear susceptibility of conversion
media, but the origins are markedly different for second- andthird-order susceptibilities. An essential requirement forsecond-order nonlinear processes to occur is inversion
symmetry breaking, and typical materials include dielectric
crystals (for example, lithium niobate and β-barium borate),
III−V semiconductors, and organic crystals.10 Althoughsecond- and third-order nonlinearities can coexist in nanoscale
structures such as dielectric nanoparticles,11 nanocrystals,12and layered nanomaterials,13,14 conventional nonlinear opticalmaterials with broken inversion symmetry exhibit strong
second-order susceptibility that overwhelms other higherorder nonlinearities. Conversely, second-order nonlinear© 2024 The Authors. Published byAmerican Chemical Society

susceptibility 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 hybrid
system by combining a noncentrosymmetric nonlinear material
with 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 noncentrosymmetric 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 silica
microcavities decorated by transition metal dichalcogenides(TMDs). Silica whispering-gallery microcavities boast ultrahigh Q values (>108) that ensure high-circulating opticalintensities essential for inducing various optical nonlinear
processes16−23 with a moderate continuous-wave (CW)
excitation. Meanwhile, monolayer TMDs possess a magnitude
Received: January 18, 2024
Revised: March 19, 2024
Accepted: March 20, 2024
Published: April 1, 20244209

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LetterFigure 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 slightly
higher 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, chargedcoupled device installed in a spectrometer. (e and f) Histograms of cavity line widths in pristine and decorated microcavities, respectively. The
degradation of Q factors is mainly attributed to an increase in surface scattering loss.

of second-order nonlinearity comparable to that of commonlyused nonlinear crystals24−26 and are thus expected to be used
for practical nonlinear applications.27−32 Their atomically thinnature gives them mechanical flexibility to conform to thesurface of the optical microcavities, and the van der Waals
character makes them compatible for the heterogeneousinterface.33−36
Here, we demonstrate a novel nonlinear photonic platformby decorating ultra-high-Q silica microspheres with tungsten
diselenide (WSe2). Atomically thin layers of the two-dimensional (2D) material are transferred onto the cavity with aminimal level of scattering loss. Cavity-enhanced secondharmonic (SH) generation is achieved by CW excitation with
only 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 twocolor excitation scheme. In addition, the pump power
dependence shows self-locking of the SH output, revealing
the mechanism of the dynamic phase-matching process. It is
confirmed that the SH process occurs for only odd layer
numbers, 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 materialdecorated silica microcavity, capable of serving as a secondorder 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. The
frequency-converted light that resonates with another longitudinal resonance mode, in a situation termed a doubly

resonant 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 mode
profile (the relationship between the evanescent field ratio andthe cavity radius is further detailed in the Supporting
Information).
We first decorate a silica microsphere cavity (diameter of∼80 μm) by transferring mechanically exfoliated monolayer
WSe2 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 microscope
images prior to the transfer.39 Figure 1c shows a false-color
image of the WSe2-decorated silica microsphere cavity (detailsof sample fabrication and interaction length presented in the
Supporting Information).
To characterize the influence of the WSe2 flake on the Q
factor of a microcavity, we compare the transmission spectra
before 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 loaded
full 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 WSe2
flake, the most probable loaded line width broadens toapproximately 40 MHz, corresponding to a Q factor of 5 × 106
even though the highest Q values are ∼107.
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LetterFigure 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 frequencydoubling 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 simultaneous
generation 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 excites
excitonic 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.

The degradation in the Q factor is likely due to an increasein scattering loss resulting from the decoration, which is also
observed in the integration of materials into other nanophotonic cavities.33,40,41 We anticipate a minimal effect on the
Q factor from the absorption loss caused by the WSe2 flake
because the telecom band photon energy is significantly lower
than the bandgap of monolayer WSe2 (∼1.75 eV). For thesame reason, we do not expect significant damage to the 2D
material. It should be noted that the uniformity of transferred
flakes is the key to maintaining high Q factors as well as theflake size and the transferred position, and placing a small flake
away from the equator of a microcavity would greatly reduce
the scattering loss in high-Q modes. For most of this study,
however, we place priority on using uniform and large WSe2
flakes and transfer onto the equator of the device to maximize
the interaction length between the optical modes and the WSe2
material.
Figure 2 presents optical spectra in the visible andcorresponding pump wavelength bands. By carefully tuning
the pump laser wavelength to a cavity resonance with a pump
power 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-order
optical nonlinearity. We stress that other third-order (Kerr)
nonlinear processes, which could arise from bulk silica
microcavities, are absent in this experiment, because thethreshold powers are far beyond our pump power level. The
required pump powers for FWM and Raman oscillation are
12.6 and 36.1 mW, respectively, in the case of a loaded Q
factor of 5 × 106, as threshold powers of these processes scale
as V/Q2.17,18
Next, we pump the device by using two CW lasers withdifferent frequencies (i.e., two-color excitation) at submilliwatt
pump powers. This scheme allows us to observe SFG as shownin panels c and d of Figure 2. A two-color pump imposes a
triply 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 and4211

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LetterFigure 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 of
the dynamic phase-matching process. (c−e) Spectral mapping for different pump powers. The SH power exhibits a significant dependence on
pump powers of 0.5, 1, and 2 mW, respectively. The color maps are normalized to a common scale.

one SF light are generated from two laser inputs because of 5fold resonant triple-phase matching.
In addition to second-order nonlinearities, we also observed
excitonic photoluminescence (PL) from the monolayer WSe2.
Figure 2g shows a spectrum of SHG at a wavelength of 715 nm
and the associated PL emission when the device is pumped at awavelength of 1530 nm. The multiple spikes seen in the PL
spectrum indicate that broad excitonic PL couples to the highQ cavity modes and the intensities are enhanced due to the
Purcell effect or modulated by the differing collection
efficiencies. The energy diagram is depicted in the inset of
Figure 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 simultaneously
occurring under the infrared excitation,42 while it is difficult todistinguish these processes from optical spectra. This result
also proves the strong interaction between a monolayer WSe2
and whispering-gallery modes via an evanescent field.
The dynamic phase matching is highlighted in the pump
power dependence of the SH power, as shown in Figure 3a(see the Supporting Information for details of the dynamic
phase-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 phasematching condition at a certain pump power, which can be
dynamically altered by the nonlinear resonance shifts. The
double logarithm plot is presented in the inset of Figure 3a,where three distinct regimes can be recognized. Below a pump
power 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 SHG
process. As the pump power is increased from 2 to 4.5 mW,
the fitted slope drastically changes to ∼5.5, and a further
increase in the pump power (>4.5 mW) induces saturation of
the SH power. Such a kink behavior of the SHG intensity has
not been reported in conventional SHG measurements of
TMD flakes on substrates43,44 or photonic nanostructures.31,41,45,46
We therefore consider the influence of the dynamic phasematching condition in a double-resonance system. Figure 3b
shows the schematic for the mechanism under consideration,
where the SH light is blue-detuned at low pump powers. In
this scenario, SH light is considered to be almost in an offresonance condition with a large detuning [state (i)], yielding
a moderate conversion efficiency with a slope of ∼2. As thepump power increases, thermal and Kerr nonlinearities induce
a 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 mode
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LetterFigure 4. Layer dependence of SH light intensity. (a) Spectral mapping of the signal intensity in the visible wavelength region in a pristine silica
microsphere. Third-order nonlinear processes (e.g., THG and TSFG) are observed in the wavelength range of 500−600 nm, whereas no strong
signal appears in the wavelength range of 740−810 nm due to the absence of the second-order nonlinearity. (b) Spectral mapping in a monolayer
WSe2-decorated microcavity. The strong SH signals are observed over a wide range of pump wavelengths (1500−1600 nm). (c) Mapping in a 2LWSe2-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.

overlap between the pump and SH modes.48 The detuning of
the 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 cavity
modes exactly match the on-resonance condition [state (iii)], a
further increase in intracavity power results in the red detuningof SH light, which would reduce the output [state (iv)]. The
maximum SH power for higher pump powers would then be
obtained for the specific intracavity power, where the doubleresonance condition is retained. Because the intracavity poweris almost constant, the SH power saturates despite a further
increase in pump power. Such a complex power dependency isclearly observed in a separate experiment, where we record SH
signals while continuously scanning the pump laser frequency
at a certain pump power. As shown in Figure 3c−e, the SH
signal 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 presented
in the Supporting Information).
It is possible to calculate the conversion efficiency from thedata depicted in Figure 3a. When we define PSH as the detected
SH power, the calculated maximum conversion efficiency(PSH/Pp2) is 6.6 × 10−4 % W−1 with a pump power Pp of 4.5
mW. It should be noted that the internal (intracavity)
conversion efficiency is expected to be >1 order of magnitude
higher 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 coupling
efficiency 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 an
additional nanofiber designed for SH wavelengths, i.e., add−
drop configuration,22,48 or by exploiting a chaotic channel in
deformed microcavities.50
As mentioned above, symmetry plays an important role indetermining the nonlinear susceptibility, and therefore, thenumber of layers in the two-dimensional material is a crucial
factor. The WSe2 crystals used in this work possess the 2Hphase (semiconducting) structure, which is more stable thanother crystal phases. The 2H-phase TMD crystals belonging tospace group D3h exhibit substantial second-order nonlinearity
for only odd layer numbers, whereas the χ(2) nonlinearity
vanishes in even layer numbers because the net nonlinear
dipoles are canceled out due to inversion symmetry.43,44
Considering these selection rules, we performed a comparative
experiment in four different devices.
Figure 4 shows the mapping of SH spectra in the visible
wavelength 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 3LWSe2 devices (Figure 4b,d), whereas there is no distinct signal
in the pristine and 2L-WSe2 devices (Figure 4a,c). This is clear
evidence 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 with4213

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pump, FWM, and stimulated Raman scattering (SRS) are
observed 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 approximately
the same for each experiment. We attribute the efficient, highly
populated SHG to giant second-order nonlinearity of TMD
materials and relaxed resonant phase-matching condition due
to 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 to
substantially increase; nevertheless, the resonant phasematching condition would become stricter as a trade-off.
We have shown thus far the results focused on theemergence of second-order nonlinearity, but one key
advantage of this technique is its flexible controllability of
nonlinear susceptibility. By carefully controlling the transferred
position and the flake size of materials, we can tune the balance
between second- and third-order nonlinearity. Here, we
intentionally 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 few
micrometers (corresponding to the scale of the cavity mode
profile) 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 WSe2decorated microcavity. In the pump wavelength band, FWM
sidebands are observed in the vicinity of the pump light and a
few 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 arise
from THG and third-order SFG processes involving the peaks
seen in the pump band. In particular, the pump and several
Raman peaks allow a variety of sum-frequency combinations,
resulting in multiple emissions in this regime. The signals
around 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 nm
corresponds 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 recognized
as a result of the simultaneous generation of second- and thirdorder processes. The signals around 780−800 nm come fromthe second-order SFG process of the pump and Raman
components, which are not observed in the previous
experiments (Figure 4).
In conclusion, we have demonstrated a novel approach for
introducing second-order optical nonlinearity in ultra-high-Q
silica microcavities through decoration by a two-dimensional
material. Via integration of atomically thin TMD layers withbroken crystal inversion symmetry onto the surface ofamorphous silica microspheres, cavity-enhanced SHG and
SFG arise from strong light−matter interactions via evanescentfields. The cavity-enhanced PL emission mediated by the SHG
process reveals the distinct optical coupling between SH lightand the excitonic resonance of the monolayer WSe2. The
conversion 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 SH

LetterFigure 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 spectrum
in the visible band. (c) Spectral mapping of signal intensities in thetwo different wavelength bands, from 740 to 820 nm and from 500 to
560 nm. The distinct signals are observed in both bands, revealing thecoexistence of second- and third-order nonlinearities.

power. A carefully coordinated clean-stamp transfer techniqueallows for investigation of the layer number dependence, as
well 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) materials
have long been strongly desired. Surface symmetry breaking48,51 and photoinduced effects52,53 can introduce secondorder nonlinear susceptibility but are limited in various aspects.
In comparison, this study offers a powerful way to controllably
enhance optical nonlinearity in high-Q microcavities throughthe size and placement of the 2D material, which would cause
break throughs in nonlinear optics. The results presented inthis work lead to an anticipation that optical nonlinearity canbe artificially designed in hybrid systems, where various
nonlinear processes are combined to implement unconventional functionalities.
In addition, we note that this approach can be extended toother centrosymmetric high-Q cavity devices, including
integrated ring resonators made of silicon or silicon nitride(Si3N4), and thus paves the way to few-photon coherent
nonlinear optics and quantum photon manipulation in various
platforms. The combination of ultra-high-Q cavities withnanomaterials opens up a novel regime in the investigation of
optical processes at high fields under CW excitation,
potentially leading to intriguing physical phenomena as wellas nanophotonic applications.
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ASSOCIATED CONTENT

Manufacturing Support Team at RIKEN for technicalassistance and H. Kumazaki for preparing experimental setups.

sı Supporting Information*■

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c00273.

AUTHOR INFORMATION

Corresponding Authors

Shun 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.jp
Yuichiro K. Kato − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,
Japan; Nanoscale Quantum Photonics Laboratory, RIKEN
Cluster for Pioneering Research, Saitama 351-0198, Japan;
orcid.org/0000-0002-9942-1459; Email: yuichiro.kato@
riken.jp
Authors

Nan Fang − Nanoscale Quantum Photonics Laboratory,
RIKEN Cluster for Pioneering Research, Saitama 351-0198,
Japan; orcid.org/0000-0002-1053-1900
Daiki Yamashita − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,
Japan; Platform Photonics Research Center, National
Institute of Advanced Industrial Science and Technology(AIST), Ibaraki 305-8568, Japan; orcid.org/0000-00026970-4677
Daichi Kozawa − Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,
Japan; Nanoscale Quantum Photonics Laboratory, RIKEN
Cluster for Pioneering Research, Saitama 351-0198, Japan;
Research Center for Materials Nanoarchitectonics, National
Institute for Materials Science, Ibaraki 305-0044, Japan;
orcid.org/0000-0002-0629-5589
Chee Fai Fong − Nanoscale Quantum Photonics Laboratory,
RIKEN Cluster for Pioneering Research, Saitama 351-0198,
Japan; orcid.org/0000-0003-1676-4665
Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.4c00273
Author Contributions

S.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 of
the measurement setup. S.F. and Y.K.K. wrote the manuscriptwith input from all authors. Y.K.K. supervised the project.
Notes

The authors declare no competing financial interest.

■

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the SH intensity (PDF)■

LetterACKNOWLEDGMENTS

This work is supported by JSPS (KAKENHI JP22H01893,
JP22K14623, JP22K14624, JP22K14625, and JP23H00262).
C.F.F. is supported by the RIKEN Special Postdoctoral
Researcher Program. The authors thank the Advanced
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