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ArticleSelf-Aligned Hybrid Nanocavities Using Atomically Thin Materials
Chee Fai Fong,* Daiki Yamashita, Nan Fang, Shun Fujii, Yih-Ren Chang, Takashi Taniguchi,
Kenji Watanabe, and Yuichiro K. Kato*
Cite This: ACS Photonics 2024, 11, 2247−2254

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sı Supporting Information*

ABSTRACT: Two-dimensional (2D) materials are increasingly
being adopted in hybrid photonics via integration with photonic
structures, including cavities. The utility of 2D materials fordielectric environment engineering in hybrid nanophotonic devices
remains largely unexplored. We demonstrate self-aligned hybrid
nanocavities in which 2D material flakes are used to form cavities
locally wherever they are placed along the PhC waveguide
postfabrication. We successfully fabricated such hybrid nanocavities with various 2D materials on silicon PhC waveguides,
obtaining Q factors as high as 4.0 × 105. Remarkably, evenmonolayer flakes can provide sufficient local refractive index
modulation to induce high Q nanocavity formation. We have also
observed cavity PL enhancement in a self-aligned MoTe2 cavity
device with an enhancement factor of about 15. Our results highlight the prospect of using such 2D material-induced PhC
nanocavities to realize a wide range of photonic components for hybrid integrated photonic circuits.
KEYWORDS: photonic crystal, hexagonal boron nitride, transition metal dichalcogenide, light−matter interaction■

INTRODUCTION
Two-dimensional (2D) van der Waals layered materials suchas graphene, hexagonal boron nitride (hBN), and transition
metal dichalcogenides (TMDCs) are garnering significant
attention for both fundamental science and device applications.1−7 In particular, semiconducting TMDCs have a direct
band gap at monolayer thickness.8,9 They exhibit bright opticalemission which is governed by their exciton (Coulomb-boundelectron−hole pair) responses even up to room temperature
due to the large exciton binding energies of hundreds ofmeV.10 Many 2D materials exhibit large optical nonlinearities,11 which could also be intricately linked to thevalley polarization.12,13 The amenability to strain and defectengineering also makes 2D materials promising for the creationof emitters for quantum light sources.4,6,14
The integration of 2D materials with nanophotonicarchitectures15−27 offers a promising avenue to manipulate
light−matter coupling, thereby facilitating the realization of
hybrid devices such as lasers,28−30 quantum emitters,31,32 andphotodetectors.22,33,34 To achieve optimal device performance,
it is imperative to couple 2D materials with cavities exhibiting ahigh quality factor over the mode volume ratio (Q/Vmode). The
planar air hole photonic crystal (PhC) nanocavities35−37
promise strong light confining in an ultrasmall mode volume,rendering them highly attractive for the aforementioned hybrid
device applications.
In previous studies, the conventional approach to hybrid
devices involved the transfer of 2D materials, such as TMDC
© 2024 The Authors. Published byAmerican Chemical Society

mono- and few-layer flakes onto prefabricated nanocavities.
Despite being ultrathin, the transfer of a TMDC flake alters thedielectric environment of the cavity and could induce otheroptical losses which results in significant degradation in thecavity quality (Q) factor.34,38,39 Interestingly, the impact of the
TMDC flake on the dielectric environment is usually ignoredor regarded as a small perturbation that needs to be slightly
compensated for by adjusting the design of the nanophotonic
cavity. The potential of 2D materials for dielectric environmentengineering in hybrid nanophotonic devices is still largely
untapped. Instead of being a detriment, the change in the
dielectric environment caused by the 2D material can beutilized to enable alternative methods for enhanced cavity
light−matter coupling.
In this work, we demonstrate self-aligned hybrid nanocavities in which 2D material flakes are used to form cavities
locally wherever they are placed along the PhC waveguide
postfabrication. The presence of the flake modulates the local
refractive index, changing the waveguide dispersion, leading to
optical confinement and thus cavity formation. We successfully
Received: December 30, 2023
Revised: May 8, 2024
Accepted: May 8, 2024
Published: May 17, 2024

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ArticleFigure 1. 2D material hybrid PhC mode gap nanocavity. (a) Schematic of a device structure showing cavity formation at the location of the 2D
material. (b) Photonic bands of the PhC waveguide showing the odd and even guided modes as labeled, as well as the mode gap in green and thecavity mode within. The solid (dashed) lines represent the guided modes with (without) the 2D material. (c) Electric field Ey profile of the cavity
mode in the xy- and xz-planes. The green shaded region represents the 2D material flake. (d) Dependence of the cavity Q factor against thicknessof WSe2. (Inset) Plot of the Q factor against the number of layers.

fabricated such nanocavities with various 2D materials
including hexagonal boron nitride (hBN), tungsten diselenide(WSe2), and molybdenum ditelluride (MoTe2), achieving highQ factors of 104 to 105. Remarkably, we discovered that even amonolayer flake could give rise to sufficient local refractive
index modulation to form a hybrid nanocavity. In fact,according to simulation results, the thinner the flake, themore moderate the spatial modulation of the refractive index,
the higher the Q factor. In such hybrid systems, the monolayer
represents the extreme limit of index modulation, promisingcavities with high Q factors. As the flake is spatially self-aligned
to the cavity, we have further observed the self-aligned
coupling of MoTe2 photoluminescence (PL) in a hybrid
nanocavity device, giving rise to enhanced emission and a
lifetime reduction of about 15 times. Our results demonstrate
the utility of 2D materials for dielectric environmentengineering to create functional hybrid devices, with potential
applications in hybrid photonic integrated circuits.
Design of Hybrid Nanocavity. For simulating our device
structure, we employ the air-suspended W1 line defect PhC
waveguide40 made of silicon (refractive index, nSi = 3.48),
consisting of a triangular array of air holes with lattice period a,with a 2D material flake covering a section of the waveguide(Figure 1a). We typically consider a PhC waveguide with 48
and 14 air holes along the Γ−K and Γ−M directions,
respectively, with the air hole radius, r = 0.28a and a = 340
nm. The PhC slab thickness is 200 nm. The photonic band
structure of the transverse-electric-like (TE-like) modes of the
PhC waveguide is shown in Figure 1b. There are two guided

modes, termed odd and even in accordance with the symmetryof the Ey field distribution about the x-axis.
When a 2D material flake is present on the PhC waveguide,
the local effective refractive index increases, red-shifting thefrequencies of the guided modes. The frequency mismatch
between the regions with and without the 2D material flake
gives rise to field confinement and thus cavity formation. Thereare corresponding cavity modes for each of the guided modesin the near-infrared (NIR) regime. The frequencies of theresulting hybrid nanocavity modes are usually lower than theband edge (frequency at kx = 0.5) of the corresponding guided
modes. The even cavity modes exhibit much higher Q factors
as the modes are within the mode gap�the frequency range
between the even guided mode edge and the lower edge of thephotonic band gap (green region in Figure 1b). In this work,
we will mainly focus on even cavity modes.
To investigate the cavity mode properties, we consider arectangular WSe2 flake (refractive index, nWSe2 = 3.9541) of 10
nm thickness with a lateral width of 14a, partially covering thesurface of the PhC waveguide. The flake is assumed to cover
the PhC structure completely along the y-direction. The
simulated Ey field amplitude profile of the fundamental (i.e.,
lowest energy) even cavity mode is shown in Figure 1c. The
cavity mode profile is largely reminiscent of the even guided
mode profile, and it remains similar for the range of WSe2
thicknesses considered in our simulations.
The simulated fundamental cavity mode Q factor dependence on the thickness of the WSe2 flake is summarized in
Figure 1d. At a thickness of 30 nm, the cavity Q is about 1 ×
104. As the thickness of the WSe2 flake decreases, the Q factor
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ArticleFigure 2. hBN-induced hybrid nanocavity. (a) Optical micrograph of an hBN hybrid nanocavity device. The scale bar represents 5 μm. The “x”
symbol marks the location of the PL measurement in (b). The dotted box outlines the area of the 2D imaging shown in (c). (b) PL spectrum
showing signatures of cavity emission. (c) 2D image of the integrated PL intensity over the wavelength range of 1325−1336 nm showing localized
cavity emission at the hBN flake (the flake is indicated by the darkened region). (d) Transmission spectra showing the cavity peak. (Inset) A closer
look at the narrow peak fitted to a Lorentzian function (magenta line).

increases, reaching a theoretical value as high as 106 for fewlayer flakes. Simulation results indicate that not only can amonolayer form a cavity but also it promises an ultrahigh Q
factor of the order of 107. The atomically thin nature of theflake induces a more gradual transition in the refractive index
both vertically along the PhC slab/2D material/air interface
and the edges of the 2D materials. Sudden changes in therefractive index can give rise to optical fields with wavevector
components falling within the leaky region in momentum
space.42 Consequently, a thin flake introduces a “gentle”
perturbation to the fields, reducing the number of components
within the leaky region (refer to Supporting Information
Figure S1). Compared to thicker flakes, a thinner flake also
gives larger refractive index contrast in the PhC slab/2D
material/air interface, thus resulting in better confinement of
optical fields. These effects lead to diminished loss throughcoupling to free space, resulting in ultrahigh Q factors. The
dependence of the cavity wavelength, mode volume, as well as
the ratio of the field intensity at the PhC surface to the
maximum mode field intensity on the WSe2 flake thickness isprovided in Supporting Information Figure S1. For a given setof PhC waveguide parameters and flake width, there is an
optimum refractive index modulation to obtain a hybrid
nanocavity with the optimum Q factor, which occurs at
different thicknesses for different 2D materials (see Supporting
Information Figure S2). Further details about other factors that
affect the cavity Q factor such as the flake lateral width andrefractive index are also provided in Supporting Information
Figure S2.
hBN-Induced Hybrid PhC Nanocavity. DielectrichBN�which is often used to encapsulate TMDC to prevent
exposure to air�is also a suitable 2D material to form hybrid
nanocavities. In particular, high crystal quality hBN flakes withlow defect densities promise high Q factors. Figure 2a shows an
optical micrograph of a fabricated hBN-on-PhC waveguidedevice. The PhC slab thickness is 200 nm, and the waveguideconsists of 96 and 14 air holes along the Γ−K and Γ−Mdirections, respectively. The lattice and the air hole radius are a= 360 nm and r = 0.27a, respectively. The hBN flake istransferred onto the PhC waveguide postfabrication. The
thickness of the hBN flake is estimated to be about 20 nm
based on the optical contrast, and its lateral width measured
along the waveguide is about 12a.
By optically exciting the silicon PhC device above the bandgap, the weak emission from the silicon substrate can couple to

the guided and/or cavity modes, manifesting as peaks in the PL
spectra. For this particular sample, emission peaks appear near
the frequency of the even guided mode edge (Figure 2b). By
performing 2D PL imaging, we confirm that the emission peaks
only appear when excited at the hBN flake along the
waveguide, further indicating the formation of a cavity (Figure2c). The nonuniformity of the 2D PL intensity map could be
due to the nonuniformity of the flake and also the presence ofmultiple cavity modes with different mode profiles (seeSupporting Information Figure S5). The polarization properties of the peak are broadly consistent with expectations inaccordance with the mode profile (see Supporting Information
Figure S3). However, the actual line width of the peaks, andthus the Q factors, cannot be determined using thespectrometer due to insufficient resolution.
We then carried out transmission measurements by excitingat the hBN flake and detecting the scattered light from thesemicircular outcoupler to the right of the waveguide. A few
features can be seen in the transmission spectrum (Figure 2d):the guided mode edge can be seen at about 1330.00 nm, as
well as a relatively broad peak centered at 1330.34 nm and anarrow peak at 1330.61 nm. The broad peak has a line width of
60.2 pm, corresponding to Q = 2.2 × 104 while the narrow
peak has a line width of 3.54 pm, giving Q = 3.8 × 105. Given
the relative position and the Q factors of the peaks, the narrowand broad peaks should correspond to the fundamental andfirst higher-order cavity modes.
The extracted Q factors from the resonant peaks of thetransmission spectra are the so-called loaded Q factors which
consist of the intrinsic cavity Q (Qi) and cavity−waveguide111
coupling Q (Qc):42 Q = Q + Q . For this hBN device, bylicchanging the transmission measurement configuration to excite
at the left output coupler and detecting light from the right
output coupler, we are able to observe the resonant dip that
corresponds to the absorption of light by the cavity, allowing
us to calculate Qi of this device to be about 4.2 × 105(Supporting Information Figure S4). Overall, our results
show that the flake and the nanocavity are colocalized,
essentially forming a self-aligned device. The enhanced Si PL
emission also indicates a cavity light−matter coupling.
Hybrid PhC Nanocavities Using Atomically Thin
WSe2. Compared with hBN, it is easier to obtain and identify
mono- and few-layer flakes with TMDCs. Motivated by our
simulation results, we conduct experiments using WSe2 as it is2249

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ArticleFigure 3. Mono- and few-layer WSe2 hybrid nanocavity. (a−c) Optical micrograph of mono-, bi-, and trilayer WSe2-induced nanocavity device. The
waveguide consists of 96 and 14 air holes along the Γ−K and Γ−M directions, respectively, to facilitate ease of 2D material flake transfer. The
brightness and contrast of the images have been adjusted to improve the visibility of the WSe2 flake. For the mono- and trilayer device, the PhC
waveguide has a = 348 nm while the bilayer device is of a = 356 nm. The lateral widths along the waveguide of the monolayer, bilayer, and trilayer
flakes of the devices are 13a, 36.5a, and 10a, respectively. The cavity resonance of the bilayer device appears at a longer wavelength since the PhC
lattice period is larger. The slightly longer cavity resonant wavelength of the monolayer device can be attributed to the larger flake width. Some
residues could be left on the sample during the transfer process in the monolayer device. However, there is no significant effect on the transmission
spectra. (d−f) Corresponding cavity peak in the transmission spectra for each of the devices. The transmission measurements are performed byexciting the left output coupler and detecting at the right output coupler for the mono- and trilayer devices. As for the bilayer device, the laser
excitation is focused on the sample and the transmitted signal is detected at the right output coupler. The peak in (d) is fitted with the Lorentzianfunction, while the peaks in (e,f) are fitted with the Fano resonance. The scale bars in (a−c) represent 5 μm.

Figure 4. PL enhancement in the MoTe2 hybrid nanocavity. (a) Optical micrograph of the MoTe2 nanocavity device. The scale bar indicates 5 μm.
(b) Transmission spectra obtained by exciting at the left and detecting at the right output coupler, showing the cavity peaks. The green curves are
the individual Lorentzian fits, and the magenta curve is the cumulative fit. (c) PL spectrum showing the emission from the MoTe2 and cavity.
(Inset) A zoom-in of the region with the cavity emission peak. (d) Time-resolved PL decay curves of the emission from the cavity and the bare
MoTe2 flake [positions 1 and 2 in (a), respectively] showing the reduced lifetime due to the cavity effect. The cavity and the bare MoTe2 emission
decay curves are fitted using a single and double exponential reconvolution function, respectively. The gray shaded region shows the IRF.

relatively stable in air. The discussion here will focus on aselection of three exemplary devices with monolayer, bilayer,
and trilayer flakes (Figure 3).The transmission spectrum foreach device is shown in Figure 3e,f. The cavity peaks appear at
the expected wavelengths in accordance with the PhC
waveguide lattice period. Due to intermode scattering, there
is nonzero transmission signal even at frequencies below (i.e.,

wavelengths above) the transmission edge. The interference of
this background transmission signal with the cavity peak could
result in an asymmetric line shape. By fitting the peaks to either
the Lorentzian (symmetric line shape) or Fano resonance(asymmetric line shape),43 Ql is determined to be 8.6 × 104,
1.0 × 105, and 4.0 × 105 for the monolayer, bilayer, and trilayer
devices, respectively. Only for the bilayer sample are we able to2250

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obtain the necessary transmission spectra with the cavity
resonant dip and peak in the different measurement
configurations to extract the Qi to be about 3.7 × 105. The
cavity Q factors do not show an obvious increase with thedecrease in flake thickness even after considering the flake
lateral widths. Various loss mechanisms could affect theextracted Q factors; further descriptions of the possible
mechanisms are provided in the Discussion section. Our
results here show unambiguously that even a monolayer flake
could give rise to sufficient local refractive index modulation toform a high Q hybrid nanocavity, despite being only one-atomthick.
Cavity PL Enhancement in Self-Aligned Hybrid
Nanocavity with MoTe2. MoTe2 is optically active in the
NIR, making it an ideal TMDC to induce a nanocavity in PhC
and simultaneously couple its exciton emission to the cavity.
Figure 4a shows an optical micrograph of a MoTe2 hybrid
nanocavity device. The flake is about 6−8 layers thick based on
optical contrast and the PL peak position. The lateral width of
the flake along the waveguide is about 6a. Despite not fully
covering the PhC along the y-direction, the transmission
spectra taken before and after MoTe 2 flake transfer(Supporting Information Figure S5) indicate that the flake
managed to induce the formation of a cavity. Multiple peaks
are visible in the transmission spectrum (Figure 4b) with Q
factors of the order of 103 to 104. The lower Q factors are due
to a combination of the larger flake refractive index, the small
lateral width, and the absorption by the flake. Modeling a
structure with similar properties as the actual device in FDTD
and FEM simulations, we confirmed that the formation of thecavity that could support multiple modes is indeed possible(Supporting Information Figure S5). In fact, a local refractive
index modulation of a small area adjacent to the waveguide issufficient to induce strong optical confinement (Supporting
Information Figure S6).
A broad background of MoTe2 exciton emission can be seenin the PL spectrum (Figure 4c). Peaks close to 1200 nm
correspond to the Fabry−Perot-like emission arising from theodd guided mode, while the narrow peaks at around 1310 nm
correspond to the cavity modes. The bright intensity of thesepeaks indicates that the MoTe2 emission is indeed coupled toand enhanced by the cavity. We further performed timeresolved PL measurements. A comparison of the decay curvesof the PL from the cavity and bare MoTe2 (positions 1 and 2,
respectively, in Figure 4a) is shown in Figure 4d. From thedecay curves, the bare MoTe2 PL lifetime is extracted to be
46.1 ± 0.3 ps, whereas the cavity emission lifetime is 3.0 ± 0.6
ps. The cavity decay curve is largely similar to that of the
instrument response function (IRF), suggesting that the actual
lifetime is shorter than the extracted value. Nonetheless, the PL
lifetimes indicate an enhancement factor of 15 ± 3. Our resultsunambiguously show that the spatial self-alignment of the flaketo the nanocavity facilitates light−matter coupling.

■

Articlethe coupling losses to free space and is comprised of thetheoretical (Q t h ) and experimental (Q e x ) losses:
11
1= Q + Q . The experimental Q factors of the devices
Q
i

th

ex

are lower than the theoretical values due to fluctuations in the
fabricated structure parameters, for example, nonuniformity of
air hole radii and lattice period, leading to scattering losses.
The broken vertical symmetry of the device could also
contribute to loss via the coupling of transverse electric andtransverse magnetic modes.44,45 The irregular shape of theflake could also lead to losses as more of the flake edges
overlap with the air holes. 46 Wrinkles, bubbles, andcontaminants could also lead to air gaps between the 2D
material flake and the PhC waveguide, affecting cavity Q.
Nonetheless, the surface roughness of the flakes is mostly
smooth and the interface between the flake and the PhC
substrate is clean, especially for the WSe2 devices which arefabricated using the anthracene-assisted transfer method.47 As
such, the surface condition of the flake is likely not the main
factor that affects the Q factor. From the scanning electron
micrograph of the fabricated PhC waveguide (Supporting
Information Figure S7), the PhC air holes show imperfectroundness with slight ellipticity, which we believe to be themain cause that limits the experimental Q factors.
Despite these issues, high Q factors have been achieved inour 2D material-induced hybrid PhC nanocavities. The highQ/Vmode of our hybrid nanocavity devices also enabled theobservation of nonlinear effects such as the optical bistability at
low excitation powers (Supporting Information Note 2 andFigure S8). Recently, there is a study reporting thedemonstration of nonlinear frequency conversion using suchself-aligned hybrid nanocavities.48 By comparing the experimentally observed guided mode red shift caused by the 2D
materials and the required change in the slab refractive index of
the bare PhC waveguide to produce the same red shift in
FDTD simulations, it is estimated that a monolayer flake
results in 0.1−0.2% change in the local refractive index. The
small change in the refractive index facilitates the formation ofhigh Q cavities, consistent with previous numerical modeling
results.40,49 Simulation results in Figures S2 and S6 also imply
that hybrid nanocavities can be formed with only a minimal
amount of material while achieving high Q without requiring
stringent fine-alignment. 2D materials, with their ease ofheterogeneous integration, are uniquely suitable to achieve
such small refractive index modulation to form high Q cavities.
The many degrees of freedom in the 2D material flake and the
PhC waveguide enable a broad range of Q factor tunability to
tailor to the needs of the application. Furthermore, the devices
are stable, with the cavities showing no significant degradationeven after being stored in ambient conditions for a time periodof several months (see Supporting Information Figure S9).

■CONCLUSIONSIn conclusion, we have experimentally demonstrated selfaligned hybrid nanocavities by dielectric environment engineering with 2D materials. The cavity can be formed at anarbitrary location along a PhC waveguide by integrating
suitably sized 2D materials postfabrication. Since the flake isnaturally aligned to the cavity, the flake could readily couple tothe cavity for stronger light−matter coupling. Even amonolayer flake can provide sufficient local refractive index
modulation to induce the formation of a nanocavity. It is worthemphasizing that 2D materials�with their atomic thicknesses

DISCUSSION

We have successfully fabricated hybrid nanocavities using hBN,
WSe2, and MoTe2 which exhibit high Q factor values. A
summary of all the fabricated devices is presented in Figure
S10 and Supporting Information Table S1. The cavity modes
consistently appear at the location of the flakes, indicating thatthey are not caused by disorder in the PhC waveguide. It is
worth reiterating that the extracted Q factors are Ql, which islimited by either Qc or Qi. The Qi is in turn determined only by2251

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and ease of heterogeneous integration�are uniquely suitableto achieve the small refractive index modulation necessary toform high Q cavities. Our versatile approach that utilizes
dielectric environment engineering can be extended toencompass a wide variety of 2D materials, with prospects oftransforming passive PhC waveguides into cavity-enhanced
functional components on hybrid photonic integrated circuits.

Articleup the WSe2 flake. The WSe2 flake and anthracene crystal werethen transferred together onto the target PhC waveguide.
Finally, the anthracene crystal is heated to about 80 °C or left
in ambient conditions to sublime, leaving behind clean flakes.
Optical Spectroscopy. PL measurements are performed
with a home-built confocal microscopy system. A Ti/sapphirelaser (Spectra Physics 3900S) is used for excitation, usually at awavelength of 760−780 nm. The excitation power is controlledusing neutral density filters, while the polarization of the laser
is adjusted using a half-wave plate to match the polarization of
the cavity mode. The laser beam is focused on the samples
using an objective lens (Olympus) of 50× magnification with anumerical aperture (NA) of 0.65. The emission from thesample is collected with the same objective lens, directed to aspectrometer (Princeton Instruments Acton SP2300), dispersed by a 150 lines/mm grating, and then detected with aliquid nitrogen-cooled InGaAs detector (Princeton Instruments PyLoN IR).
For transmission measurements, a wavelength-tunable
continuous-wave laser (Santec TSL-550) is used. A steering
mirror and a 4f system are used to displace the laser excitation
spot while keeping the same detection spot. The light scatteredfrom the sample is collected by the objective lens and coupled
into an optical fiber to direct the signal to a photoreceiver(New Focus 2011).
For time-resolved measurements, a Ti/sapphire laser(Coherent MIRA) operating in pulsed mode is used for
excitation. The excitation wavelength is set to 780 nm. The
laser beam is focused on the sample with an objective lens(Olympus) of 100× magnification and NA of 0.85. The PL
emission is collected by the same objective lens and thenfiltered spectrally with a long-pass filter to direct light of a
specific wavelength range into an optical fiber connected to asuperconducting nanowire single-photon detector (Quantum
Design Eos). All measurements are carried out at room
temperature. The samples are kept in a nitrogen gas
environment in order to reduce the oxidation rate of the
TMDC flakes.

■

METHODS
Numerical Simulations. The photonic band structure is
calculated using MIT Photonic Bands (MPB),50 considering aunit cell of the PhC waveguide with periodic boundary
conditions in the x- and y-directions. The finite-differencetime-domain (FDTD) simulations are performed using theopen-source package MEEP51 on a computing cluster. The
grid resolution is usually set to at least a/24 or higher
depending on the thickness of the simulated 2D materials.
With subpixel averaging, we are able to simulate 2D material of
thicknesses down to 5 nm and obtain reliable results.
Numerical finite element method (FEM) simulations arecarried out with COMSOL to simulate PhC structures withmono- to few-layer thick 2D materials. The simulated PhC
waveguide consisting of 48 (14) air holes along the Γ−K (Γ−M) direction is sufficiently large such that any further increasein the size does not change the Q factor, i.e., the Q factor islimited only by out-of-plane losses. The type of simulated 2D
materials is controlled by setting the value of the dielectric
constant, obtained from Laturia et al.41 We assume amonolayer effective thickness of 0.7 nm in our simulations.
Silicon has minimal absorption at the NIR regime, where thatcavity mode is. Aside from MoTe2, hBN and WSe2 are notexpected to have strong absorption in the NIR and thus we do
not include absorption losses in the simulations. We also
performed simulations using isotropic or anisotropic dielectric
constants and found no significant difference in the results.
Silicon PhC Waveguide Fabrication. The PhC waveguides are fabricated on a silicon-on-insulator substrate with a
200 nm-thick top silicon layer and a 1 μm-thick buried oxide
layer. The PhC pattern is first defined on a resist mask byelectron beam lithography, and then the pattern is transferred
onto the substrate via inductively coupled plasma using C4F8
and SF6 gases. Following resist removal, the buried oxide layer
is etched away using a solution of 20% hydrofluoric acid toform air-suspended PhC waveguide structures.
2D Material Dry Transfer. The hBN flakes (NIMS) areprepared on a polydimethylsiloxane (PDMS) sheet (Gelfilm byGelpak) by mechanical exfoliation of bulk crystals. Suitableflakes are identified using an optical microscope and thentransferred onto the target PhC waveguide using a home-built
micromanipulator setup. MoTe2 flakes (HQ graphene) areprepared and transferred using the same method.
WSe2 flakes (HQ graphene) are prepared on commerciallyavailable 90 nm-thick SiO2/Si substrates via mechanicalexfoliation to enable the identification of the layer number
via optical contrast. The WSe2 flakes are then placed on the
PhC waveguides using the anthracene-assisted transfer
process.47 To grow the anthracene crystals, anthracene powder
is heated to about 80 °C. The sublimated anthracene vapor willthen recrystallize on the bottom surface of a glass slide placedat about 1 mm above the anthracene powder. The growth time
is typically 10 h. A small PDMS sheet is then placed on a glass
slide, followed by an anthracene crystal on the PDMS to form
an anthracene/PDMS stamp. Next, this stamp is used to pick■

ASSOCIATED CONTENT* Supporting Information
sı

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsphotonics.3c01927.
Further simulation results; polarization dependence
measurement of the cavity modes; summary of hybrid
nanocavity devices; and details of the analysis of thecavity mode intrinsic Q factor and the optical bistability
(PDF)

■

AUTHOR INFORMATION

Corresponding Authors

Chee Fai Fong − Nanoscale Quantum Photonics Laboratory,
RIKEN Cluster for Pioneering Research, Saitama 351-0198,
Japan; orcid.org/0000-0003-1676-4665;
Email: cheefai.fong@riken.jp
Yuichiro K. Kato − Nanoscale Quantum Photonics
Laboratory, RIKEN Cluster for Pioneering Research, Saitama
351-0198, Japan; Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198,
Japan; orcid.org/0000-0002-9942-1459;
Email: yuichiro.kato@riken.jp
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Authors

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
Nan Fang − Nanoscale Quantum Photonics Laboratory,
RIKEN Cluster for Pioneering Research, Saitama 351-0198,
Japan; orcid.org/0000-0002-1053-1900
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
Yih-Ren Chang − Nanoscale Quantum Photonics Laboratory,
RIKEN Cluster for Pioneering Research, Saitama 351-0198,
Japan
Takashi Taniguchi − Research Center for Materials
Nanoarchitectonics, National Institute for Materials Science,
Tsukuba 305-0044, Japan; orcid.org/0000-0002-14673105
Kenji Watanabe − Research Center for Electronic and Optical
Materials, National Institute for Materials Science, Tsukuba
305-0044, Japan; orcid.org/0000-0003-3701-8119

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Complete contact information is available at:https://pubs.acs.org/10.1021/acsphotonics.3c01927
Author Contributions

C.F.F. conceived the idea and performed the numericalsimulations. C.F.F. fabricated the PhC waveguide under theguidance of D.Y. C.F.F. carried out the 2D materials transfer
with assistance from N.F. and Y.R.C. C.F.F. performed theoptical measurements with some assistance from D.Y. and S.F.
K.W. and T.T. provided the bulk hBN crystals. C.F.F. analyzed
the data and wrote the manuscript. All authors contributed tothe discussion of the results and the manuscript. Y.K.K.
supervised the project.
Notes

The authors declare no competing financial interest.
C.F.F., D.Y., N.F., S.F., Y.-R.C., T.T., K.W., and Y.K.K., SelfAligned Hybrid Nanocavities Using Atomically Thin Materials.
2023, 2308.10566 arXiv http://arxiv.org/abs/2308.10566(accessed May 8, 2024).

■

ArticleACKNOWLEDGMENTS

This work was supported in part by JSPS KAKENHI(JP22K14623, JP20H02558, JP20J00817, JP22K14624,
JP22K14625, JP22KF0407, JP23H00262, JP24K08296, andJP24K17627) and ARIM of MEXT (JPMXP1222UT1138), as
well as the RIKEN Incentive Research Projects. K.W. and T.T.
acknowledge support from the JSPS KAKENHI (grant nos.
JP21H05233 and JP23H02052) and World Premier International Research Center Initiative (WPI), MEXT, Japan. C.F.F.
was supported by the RIKEN SPDR fellowship. Y.-R.C. was
supported by the JSPS Postdoctoral Fellowship. We acknowledge support by the RIKEN Information Systems Division for
the use of the Supercomputer HOKUSAI BigWaterfall andSailingShip, as well as the use of the COMSOL license.
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