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## Creator

[Keisuke Watanabe](https://orcid.org/0000-0002-4285-2135), [Tadaaki Nagao](https://orcid.org/0000-0002-6746-2686), [Masanobu Iwanaga](https://orcid.org/0000-0002-8930-6940)

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[Low-Contrast BIC Metasurfaces with Quality Factors Exceeding 100,000](https://mdr.nims.go.jp/datasets/4cdd2cc9-2889-4825-b97d-c55e8fbe9452)

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Low-Contrast BIC Metasurfaces with Quality Factors Exceeding 100,000Low-Contrast BIC Metasurfaces with Quality Factors Exceeding100,000Keisuke Watanabe,* Tadaaki Nagao, and Masanobu IwanagaCite This: Nano Lett. 2025, 25, 2777−2784 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Dielectric metasurfaces operating at quasi-boundstates in the continuum (qBICs) can achieve exceptionally highradiative quality (Q) factors by introducing small asymmetries intotheir unit cells. However, fabrication imperfections often imposemajor limitations on the experimentally observed Q factors. In thisstudy, we experimentally demonstrate BIC metasurfaces with a Qfactor of 101,000 under normal excitation of light in the telecomwavelength range achieved by employing low-contrast silicon pairs.Our findings show that such free-space accessible ultrahigh-Qfactors can be attained by leveraging both the high radiative Qfactors of higher-order qBIC modes and reduced scattering losses inshallow-etched designs. Additionally, we demonstrate stable sub-picometer-level wavelength fluctuations in water, with a limit ofdetection of 10−5 for environmental refractive index changes. The proposed approach can be extended to BIC metasurfaces withmany other configurations and operating wavelengths for ultrahigh-Q applications in both fundamental physics and advanceddevices.KEYWORDS: metasurfaces, bound states in the continuum, Q factors, all-dielectric, silicon, biosensorsStrong confinement and localization of light in nano-fabricated structures are of great importance for diverseapplications, including high-efficiency lasers,1 sensors,2 bio-imaging,3 nonlinear enhancement,4 non-Hermitian optics,5 andtopological photonics.6 The energy dissipation of confinedlight is quantified by the quality (Q) factor, and both thematerial selection and structural design play major roles formaximizing the experimental Q factors. To date, nanostruc-tures with periodic arrays have been successfully employed toachieve high Q factors. Such nanostructure arrays includephotonic crystals (PCs),7 plasmonic cavities,8 and metallic ordielectric metasurfaces that support surface lattice resonan-ces9,10 and bound states in the continuum (BICs).11Remarkably, BICs in dielectric materials have recentlyattracted considerable attention owing to their design flexibilityin controlling radiative losses through precise structural design.A symmetry constraint dictates that symmetry-protected BICsare not accessible from free-space. However, true BICs can betransformed into quasi-BICs (qBICs) by breaking thesymmetry of the unit cell, yielding finite radiative Q factors.This symmetry breaking enables the observation of sharpresonances under normal-incidence excitation. The highradiative Q factors and resonantly enhanced electric fields arethe key features of the qBIC modes, offering novel strategies toenhance the functionality of optical devices.12−14 However,BIC metasurfaces, which often have a large ratio ofnanostructure depth to in-plane dimensions, are susceptibleto scattering losses owing to fabrication imperfections, thuslimiting their experimental Q factors to the range of severalhundreds to thousands. More specifically, the localized electricfields within high-index nanostructures overlap with thesidewalls, on which light scattering caused by surfaceroughness from nanofabrication drastically reduces theexperimental Q factors. In other words, experimental Q factorscannot improve under large scattering losses, despite manyresearchers focusing on increasing the radiative Q factorsobtained from numerical calculations. To resolve this problem,two approaches can be considered. The first approach involvesutilizing imperfection-tolerant designs.15,16 Jin et al. proposedthat merging off-Γ BICs with multiple topological charges intoan isolated symmetry-protected BIC at the Γ point canpreserve high Q factors over a broad range in the k-space,substantially suppressing out-of-plane scattering.15 Thisapproach enabled the experimental demonstration of ultra-high-Q factors up to 4.9 × 105 in PC structures. However,topological charge engineering requires precise control infabrication, because the merging BICs are extremely sensitiveto structural parameters.17 Zhong et al. recently demonstratedReceived: November 20, 2024Revised: January 23, 2025Accepted: February 4, 2025Published: February 7, 2025Letterpubs.acs.org/NanoLett© 2025 The Authors. Published byAmerican Chemical Society2777https://doi.org/10.1021/acs.nanolett.4c05880Nano Lett. 2025, 25, 2777−2784This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on February 19, 2025 at 11:35:14 (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="Keisuke+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tadaaki+Nagao"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masanobu+Iwanaga"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.4c05880&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/25/7?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/7?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/7?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/7?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c05880?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/toroidal dipole BIC metasurfaces with Q factors that are robustagainst variations in the shape of dimer nanoholes.18 Theexperimentally observed Q factors benefit from the fabricationtolerance of the toroidal dipole mode but remain limited to theorder of 104. The second approach focuses on imperfection-reduced designs. Huang et al. recently demonstrated a simplestructure with a thin patterned photoresist layer on top of asilicon-on-insulator (SOI) wafer,19 achieving an ultrahigh-Qguided mode resonance with a Q factor as high as 2.4 × 105 forPC structures. In this design, the absence of nanopatterning inthe silicon layer minimized scattering losses typically caused bysurface roughness from silicon etching. However, this ultra-high-Q factor has not been demonstrated in BIC metasurfacesto date. More importantly, nanopatterned photoresist layerssuffer from poor durability because they can be easily damaged,degraded, and dissolved in many solvents. Therefore, achievingmonolithic high-Q metasurfaces with stable patterned layersremains challenging, which is essential for a wide range ofoptical device applications.In this study, we propose and experimentally demonstrate amethod to minimize fabrication imperfections in siliconmetasurfaces by employing shallow-etched structures.20−22Our approach increases the experimental Q factors byapproximately 1 order of magnitude compared with conven-tional designs, pushing the limits of Q factors over 105. Wedesign, fabricate, and characterize BIC metasurfaces composedof arrays of silicon pairs with varying etching depths. Ourresults show that shallower etching simultaneously enhancesthe radiative Q factors and reduces scattering losses caused byfabrication imperfections in higher-order qBIC modes, leadingto substantially improved experimental Q factors. Finally, wedemonstrate highly stable refractometric sensing, achieving alimit of detection (LOD) at the 10−5 level using low-contrastBIC metasurfaces. We expect that the developed ultrahigh-Qsilicon metasurfaces will find broad applications in fieldsrequiring strong light−matter coupling at the nanoscale.Figure 1a illustrates the proposed nanostructures, which areshallow-etched silicon pairs with etching depth d, fabricated onSOI wafers with a thickness of 400 nm. Silicon is chosen as thematerial for light confinement due to its high refractive indexand transparency over a broad wavelength range. The buriedoxide (BOX) layer is 2000 nm, which is thick enough tosufficiently suppress leakage losses to the bottom siliconsubstrate.23 The dimensions of the unit structure are period P= 760 nm, shallow rod length L = 610.8 nm, and shallow rodwidth w = 235.5 nm. The symmetry of the unit structure isbroken by changing asymmetry parameter α = 2ΔL/L, whichcontrols the radiation losses of the resonance mode emittedinto the far-field. Figure 1b shows the simulated transmittancespectra for an infinite periodic structure computed using thefinite-difference time-domain (FDTD) method (Ansys Lumer-ical). The resonance modes are excited by a normally incidentx-polarized plane wave parallel to the major axis of therectangular structures (see Supporting Information S1 formethods). For d = 0 (i.e., unpatterned silicon), thetransmittance spectrum exhibits interference patterns owingto the multilayer configuration of the SOI wafer. Whenshallow-etched nanostructures with d = 82.7 nm and α = 0%(i.e., no asymmetry) are introduced, a large transmittance dipappears around the wavelength of 1870 nm caused by thedestructive interference between the leaky guided-moderesonance and background continuum. When α = 5%, threeBIC modes are transformed into qBIC modes. The cross-sectional Ex profiles shown in Figure 1c indicate that themetasurfaces support a fundamental mode (qBIC1) andhigher-order modes (qBIC2 and qBIC3) in the wavelengthrange of interest (see Supporting Information S2). Figure 1d(left) presents the transmittance maps for different asymme-tries α when d = 82.7 nm. The three qBIC modes retainnarrow line widths even as α increases, suggesting that the low-contrast matesurfaces maintain large radiative Q factors acrossa broad range of α. Notably, the peak wavelengths of the threeqBIC modes remain nearly constant as α changes because theoverall volume of the shallow pair-rod remains unchangeddespite changes in the lengths of the upper and lower rods.Figure 1d (right) shows the transmittance maps for different dwhen α = 5%. As d increases, the line widths graduallybroaden, and the resonance wavelengths undergo a blueshift,as the modes penetrate more into surrounding air. Based onthe calculation of field confinement factors in silicon f Si (seeSupporting Information S3), we find that the qBIC1 modeexhibits the highest f Si. However, the qBIC1 mode arises in thelonger wavelength regime around 2.2 μm, necessitating smallerfeature sizes to bring the resonance wavelength closer to 1.55μm. This makes the qBIC1 mode more susceptible tofabrication imperfections. While the qBIC3 mode is close toFigure 1. Low-contrast BIC metasurfaces fabricated on SOI wafers. (a) Schematic of structure and its dimensions. (b) Simulated spectra forunpatterned silicon (blue) and metasurfaces with d = 82.7 nm for α = 0% (black) and α = 5% (red). (c) Cross-sectional Ex distributions for threeqBIC modes supported by proposed metasurfaces with d = 82.7 nm and α = 5%. (d) Simulated transmittance maps for different α when d = 82.7nm (left) and for different d when α = 5% (right).Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c05880Nano Lett. 2025, 25, 2777−27842778https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c05880?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asFigure 2. Fabricated low-contrast BIC metasurfaces with d = 82.7 nm. (a) Tilted and (b) yz-plane cross-sectional SEM images. (c) Simulated (red)and experimental (blue) transmittance spectra for different α. The gray regions indicate the wavelengths where the qBIC2 mode appears. Each insetshows a schematic of the unit structure with broken symmetry. (d) Representative SEM image (top) and transmittance spectrum (bottom) of ametasurface with α = 1%. The Q factor is extracted by fitting the transmittance spectrum with a Fano function.Figure 3. Characteristics of low-contrast BIC metasurfaces with different etching depths d. (a) Transmittance maps. The dashed lines indicate αvalues that satisfy the critical coupling conditions (αCC). (b) Q-analysis results. The radiative Q factors (Qr, filled circles) with their fitted lines andQscat obtained by fitting the experimental Q factors (black open circles) are shown. The Q factors were measured in air. (c) Coefficient Q0 obtainedby fitting the inverse square relation of Qr. (d) Extracted Qscat. The gray region indicates the approximate upper limit of Qscat. (e) Experimentalspectra and corresponding Fano fitting for metasurfaces near critical coupling conditions: α = 3% for d = 82.7 nm, α = 4% for d = 116.1 nm, and α= 5% for d = 149.5 nm. The normalized electric field |E| distributions for each condition are also shown.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c05880Nano Lett. 2025, 25, 2777−27842779https://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c05880?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe qBIC2 mode in wavelengths, its smaller f Si is notconductive to achieving higher Q factors. Therefore, theqBIC2 mode emerges as the most suitable choice for theultrahigh-Q factors, balancing a sufficiently large f Si and apractical feature size.For the experimental realization of the designed low-contrastmetasurfaces, electron beam lithography with a positive resistwas employed, followed by dry etching using the Boschprocess with SF6 and C4H8 gases, allowing the precise controlof the silicon etching depth (Figure S1). Figure 2a, b presentsscanning electron microscopy (SEM) images of the fabricatedmetasurfaces under highly controlled etching depth conditions.The fabricated device was characterized using a custom-builtsetup comprising a tunable laser and photodiode (Figure S2).Figure 2c shows the simulated and experimental transmittancespectra for d = 82.7 nm when normally incident x-polarizedlight was coupled with the metasurfaces with different α. Thesimulations and experiments were in good agreement, showingthe qBIC2 resonance mode even for a small asymmetry α of0.5%. Because the length difference between the upper andlower shallow rods for α = 0.5% is given by (L + 2ΔL)−(L−2ΔL) = 6.1 nm, the fabrication disorder is considered to besmaller than this value (see Supporting Information S4). Asseen in the spectra, the resonance amplitude increased withincreasing α, which is a typical behavior of qBIC modes.However, small sidebands occasionally appeared in thespectrum, possibly due to periodic size variations in thefabricated metasurfaces. These variations may arise from thewriting order with regularity during EB lithography (seeSupporting Information S5). Additionally, a slight blueshift inthe experimental resonance wavelengths compared with thesimulations was observed, which can be attributed to therounded corners from lithography and/or the undercutgeometry of sidewalls. Figure 2d shows a representativeenlarged SEM image and the corresponding spectrum for α= 1%, showing a clear ultrasharp Fano resonance. The Q factorwas extracted by fitting the transmittance spectrum with aFano function, yielding a Q factor of approximately 101,000 atresonance peak wavelength of λ = 1560.3 nm.To further elucidate the physics underlying the ultrahigh-Qfactors, low-contrast BIC metasurfaces with etching depths d of82.7, 116.1, and 149.5 nm were fabricated, and their Q-analysiswas conducted by varying asymmetry α. First, we comparedthe simulated transmittance spectra for the metasurfaces withthree etching depths, as shown in Figure 3a. As α increased,the resonance peaks blueshifted, and the resonance linewidthsbroadened, corresponding to an increased radiative componentcoupling into the external medium. The experimentallymeasured Q factors as a function of α are shown in Figure3b. The black curves overlapping the experimental Q factorswere fitted using Q−1 = Qr−1 + Qscat−1, where Qscat−1 representsthe scattering losses arising from fabrication imperfections.Qscat was determined using nonlinear least-squares curve-fittingmethod and assuming that Qscat was independent of α (seeSupporting Information S6). The loss component that doesnot contribute to far-field coupling is expressed as the sum ofthe material absorption and scattering losses owing tofabrication imperfections. However, in this study, we onlyconsidered scattering losses because the silicon and BOX layersexhibit negligible absorption in the wavelength range ofinterest. Qr denotes the radiative Q factor calculated fromcomplex eigenfrequencies obtained from the finite elementmethod (FEM) method (COMSOL). Here, Qr follows thetypical inverse square relation, Qr = Q0α−2 for qBIC modes,24with Q0 being a constant that depends on the metasurfacedesign and mode radiation characteristics (Figure 3c). Asshown in Figure 3b, for small α, the experimental Q factorslargely deviated from the inverse square relation for all etchingdepths and approached a fix value determined by Qscat. Thesebehaviors indicated that the experimental Q factors werelimited by an inherent Qscat. The difference between theexperimental Q factors and Qr widened as d increased,indicating an increase in scattering losses (i.e., decrease inQscat) for larger etching depths. Figure 3d shows theexperimentally determined Qscat as a function of d, demonstrat-Table 1. Experimentally Observed Free-Space Accessible Q Factors Reported to Date in All-Dielectric NanostructuresaUnit structure Resonance type Material θinc (deg) λ (nm) Exp. Q Ref.Circular hole GMR Si3N4 on SiO2 0.2 490 32,000 33Circular hole GMR Resist on SOI 0 1551 239,000 19Square hole GMR Resist on Si3N4 0 779 1,100,000 29Circular hole Merging-BIC Si slab 1.2 1568 490,000 15Circular hole Mini-BIC Si slab 5.4 1573 1,090,000 28Tilted bars Chiral-BIC TiO2 on SiO2 0 612 1250 34Cuboid Accidental-BIC SOI 0 1538 5305 35Square nanodisk with hole TD-BIC SOI 0 1497 4990 36Nanodisk dimer TD-BIC SOI 0 1480 3142 37Nanohole dimer TD-BIC SOI 0 1518 22,633 18Nanodisk SLR a-Si on silica 0 1183 2750 38Cylinder SP-BIC SOI 0 1425 1946 39T-shape block SP-BIC Si on quartz 0 1588 18,511 40U-shape block SP-BIC Si on sapphire 0 1548 3534 41Block with nanogaps SP-BIC Si on quartz 0 1553 1233 42Shallow tetramer SP-BIC Si3N4 on quartz 0 828 6061 22Nanorod SP-BIC a-Si on fused silica 0 1505 4130 43Double holes SP-BIC SOI 0 1553 36,964 44Square nanopillar SP-BIC SOI 0 1685 2476 45Shallow pair-rod SP-BIC SOI 0 1560 101,000 This studyaSP, symmetry-protected; TD, toroidal dipole; a-Si, amorphous silicon; GMR, guided-mode resonance; SLR, surface lattice resonance.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c05880Nano Lett. 2025, 25, 2777−27842780https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c05880?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asing that metasurfaces with smaller d exhibited reducedscattering losses, thereby yielding higher experimental Qfactors. Although the highest Q factor exceeding 105 wasachieved for d = 82.7 nm in our experiments, furtherreductions in d below 50 nm did not yield any additionalincrease in Qscat (see Supporting Information S7). Thissuggests that once a certain Qscat is reached, furtherimprovements in Q factors are unlikely due to inevitablefabrication errors and structural disorder.As discussed below, the condition where Qr = Qscatcorresponds to the critical coupling condition, which plays acrucial role in various applications such as sensing.25−27 In ourexperiment, we selected and measured metasurfaces with α =3%, 4%, and 5% for etching depths d = 82.7, 116.1, and 149.5nm, respectively, as these structural conditions were close tothe critical coupling condition (αcc). The representative spectraare shown in Figure 3e. As expected, the experimental Qfactors increased as d decreased. Figure 3e also shows thecross-sectional electric field distributions, indicating that theoverlap between the localized electric fields and sidewalls ofthe shallow pair-rod structures reduces with decreasing d. Thisobservation supports our conclusion that the increased Qfactors for smaller d are attributable to the reduced ratio ofnanostructure depth to in-plane dimensions, resulting in lowerscattering losses due to nanostructured sidewall roughness.Considering that Q0 is larger for smaller d, we can concludethat the experimental Q factors increased by both the high Qrand Qscat values for the low-contrast metasurfaces.Table 1 compares the ultrahigh-Q factors achieved in thisstudy with previously reported experimental Q factors for free-space accessible all-dielectric nanostructures. While somereports demonstrate Q factors exceeding those achieved inthis work, they rely on topological charge engineering andprecise control of the incident angles,15,28 or suffer from poordurability of nanopatterned photoresist.19,29 In contrast, ourlow-contrast BIC metasurfaces supporting symmetry-protectedBICs offer potential advantages in simpler fabrication andexhibit an experimental Q factor that is an order of magnitudehigher than the highest Q factor reported to date. Given thatmost reported Q factors were 1000 or less,14,30−32 the Q factorof our metasurface was one or two orders of magnitude higherthan typical values.Next, we characterized the sensing properties of the low-contrast BIC metasurfaces. Previous studies have reported thatthe lowest LOD for environmental refractive index changes canbe achieved under critical coupling conditions.27,46 Therefore,we compared the refractometric sensing performance ofmetasurfaces near the critical coupling conditions for differentetching depths d. Figure 4a shows the experimental results,where aqueous solutions with different bulk refractive indices,adjusted by mixing isopropyl alcohol (IPA) and heavy water(D2O), were introduced into a polydimethylsiloxane (PDMS)microfluidic channel. The resonance peak wavelengths wererecorded in real-time (see Supporting Information S8 for theoriginal data). Here, we used D2O instead of H2O to avoidabsorption loss of water in the wavelength range of interest,thus simplifying the analysis of sensing performance. As shownin the figure, the resonance peak wavelengths redshifted withincreasing refractive index of the solution. Figure 4b shows therelation between the resonance peak wavelength shift Δλ andrefractive index change Δn. The environmental refractive indexsensitivity S was calculated from the slope and found to beapproximately 26 nm/RIU, which was smaller than the typicalS of several hundred of nm/RIU for photonic sensors.47−51Although S increased slightly with increasing d, the incrementwas small, being consistent with the simulation results (seeSupporting Information S9). This reduced sensitivity is alsoattributed to the intrinsically strong confinement of the higher-Figure 4. Refractometric sensing in low-contrast BIC metasurfaces with varying etching depths d. (a) Real-time measurement results. D2Osolutions with different refractive indices were introduced sequentially, with refractive index variations of Δn = 0.001, 0.0025, 0.005, and 0.01. Aftereach step, the metasurfaces were rinsed with D2O. (b) Wavelength shift as a function of refractive index variation. Sensitivity S was derived from thecurve slope. (c) Representative real-time fluctuations in resonance peak wavelength measured in D2O. (d)-(g) Comparison of S, Q, FOM, andLOD for different d.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c05880Nano Lett. 2025, 25, 2777−27842781https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c05880/suppl_file/nl4c05880_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c05880?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asorder qBIC2 mode, which has weaker mode overlap with theexternal medium. In fact, the qBIC2 mode in low-contrastmetasurfaces exhibits a field confinement factor in water that isten times smaller than that of the fully etched BICmetasurfaces (see Supporting Information S10). Figure 4cshows real-time measurements of peak wavelength fluctuationsδλ (= 3σ, where σ is the standard deviation) in D2O over aperiod of approximately 1 min. The δλ increased slightly aslinewidths widened with increasing d. Here, the shortevaluation period (approximately 1 min) was chosen tominimize the effects of wavelength drift. The minimum δλ was0.79 pm for the metasurface with d = 82.7 nm (α = 3%).Figure 4d-g compares the experimental Q factors, sensitivity S,figure-of-merit (FOM), and LOD for different d. The FOMdefined as FOM = S/fwhm, where fwhm is the full width athalf-maximum, reached a maximum of 825 when d = 82.7 nm(Figure 4f). Thanks to our ultranarrow linewidths, this FOMwas among the highest reported for experimentally demon-strated all-dielectric metasurfaces, such as asymmetric doublebars (FOM ∼200),52 nanogap-enhanced blocks (FOM =239),42 and dual-rectangular pillars (FOM = 418).53 The LODgiven by δλ/S slightly reduced with decreasing d (Figure 4e),yielding values of 3.00 × 10−5, 3.28 × 10−5, and 3.51 × 10−5 ford = 82.7, 116.1, and 149.5 nm, respectively. Although thewavelength fluctuations were small at the sub-picometer scale,sensitivity S was also small. Therefore, increasing the Q factorsdoes not lead to substantial LOD improvements due to thegeneral trade-off between the Q factor and sensitivity.Nevertheless, our ultrahigh-Q silicon metasurfaces hold greatpotential for applications that require detection of localperturbations, such as single-molecule detection at highconcentrations, leveraging both their high Q factors andsmall mode volume.54 Although a rigorous quantification ofthe mode volume warrants further investigation, the Q/V ratiocould potentially be optimized through geometric modifica-tions, including adjustments to the number of unit structures.Moreover, the proposed metasurfaces offer substantialadvantages, including simple measurements based on posi-tion-insensitive vertical excitation from free-space, broadwavelength tunability (see Supporting Information S11), andavailability of well-established complementary metal−oxide−semiconductor compatible fabrication processes, facilitatingthe practical implementation of low-cost sensing systems.In conclusion, we have experimentally demonstrated ultra-high-Q factors exceeding 105 in low-contrast siliconmetasurfaces supporting higher-order qBIC modes. By design-ing the metasurfaces to minimize the overlap between thelocalized electric fields and sidewalls in shallow-etchednanostructures, we achieved high radiative Q factors whilereducing scattering losses from fabrication imperfections.Specifically, for an etching depth of 82.7 nm and asymmetryparameter of 1%, we obtained a record-high Q factor of101,000, which was an orders of magnitude higher than that oftypical dielectric metasurfaces governed by symmetry-pro-tected BICs. Additionally, we observed sub-picometer peakwavelength fluctuations in an aqueous solution, demonstratingan improved limit of detection in the order of 10−5 for changesin the environmental refractive index. Given that qBIC modescan be easily coupled to normally incident light without theneed for delicate coupling systems, we believe that ourultrahigh-Q metasurfaces offer a promising platform for a widerange of applications requiring strong light−matter couplingincluding strong coupling, nonlinear frequency conversion, andquantum photonics.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c05880.Additional details about simulation, fabrication, andoptical characterization; electric field distributions;calculation of field confinement factors; characterizationof fabrication imperfections from the SEM image;possible cause of sidebands in the spectrum; analysisof the measured spectra with the temporal coupledmode theory; Q-analysis for metasurfaces with shalloweretching depth; original data of the refractometric sensingresults; simulation results of refractometric sensitivity;direct comparison with fully etched metasurfaces;simulated transmittance spectra for varying geometricalparameters (PDF)■ AUTHOR INFORMATIONCorresponding AuthorKeisuke Watanabe − International Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0002-4285-2135;Email: watanabe.keisuke@nims.go.jpAuthorsTadaaki Nagao − International Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; Department of Condensed Matter Physics, GraduateSchool of Science, Hokkaido University, Sapporo 060-0810,Japan; orcid.org/0000-0002-6746-2686Masanobu Iwanaga − Research Center for Electronic andOptical Materials, National Institute for Materials Science(NIMS), Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-8930-6940Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.4c05880NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was financially supported by JSPS KAKENHI GrantNumber JP22K20496, Iketani Science and TechnologyFoundation (Grant Number 0361252-A), and “AdvancedResearch Infrastructure for Materials and Nanotechnology inJapan (ARIM)″ of the Ministry of Education, Culture, Sports,Science and Technology (MEXT). 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