Silicon BIC Metasurface

• Silicon BIC metasurfaces are 2D arrays of nanoresonators that trap light via symmetry protection and destructive multipolar interference, yielding ultrahigh-Q modes.
• Precision in geometrical design enables tuning of resonance frequencies and enhancement of nonlinear effects like third harmonic generation and four-wave mixing.
• The platform supports applications in sensing, filtering, and on-chip optics while balancing performance trade-offs and fabrication tolerances.

A silicon bound-state-in-the-continuum (BIC) metasurface is a two-dimensional periodic array of silicon nanoresonators engineered to support electromagnetic modes that remain localized, despite lying within the radiation continuum. These states suppress radiative loss via symmetry protection or multipolar interference, enabling extremely high quality factors (Q), subwavelength field confinement, and substantial electromagnetic field enhancement. In silicon-based platforms, the BIC mechanism is central to the realization of ultranarrow nonlinear, sensing, and modulation functionalities in nanoscale photonic devices.

1. Physical Mechanisms of BICs in Silicon Metasurfaces

Bound states in the continuum in silicon metasurfaces arise mainly via two mechanisms:

• Symmetry-protected BICs: When the unit cell and excitation conditions are such that the eigenmode’s symmetry is orthogonal to all accessible free-space radiation channels (e.g., even symmetry under C₂ rotation for electric field, odd incoming plane wave), the associated electromagnetic mode cannot couple out, yielding infinite (in principle) radiative Q. This decoupling is exact at high-symmetry points in momentum space (usually the Γ-point, $k_{\|}=0$), enforced by the underlying point-group symmetry of the metasurface (e.g., C₂ᵥ, C₄, C₄ᵥ) (Liu et al., 2022, Yang et al., 2024, Fang et al., 2022).
• Accidental (Friedrich–Wintgen) BICs: These result from destructive interference between two leaky multipoles (such as electric and toroidal dipoles), canceling radiation within a specific parameter regime (Liu et al., 2022, Islam et al., 2023).

Small deviations, such as structural imperfections or oblique incidence, lift the symmetry protection, yielding quasi-BICs with finite but large Q ($Q \sim 10^3$–$10^5$ in experiment) and extremely sharp resonances.

2. Geometrical Design and Resonance Engineering

Silicon BIC metasurfaces are realized through precise control over resonator geometry and lattice parameters:

• Unit Cells: Designs include nanodisk dimers (period $p=960$ nm, radius $r=210$ nm, height $h=280$ nm, gap $d=40$ nm) (Liu et al., 2022); notched nanoblocks (Fang et al., 2022); arrays of squares with varying lateral sizes for C₄ symmetry (Yang et al., 2024); dome-shaped nanobars oriented at a tilt angle $\theta$ (Islam et al., 2023); and core-shell spheres for inverse design (Gladyshev et al., 2023).
• Symmetry Breaking: Optical coupling is enabled by in-plane shape asymmetry, lateral displacement, angular perturbation, or dimensional mismatch (e.g., $dL = A_1 - A_2$ between squares (Yang et al., 2024); tilt angle $\theta$ in dome metasurfaces (Islam et al., 2023); defect size $a$ in notched blocks (Fang et al., 2022); or bar length asymmetry $\alpha = \Delta L / L_1$ in nanobars (Zheng et al., 2021)). The radiative Q universally obeys $Q \propto 1/(\text{asymmetry})^2$ for small perturbations (Liu et al., 2022, Yang et al., 2024, Fang et al., 2022, Zheng et al., 2021).
• Resonance Tuning: Resonance frequencies are shifted by scaling in-plane feature sizes ($r$, $h$, $d$), lattice period $p$, or by adjusting the gap between dimerized elements. Global scaling is employed for coarse wavelength positioning (e.g., scaling factor $S$ in (Yang et al., 2024)).

The result is the ability to selectively dial resonance spectral positions, Q, and mode profiles, supporting the integration of multiple quasi-BICs within single devices with typical footprints of a few square microns (Liu et al., 2022).

3. Nonlinear Optical Enhancement

The intense electromagnetic field confinement in silicon BIC metasurfaces results in dramatic enhancement of third-order nonlinearities. Key processes include:

• Third Harmonic Generation (THG): The nonlinear polarization at $3\omega$ is given by $P^{(3)}(3\omega)=\epsilon_0 \chi^{(3)} E(\omega)^3$, where the local field $E(\omega)$ is resonantly enhanced by the quasi-BIC. THG output scales as $$I_{THG} \propto |\chi^{(3)}|^2 (Q/V)^3 I_\text{pump}^3$$ (Liu et al., 2022, Fang et al., 2022, Koshelev et al., 2019). Enhancements up to 368× over unpatterned silicon films are observed (Fang et al., 2022), and up to $1.48 \times 10^{-5}$ THG efficiency is reported for dome-type BIC metasurfaces (Islam et al., 2023).
• Four-Wave Mixing (FWM): Multiple quasi-BICs enable simultaneous triple-resonant excitation, maximizing local fields at each pump frequency $\omega_1, \omega_2, \omega_3$, yielding degenerate and non-degenerate FWM signals (with ten new frequencies demonstrated in the visible) (Liu et al., 2022). The FWM yield is proportional to $Q(\omega_1)Q(\omega_2)Q(\omega_3)$.
• Critical Coupling: The nonlinear efficiency is maximized when the radiative and non-radiative losses are matched ($Q_\text{rad} = Q_\text{nr}$). The critical coupling condition dictates the optimal asymmetry parameter for the strongest THG response (Koshelev et al., 2019).

These enhancements underpin applications in frequency mixing, on-chip wavelength conversion, and potentially integrated photon-pair sources for quantum optics (Liu et al., 2022).

4. Sensing, Filtering, and Field-Driven Phenomena

Silicon BIC metasurfaces enable a spectrum of sensing and light-manipulation functions:

• Refractive Index Sensing: The BIC’s field enhancement yields high-sensitivity detection. Shifts in resonance are proportional to analyte RI, with sensitivities up to $85.7$ nm/RIU (glucose) (Islam et al., 2023), $171$ nm/RIU with figure-of-merit $17.6$ (Sharma et al., 4 Sep 2025), and $1.5 \times 10^7$ µm/RIU via giant Goos–Hänchen shift enhancement (Zheng et al., 2021).
• Ultra-narrow Filtering: BIC-derived quasi-BICs readily support ultrasharp transparency bands, suitable for bandpass filters and spectrally selective IR absorbers (Abujetas et al., 2020, Sharma et al., 4 Sep 2025).
• Polarization Independence: Designs with C₄ symmetry (four-square arrays) ensure identical optical response for all in-plane polarizations, with Q approaching 100 and resonance modulation ($M \approx 50\%$), supporting robust, unpolarized filtering and sensing (Yang et al., 2024).
• Optical Bistability: Thermo-optic nonlinearities at high-Q BICs enable optical switching and bistability, with controllable hysteresis and power thresholds, governed by the interplay between radiative and nonradiative damping (Barulin et al., 2023).
• Goos–Hänchen Shift: Near quasi-BIC resonances, lateral shift is enhanced by three orders of magnitude, with direct correlation between Q and GH shift, facilitating ultrasensitive displacement sensors (Zheng et al., 2021).

The platform provides tailored trade-offs between Q, modulation depth, and polarization robustness (Yang et al., 2024, Jiang et al., 23 Jun 2025).

5. Fabrication, Materials, and Inverse Design

Realization of silicon BIC metasurfaces follows established nanofabrication flows:

• Lithography/Epitaxy: Structures are typically defined with electron-beam lithography or deep-UV stepper lithography on amorphous or polycrystalline silicon layers, followed by hard-mask deposition (Cr or Al) and inductively coupled plasma (ICP) or reactive-ion etching (RIE) for pattern transfer (Yang et al., 2024, Fang et al., 2022, Liu et al., 2022).
• Substrate & Spacer Engineering: Devices may be fabricated on glass, fused silica, or silicon-on-insulator, with under-etched membranes for suspended architectures or low-index spacers for hybrid absorption (MXene, graphene) (Sharma et al., 4 Sep 2025, Xiao et al., 2020).
• Tolerance and Cleaning: Maintaining uniformity, surface smoothness, and precise dimensions is essential, with tolerances of several nanometers required to sustain Q above $10^3$–$10^4$ (Fang et al., 2022, Gladyshev et al., 2023).
• Inverse Design: Semi-analytical and machine-learning-assisted methods allow inverse calculation of metasurface geometries (core-shell, high-symmetry units) for target BIC spectral positions, with sufficient robustness to fabrication errors (Gladyshev et al., 2023).

Advances in MEMS integration enable dynamic tuning of the BIC resonance wavelength via dual-axis displacement, with tuning ranges exceeding 60 nm and maintained Q (Kovalev et al., 26 Jun 2025).

6. Performance, Limitations, and Application-Specific Trade-offs

The operational window and ultimate performance of silicon BIC metasurfaces are governed by intrinsic and extrinsic loss mechanisms, as well as environmental factors:

• Intrinsic/Extrinsic Loss: In low-loss environments (dry, air), dielectric BIC metasurfaces outperform plasmonic platforms in field enhancement and Q (Q~450, Abs~0.02), but degrade quickly when solvent absorption increases ($k_\mathrm{env} > 2 \times 10^{-3}$), at which point Au-based BICs become superior for SEIRAS (surface-enhanced IR absorbance) (Jiang et al., 23 Jun 2025).
• Critical Coupling and Absorber Bandwidth: In hybrid structures (e.g., graphene-on-Si), bandwidth and absorption can be tuned by asymmetry and carrier density, with the linewidth scaling as $\Delta\omega \propto \text{asymmetry}^2$ at critical coupling (Xiao et al., 2020).
• Trade-offs: Designs must balance field enhancement (favoring small asymmetry/high Q) with resonance modulation (visibility), detection SNR, and tolerance to angular or polarization variations (Yang et al., 2024).

Table: Typical Performance Parameters for Representative Silicon BIC Metasurfaces

| Structure | Q (exp.) | Max |E/E₀| | Nonlinear Eff. (THG) | Sensing FOM (RIU⁻¹) | Notes | |----------------------------|----------|-----------|-----------------------|---------------------|------------------------------------------| | Nanodisk dimers (Liu et al., 2022) | 10³–10⁵ | >20 | FWM/THG, 10⁻⁷–10⁻⁸ W/W | — | Triple quasi-BIC, ultracompact mixer | | Notched nanoblocks (Fang et al., 2022) | >3,000 | >10 | 368× (vs. film) | — | Topology modulates scaling exponent | | Four-square C₄ arrays (Yang et al., 2024) | ~100 | >30 | — | — | Polarization-independent | | Dome nanobars (Islam et al., 2023) | >10⁴ | — | 353× (vs. film) | ~25 | Dual-mode, glucose/THG applications | | Nanobars, GH (Zheng et al., 2021) | 10³–10⁴ | — | — | 1.5×10⁷ μm/RIU | Giant Goos–Hänchen shift sensor | | Nanodisk + MXene (Sharma et al., 4 Sep 2025) | ~150 | — | — | 17.6 | Methane sensing, >95% absorption | | Elliptical pillars (Jiang et al., 23 Jun 2025) | ~450 | 18× | — | See text | SEIRAS, Si vs. Au performance trade-off |

7. Outlook and Application Domains

Silicon BIC metasurfaces now underpin functions spanning nonlinear frequency generation (FWM, THG), on-chip optical mixing, refractometric and biochemical sensing, narrowband filtering, high-contrast switching, and ultra-narrow SEIRAS. Their fabrication is CMOS-compatible, their symmetry-based design principles are extendable to non-silicon platforms (GaP, InP, GaAs), and advanced inverse-design and MEMS-tunable architectures are emerging for flexible, robust photonic integration (Liu et al., 2022, Yang et al., 2024, Kovalev et al., 26 Jun 2025).

The continued evolution of silicon BIC metasurfaces is expected to deliver integrated quantum photonics (via spontaneous FWM), flat-optics light sources (via BIC-exciton polariton condensation (Berghuis et al., 2023)), robust in-liquid sensing, and agile MEMS-tunable photonic devices, consolidating their role as core elements in advanced photonic circuit design.