Quasi-BICs in Photonics and Beyond

• Quasi-BICs are weakly radiative resonances that emerge when bound states in the continuum are slightly perturbed, resulting in ultrahigh-Q modes with narrow Fano features.
• They are implemented in dielectric metasurfaces, whispering-gallery microcavities, and acoustic resonators, enabling precise spectral filtering and enhanced nonlinear interactions.
• Their formation mechanisms include symmetry breaking, Friedrich–Wintgen interference, and non-Hermitian perturbations, offering versatile design strategies for optical and acoustic devices.

Quasi-bound states in the continuum (quasi-BICs, q-BICs) are weakly radiative resonant states obtained when an ideal bound state in the continuum is slightly opened by a perturbation. In the ideal BIC limit, the mode lies in the radiation continuum in frequency and momentum yet cannot couple out because symmetry forbids the coupling; once exact decoupling is weakly disturbed, the mode acquires finite leakage, a very large but finite $Q$-factor, and typically a narrow Fano resonance (Overvig et al., 2020). Recent work places quasi-BICs in periodic dielectric metasurfaces, deformed whispering-gallery microcavities, twisted bilayer photonic slabs, compact acoustic resonators, coupled waveguide lattices, and interaction-modulated Bose-Hubbard models, indicating that the concept is not restricted to a single material system or a single protection mechanism (Liu et al., 2024, Huang et al., 2022, Deriy et al., 2021, Longhi, 2021, Huang et al., 2023).

1. Definition, spectral character, and quality-factor scaling

The canonical photonic picture treats a quasi-BIC as a long-lived resonance created by weakly opening a symmetry-protected or interference-protected BIC. In temporal coupled-mode descriptions, the perturbation magnitude $\delta$ sets the radiative leakage, with the usual scaling

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$

so smaller perturbations produce weaker leakage, longer lifetime, and sharper resonances (Overvig et al., 2020). In experimentally fitted transmission or reflection spectra, the line shape is typically Fano-like, for example

$$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$

which makes the quasi-BIC identifiable as a narrow interference feature on top of a broader background (Liu et al., 2024).

This narrow linewidth is the operational basis of quasi-BIC functionality. In metasurfaces it yields ultrasharp spectral filtering and wavefront control; in nonlinear platforms it increases field buildup and modal overlap; in microcavities it suppresses leakage loss; and in scattering experiments it appears as a high-$Q$ Fano resonance rather than a perfectly dark state. A quasi-BIC is therefore not an exact embedded eigenstate, but a controlled finite-lifetime descendant of one.

2. Formation mechanisms

The literature now contains several distinct routes to quasi-BIC formation. In periodic dielectric structures, the most familiar route is a small symmetry-breaking perturbation that converts a dark mode into a leaky resonance. Chiral metasurfaces extend this beyond linear eigenpolarizations by breaking the horizontal mirror symmetry with a two-interface perturbation, allowing arbitrary elliptical eigenpolarizations and full circular dichroism under the quarter-wave condition (Overvig et al., 2020). Doubly degenerate quasi-BICs can also be produced while preserving $C_{4v}$ rotational symmetry, by breaking translation symmetry in a diatomic silicon metasurface without losing polarization independence (Liu et al., 2024).

A second class is Friedrich–Wintgen interference, where two leaky resonances share the same radiation continuum and destructively interfere in loss. In a single deformed Limaçon whispering-gallery microcavity, boundary deformation creates stable unidirectional leakage channels, enabling strong external coupling between a high-$Q$ quasi-WGM and a lower-$Q$ 6-bounce mode; one hybrid branch suppresses radiation while the other becomes more lossy (Liu et al., 2024). A related avoided-crossing mechanism in finite photonic crystals is used to realize the “super quasi-BIC,” where parameter-space hybridization and momentum-space BIC merging act simultaneously (Zhang et al., 2022).

A third class relies on out-of-plane or non-Hermitian perturbations. In bianisotropic metasurfaces, perforating the top of cylinders breaks $\sigma_h$, activates coupling between electric-like and magnetic-like q-BIC resonances, and drives Rabi-like splitting in the strong-coupling regime (Máñez-Espina et al., 9 Dec 2025). In a $\mathcal{PT}$-symmetric ENZ trilayer, spontaneous $\delta$0-symmetry breaking at a singular point coincident with the BIC position turns the ideal BIC into a high-$\delta$1, nearly perfectly transmitting quasi-BIC (Novitsky et al., 2021). A closely related but topological formulation shows that preserving in-plane symmetry while breaking $\delta$2 can convert a BIC into a quasi-BIC and induce a Zak phase inversion with band inversion between quadrupole and dipole modes (Chen et al., 22 Jan 2026).

A fourth class broadens the concept beyond simple symmetry breaking. In quasi-BFICs, mirror-symmetric disorder inside a periodic supercell produces band folding, mode localization, and multiple topological charges in $\delta$3-space, so that a whole flat band above the light line exhibits quasi-BIC behavior (Qin et al., 7 Nov 2025). In a many-body Bose-Hubbard chain, the quasi-BIC is a multiparticle maximally localized Wannier state in which a bound pair is localized by the standing wave of a third particle under periodic boundary conditions (Huang et al., 2023).

Mechanism	Representative system	Reported consequence
Small symmetry-breaking perturbation	Dielectric or chiral metasurfaces	Finite leakage, large $\delta$4, narrow Fano resonance
Shared leaking continuum	Deformed microcavity; avoided-crossing PhC	Loss suppression in one branch, larger loss in the other
Out-of-plane or non-Hermitian symmetry breaking	$\delta$5-broken metasurfaces; $\delta$6-symmetric ENZ trilayers	Controlled radiation coupling and quasi-BIC formation
Disorder or flat-band engineering	quasi-BFICs; moiré flat bands	High-$\delta$7 response over extended $\delta$8-space

This diversity shows that “quasi-BIC” has become a general language for controlled partial decoupling from a continuum, not merely a label for one perturbative geometry.

3. Theoretical descriptions

Temporal coupled-mode theory (TCMT) remains the most compact description of photonic quasi-BICs. For chiral quasi-BIC metasurfaces, the scattering matrix is written as

$\delta$9

where $\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$0 is the direct scattering pathway and $\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$1 encodes resonant coupling to the external channels (Overvig et al., 2020). This form makes explicit how the high-$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$2 resonant term interferes with the broad background and produces polarization-selective Fano features.

When more than one mode participates, non-Hermitian two-mode models become central. In the single deformed microcavity, the effective Hamiltonian

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$3

distinguishes internal strong coupling from external strong coupling through a shared leaking continuum; the decisive effect is a redistribution of the imaginary parts of the eigenvalues, not merely a shift of the real parts (Liu et al., 2024). In bianisotropic metasurfaces, a two-mode TCMT with electric-like and magnetic-like resonances,

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$4

produces hybrid eigenfrequencies

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$5

which encode the avoided crossing and Rabi-like splitting of opposite-symmetry q-BIC-derived modes (Máñez-Espina et al., 9 Dec 2025).

Finite-size theories emphasize that quasi-BICs are not solely vertical-radiation phenomena. For few-cell photonic-crystal slabs, the total quality factor is decomposed as

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$6

with vertical radiation set by reciprocal-space overlap with the infinite-slab $\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$7 landscape and in-plane loss set by the finite cavity extent (Taghizadeh et al., 2017). In one-dimensional quasi-BIC gratings, coupled-mode equations for counter-propagating modes,

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$8

yield the dispersion relation

$\tau=\frac{Q}{\omega_0}\propto \frac{1}{\delta^2},$9

which makes the continuous $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$0-$$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$1 quasi-BIC branch explicit (Sun et al., 2022).

A distinct theoretical reformulation derives quasi-BICs from a two-band model with an arbitrary confining potential rather than from symmetry protection or topology. In that framework, a quasi-BIC is a bound state weakly coupled to a continuum band, and an exact BIC appears when the real-pole contribution is eliminated through the condition

$$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$2

or its multiband analogue (Rao et al., 24 Jun 2025). This does not replace symmetry-based quasi-BIC theory, but it broadens the formal criteria under which exact and approximate embedded states can be defined.

4. Reciprocal-space structure, finite size, and band generalizations

A recurrent limitation of conventional quasi-BICs is that they are confined to a very narrow range in $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$3-space and are highly sensitive to disorder (Qin et al., 7 Nov 2025). Several recent directions explicitly target that limitation. One-dimensional quasi-BICs in a binary grating form a continuous family of resonances over a broad spectral band along a dispersion line in $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$4-$$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$5 space, with $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$6 approximately following $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$7 as the ridge-width asymmetry shrinks (Sun et al., 2022). A related photonic-slab experiment reports a symmetry-protected quasi-bound band in the continuum (qBBC) extending continuously along $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$8–X, with linewidth about $$T=\left|a_1+i a_2+\frac{b}{\omega-\omega_0+i\gamma}\right|^2,\qquad Q=\frac{\omega_0}{2\gamma},$$9–$Q$0 meV through at least half the Brillouin zone (Tsoi et al., 5 Nov 2025).

Disorder-enabled flat-band engineering pushes the same idea further. In quasi-BFICs, a mirror-symmetric supercell with disorder strength $Q$1 yields a flat band whose Bloch modes all behave quasi-BIC-like above the light line; angle-resolved measurements give $Q$2-factors around $Q$3, $Q$4, $Q$5, and $Q$6, with the overall statement that $Q$7 stays near $Q$8 from $Q$9 to $C_{4v}$0 (Qin et al., 7 Nov 2025). In moiré quasi-BICs, the flat-band condition is achieved by balancing interlayer coupling and twist angle in a twisted bilayer photonic slab, and the $C_{4v}$1-point mode approaches a perfect BIC as the twist angle is reduced (Huang et al., 2022).

Finite-size engineering supplies a complementary route. A photonic-crystal slab can retain very high-$C_{4v}$2 quasi-BIC behavior with only two or three unit cells, provided the finite cavity mode in reciprocal space is matched to the infinite-slab BIC dispersion (Taghizadeh et al., 2017). The “super quasi-BIC” combines avoided crossing in parameter space with merging BICs in momentum space, changing the asymptotic behavior of the quality factor over the number of resonators from $C_{4v}$3 to exclusive $C_{4v}$4 for symmetry-protected BIC-derived modes (Zhang et al., 2022). Taken together, these results show that quasi-BIC physics can be extended from isolated singular points to lines, bands, merged topological structures, and even few-cell cavities.

5. Representative implementations and functionalities

Chiral and polarization-structured metasurfaces provide one of the clearest demonstrations of how quasi-BIC perturbations become a design resource rather than a parasitic leakage channel. In conventional planar ultrathin implementations with horizontal mirror symmetry, the q-BIC eigenpolarization is limited to linear polarization, circularly polarized illumination couples only half of the incident power to the resonance, and anomalous-reflection efficiency is capped at $C_{4v}$5. By introducing a chiral two-interface perturbation, the eigenpolarization spans arbitrary elliptical states on the Poincaré sphere; under the quarter-wave condition the eigenpolarization becomes pure RCP or LCP, and geometric-phase engineering then yields full amplitude-phase control within the resonant band, with a reported anomalous-reflection diffraction efficiency of $C_{4v}$6 (Overvig et al., 2020).

Quasi-BICs have also become a platform for analog optical computing. An all-dielectric metasurface composed of four silicon nanodisks per unit cell on an SOI platform supports a polarization-independent quasi-BIC with $C_{4v}$7 symmetry and implements isotropic two-dimensional spatial differentiation directly in free space, without a 4-f lens system. At the resonant wavelength $C_{4v}$8 nm, the transmission is near zero at normal incidence and rises approximately quadratically with $C_{4v}$9, giving a numerical aperture of about $Q$0, spatial resolution on the order of $Q$1m, and an output-to-input peak intensity ratio about $Q$2 in edge-detection imaging (Liu et al., 2024).

Nonlinear nanophotonics has adopted quasi-BICs because simultaneous linewidth narrowing and field confinement directly enhance nonlinear polarization. In an etchless lithium niobate resonant grating waveguide, dual quasi-BICs at $Q$3 nm and $Q$4 nm produce sum-frequency generation with conversion efficiency $Q$5, five orders of magnitude higher than that of LiNbO$Q$6 films of the same thickness, and a full-width at half-maximum less than $Q$7 nm (Feng et al., 2023). In a silicon metasurface with doubly degenerate $Q$8-protected quasi-BICs, a maximum THG conversion efficiency up to $Q$9 is recorded under a pump intensity of $Q$0 GW/cm$Q$1, and the THG signal is about $Q$2 times stronger than from a plain silicon film of the same thickness (Liu et al., 2024). In a twisted bilayer photonic slab, SHG assisted by moiré quasi-BICs is nearly ten times larger than that based on dispersive quasi-BICs with similar quality factors under a wide-angle optical source (Huang et al., 2022).

Quasi-BICs also support chiral emission and directional absorption. In intrinsic chiral silicon metasurfaces loaded with achiral dye, the quasi-BIC mode produces circularly polarized photoluminescence with $Q$3 over $Q$4, and the reported dissymmetry factor remains robust against variations in emission angle and dye thickness because of strong lateral field confinement (Zhu et al., 26 Aug 2025). In deeply subwavelength bianisotropic metasurfaces embedded in a reflective slab, q-BIC-enabled strong mode coupling yields dual-band asymmetric absorption with more than a $Q$5 directional absorption difference and fitted coupling strength $Q$6 THz, well above the strong-coupling threshold $Q$7 THz (Máñez-Espina et al., 9 Dec 2025).

6. Beyond planar optics: microcavities, acoustics, many-body analogs, and open questions

The quasi-BIC concept now extends well beyond planar dielectric metasurfaces. In a single deformed Limaçon whispering-gallery microcavity on a Si$Q$8N$Q$9 platform, strong external coupling through a shared leaking continuum yields a Friedrich–Wintgen quasi-BIC with highest measured $\sigma_h$0 exceeding $\sigma_h$1, more than a 3-fold increase relative to the uncoupled mode (Liu et al., 2024). In compact acoustic resonators immersed in fluid, breaking the symmetry of a genuine acoustic BIC mixes non-radiating torsional multipoles with radiative channels and produces quasi-BICs that appear in scattering spectra as high-$\sigma_h$2 Fano resonances, with absorption-limited $\sigma_h$3 estimated around $\sigma_h$4 to $\sigma_h$5 depending on the loss factor (Deriy et al., 2021). In coupled photonic waveguide lattices, breaking mirror symmetry and allowing non-nearest-neighbor couplings creates a doublet of quasi-BIC states enabling weakly damped embedded Rabi oscillations, with a reported regime where $\sigma_h$6 (Longhi, 2021). In the interaction-modulated Bose-Hubbard model, the quasi-BIC under periodic boundary conditions is a multiparticle maximally localized Wannier state, and Thouless pumping shifts its center of mass by a unit cell while the bound pair moves opposite to the standing wave (Huang et al., 2023).

Two objective clarifications follow from this broader landscape. First, it is incomplete to identify quasi-BICs only with symmetry-protected resonances at isolated high-symmetry points of planar photonic crystals. Counterexamples include shared-continuum Friedrich–Wintgen states in a single microcavity (Liu et al., 2024), quasi-BICs induced by $\sigma_h$7-symmetry breaking in ENZ trilayers (Novitsky et al., 2021), disorder-assisted quasi-BFICs spanning a flat band (Qin et al., 7 Nov 2025), and the two-band criterion for exact BIC formation that requires neither symmetry protection nor topological constraints (Rao et al., 24 Jun 2025). Second, the main practical limitations remain bandwidth, angular window, and disorder or fabrication sensitivity. The free-space edge-detection metasurface is explicitly not broadband in the absolute sense and depends on fabrication precision (Liu et al., 2024), while conventional quasi-BICs are explicitly described as highly sensitive to disorder and confined to a narrow range in $\sigma_h$8-space (Qin et al., 7 Nov 2025).

These developments suggest three converging research directions. One is the extension from point-like singular resonances to line, band, and flat-band quasi-BICs. A second is the use of additional symmetry knobs—especially $\sigma_h$9 breaking, chirality, and non-Hermitian coupling—to control not just $\mathcal{PT}$0, but also polarization, directionality, and modal hybridization. A third is the search for more general exact-BIC criteria that unify symmetry-protected, interference-based, disorder-assisted, and many-body embedded states within a common continuum-coupling framework.