Light-Cone Proximal Chiral q-BICs

• Light-Cone Proximal Chiral q-BICs are high-Q photonic resonances derived from symmetry-protected modes near the light cone that exhibit strong spin-selectivity.
• They are realized by weak symmetry breaking in metasurfaces, which opens controlled radiative leakage channels and achieves large circular dichroism.
• Quantitative design rules and topological polarization properties enable practical applications in chiral sensing, lasing, and advanced optical modulation.

Searching arXiv for the cited paper and closely related work on chiral q-BICs, light-cone proximity, Janus BICs, and chiral polaritons. Light-cone-proximal chiral quasi-bound states in the continuum are high-$Q$, weakly radiative resonances in metasurfaces or photonic-crystal slabs that descend from bound states in the continuum embedded in the radiation continuum but remain accessible to free-space excitation because they lie at, near, or very close to the light cone. Their defining feature is the coexistence of BIC-derived linewidth suppression with strong handedness selectivity: the resonant channel couples predominantly to one circular polarization, yielding large circular dichroism (CD), large degree of circular polarization (DCP), or both. In the recent literature, this concept spans $\Gamma$-point guided resonances at normal incidence, cone-proximal oblique-angle states near $k_z \to 0$, Janus and chiral BIC topologies in bilayer photonic crystals, strong-coupling platforms for valley-selective exciton polaritons, and magnetically controlled chiral BICs with real-space topological textures (Shi et al., 2021, Gromyko et al., 28 Aug 2025, Wurdack et al., 2024, Kang et al., 2024, Dong et al., 14 May 2026, Zhao et al., 26 Feb 2026).

1. Conceptual definition and placement relative to the light cone

A BIC is a spatially localized electromagnetic eigenmode whose eigenfrequency lies within the radiation continuum but whose far-field coupling is exactly forbidden by symmetry or destructive interference, so that the radiative quality factor diverges. A q-BIC is obtained when the protecting symmetry is weakly perturbed, opening a finite but small radiative channel and yielding a narrow resonance with large but finite $Q$. In metasurfaces, the perturbation can be intrinsic, through unit-cell asymmetry, or extrinsic, through oblique illumination (Shi et al., 2021).

The light-cone condition is commonly written as $k_{\parallel} \le k_0 n$, with $k_0=\omega/c$ and $k_{\parallel}=\sqrt{k_x^2+k_y^2}$. In $\Gamma$-point implementations, light-cone proximity means that the mode lies at or near $k_{\parallel}=0$ within the free-space-accessible part of the dispersion, so that only a small set of radiative channels is open and symmetry selection rules are especially restrictive. In cone-singularity implementations at large oblique angles, light-cone proximity instead refers to a mode whose in-plane wavevector approaches the light line so that $k_z \to 0$ and the relevant radiative channel sits very near the diffraction threshold (Gromyko et al., 28 Aug 2025).

Taken together, these works suggest two operational meanings of light-cone proximity. One is $\Gamma$0-point accessibility, where a symmetry-protected BIC becomes a normal-incidence or near-normal-incidence chiral q-BIC after weak symmetry breaking. The other is asymptotic proximity to the light line at oblique $\Gamma$1, where radiative coupling is strongly reshaped by the divergence of the radiative local density of states (LDOS). Both meanings retain the same core logic: the resonance remains BIC-derived, the leakage is controlled rather than generic, and chirality emerges from symmetry-engineered spin selectivity rather than from broadband circular response alone (Shi et al., 2021, Gromyko et al., 28 Aug 2025).

2. Symmetry breaking, leakage channels, and $\Gamma$2-factor scaling

The canonical route to a chiral q-BIC begins with a symmetry-protected $\Gamma$3-point BIC and then introduces a small perturbation that unlocks a single or strongly dominant spin channel. In planar chiral metasurfaces, this perturbation can be an in-plane geometric asymmetry or a slight incidence tilt. In intrinsically chiral visible-frequency structures, both in-plane and out-of-plane mirror symmetries are deliberately broken, so that the resonance couples differently to left- and right-circularly polarized light even at normal incidence without relying on polarization conversion artifacts (Shi et al., 2021, Chen et al., 2022).

Across these systems, the radiative leakage obeys the standard q-BIC quadratic law. In the planar double-sided scythe metasurface, the extrinsic asymmetry parameter is $\Gamma$4, giving $\Gamma$5, while the intrinsic asymmetry parameter is $\Gamma$6, giving $\Gamma$7. The total quality factor follows

$\Gamma$8

In the slanted visible-frequency TiO$\Gamma$9 metasurface, the same physics is expressed as $k_z \to 0$0 and $k_z \to 0$1, with the perturbation set by the in-plane deformation $k_z \to 0$2 and out-of-plane slant $k_z \to 0$3 (Shi et al., 2021, Chen et al., 2022).

Temporal coupled-mode theory provides the common reduced model. For a single resonance,

$k_z \to 0$4

where $k_z \to 0$5 is the modal amplitude, $k_z \to 0$6, and the coupling vectors $k_z \to 0$7 and $k_z \to 0$8 encode the spin-resolved in-coupling and out-coupling. Maximal chirality corresponds to the case in which one circular port is dark while the other remains bright. In the normal-incidence intrinsic chiral BIC realized in the visible, this is expressed by distinct circular-basis selection rules $k_z \to 0$9, $Q$0, or vice versa (Chen et al., 2022).

A different leakage mechanism appears in cone-proximity q-BICs at large oblique angles. There the radiative rate is written as

$Q$1

and the LDOS near the light cone scales as

$Q$2

This introduces an “LDOS lever”: even modest spin-selective overlap $Q$3 can generate strong circular selectivity when the mode approaches $Q$4 (Gromyko et al., 28 Aug 2025).

3. Polarization topology, helicity, and chiral singularities

The chiral character of these q-BICs is not merely a spectral asymmetry between left and right circular inputs. It is tied to the topology of the far-field polarization field in momentum space. In the planar double-sided scythe metasurface, the BIC sits at a vortex polarization singularity, denoted a V point, around which the eigen-polarizations are elliptical with nonvanishing helicity $Q$5. Opposite-helicity eigen-polarizations encircle the singularity, producing a chiral polarization vortex that directly underpins spin-selective coupling near the q-BIC (Shi et al., 2021).

This polarization-topological picture becomes more explicit when the parent BIC singularity splits into C points. In the visible-frequency intrinsic chiral BIC, a $Q$6-point integer V point splits into half-charged C points when the out-of-plane mirror symmetry is broken; then an additional in-plane perturbation moves one C point to $Q$7, making the resonance bright for one circular polarization and dark for the other at normal incidence (Chen et al., 2022). In Janus BIC systems, the same process is resolved separately in the upward and downward radiation channels, so that one side can carry a different net topological charge from the other (Kang et al., 2024, Dong et al., 14 May 2026).

The topological charge is defined by the winding of the polarization angle in momentum space,

$Q$8

In the Janus photonic-crystal slab, a $Q$9-point BIC with $k_{\parallel} \le k_0 n$0 coexists with two off-$k_{\parallel} \le k_0 n$1 accidental BICs of charge $k_{\parallel} \le k_0 n$2. Breaking out-of-plane mirror symmetry splits each off-$k_{\parallel} \le k_0 n$3 BIC into circularly polarized C points of charge $k_{\parallel} \le k_0 n$4, and selective merging of the downward-channel C points at $k_{\parallel} \le k_0 n$5 yields $k_{\parallel} \le k_0 n$6 and $k_{\parallel} \le k_0 n$7, producing a Janus BIC. Further in-plane symmetry breaking produces Janus chiral BICs and nearby chiral q-BICs with strong spin-selective conversion and reported $k_{\parallel} \le k_0 n$8 and $k_{\parallel} \le k_0 n$9 near the chiral BIC (Kang et al., 2024).

Real-space topology has also entered the subject. In a gyromagnetic honeycomb YIG slab, an out-of-plane magnetic field breaks time-reversal symmetry and splits a doubly degenerate $k_0=\omega/c$0-point BIC into two chiral BIC branches with opposite far-field circular polarization. Near-field scanning reveals phase vortices with measured charges $k_0=\omega/c$1 for the $k_0=\omega/c$2 components of the R-BIC branch, opposite winding for the L-BIC, spatially segregated circular polarization in the in-plane field, and skyrmionic Stokes textures characterized by

$k_0=\omega/c$3

This extends BIC topology from momentum space into real space under magnetic control (Zhao et al., 26 Feb 2026).

4. Representative platforms and quantitative performance

The field includes planar, slanted, monoclinic, bilayer, exciton-polaritonic, and gyromagnetic implementations. Their shared objective is to combine free-space accessibility with sharp linewidths and spin-pure or spin-selective radiation, but their symmetry strategies differ substantially.

Platform	Symmetry / leakage route	Reported performance
Planar a-Si double-sided scythe metasurface on fused silica	In-plane asymmetry $k_0=\omega/c$4 or incidence tilt $k_0=\omega/c$5 converts a $k_0=\omega/c$6-point BIC into intrinsic or extrinsic chiral q-BICs	Measured $k_0=\omega/c$7, measured $k_0=\omega/c$8; simulated $k_0=\omega/c$9 up to $k_{\parallel}=\sqrt{k_x^2+k_y^2}$0, simulated $k_{\parallel}=\sqrt{k_x^2+k_y^2}$1 (Shi et al., 2021)
Slanted trapezoidal TiO$k_{\parallel}=\sqrt{k_x^2+k_y^2}$2 metasurface on glass with PMMA	In-plane deformation $k_{\parallel}=\sqrt{k_x^2+k_y^2}$3 plus out-of-plane slant $k_{\parallel}=\sqrt{k_x^2+k_y^2}$4 creates intrinsic normal-incidence chiral q-BICs	Near-unity chiral dichroism of $k_{\parallel}=\sqrt{k_x^2+k_y^2}$5 and record-high quality factor exceeding $k_{\parallel}=\sqrt{k_x^2+k_y^2}$6 at visible frequencies (Chen et al., 2022)
Monoclinic dielectric metasurface for cone-proximal off-normal operation	Lattice angle tuning shifts the q-BIC near the light line; hole width $k_{\parallel}=\sqrt{k_x^2+k_y^2}$7 tunes $k_{\parallel}=\sqrt{k_x^2+k_y^2}$8	Near-unity circular dichroism in transmission and fully circularly polarized emission at angles exceeding $k_{\parallel}=\sqrt{k_x^2+k_y^2}$9; $\Gamma$0 down to $\Gamma$1 across $\Gamma$2–$\Gamma$3 nm (Gromyko et al., 28 Aug 2025)
Monolayer WS$\Gamma$4 on TiO$\Gamma$5 q-BIC metasurface	Chiral q-BIC strongly couples to valley excitons; zone folding makes high-$\Gamma$6 polaritons radiative at $\Gamma$7	$\Gamma$8, $\Gamma$9 meV, and polariton PL intensity and degree of circular polarization exceed uncoupled excitons by an order of magnitude (Wurdack et al., 2024)

The planar double-sided scythe metasurface established that sharply resonant planar chirality does not require depth-resolved stereo-nanofabrication. Its nominal symmetric unit has $k_{\parallel}=0$0 nm, $k_{\parallel}=0$1 nm, $k_{\parallel}=0$2 nm, $k_{\parallel}=0$3 nm, and thickness $k_{\parallel}=0$4 nm, and it supports a $k_{\parallel}=0$5-point guided resonance dominated by a vertical magnetic dipole in the amorphous-silicon pillar. At normal incidence with $k_{\parallel}=0$6 nm, the measured resonance occurs near $k_{\parallel}=0$7 nm with $k_{\parallel}=0$8 and $k_{\parallel}=0$9. Under oblique incidence, the sign of CD reverses between $k_z \to 0$0 and $k_z \to 0$1, consistent with opposite helicity along the two scan directions in $k_z \to 0$2-space (Shi et al., 2021).

The visible-frequency intrinsic chiral BIC platform addresses a different question: how to realize true intrinsic chirality at normal incidence with negligible cross-polarization. There the TiO$k_z \to 0$3 slab is patterned into a square lattice of slanted trapezoid nanoholes with $k_z \to 0$4 nm, $k_z \to 0$5 nm, $k_z \to 0$6 nm, representative perturbations $k_z \to 0$7 rad and $k_z \to 0$8 rad, and a PMMA cap to restore approximate vertical symmetry before intentional slanting. The reported result is a reflection-based CD of $k_z \to 0$9 near $\Gamma$00–$\Gamma$01 nm with $\Gamma$02, while the cross-polarized reflection terms are negligible (Chen et al., 2022).

5. Extensions: grazing-angle lasing, exciton polaritons, Janus radiation, and learned design

The off-normal generalization of chiral q-BICs shows that chirality need not be restricted to $\Gamma$03-point resonances. In monoclinic single-layer dielectric membranes with broken in-plane mirror symmetry, tuning the lattice angle moves the q-BIC along $\Gamma$04 toward the light line, while a rectangular-hole perturbation controls the overlap factor $\Gamma$05. The resulting states exhibit near-unity circular dichroism in transmission and fully circularly polarized emission at angles exceeding $\Gamma$06 from normal. The same study emphasizes critical coupling, $\Gamma$07, as the operating condition most relevant for grazing-angle chiral lasing (Gromyko et al., 28 Aug 2025).

Strong-coupling implementations broaden the concept from passive resonance engineering to chiral quasiparticles. In a WS$\Gamma$08/TiO$\Gamma$09 heterostructure, a chiral q-BIC at $\Gamma$10 nm and $\Gamma$11–$\Gamma$12 eV with $\Gamma$13 couples to the bright A exciton of monolayer WS$\Gamma$14 at $\Gamma$15 eV. The observed Rabi splitting is $\Gamma$16 meV, with $\Gamma$17 meV, and the Hopfield fractions at $\Gamma$18 are $\Gamma$19, $\Gamma$20, $\Gamma$21, and $\Gamma$22. The lower polariton inherits the intrinsic $\Gamma$23 chirality of the q-BIC, while the upper polariton inherits valley-exciton selection rules, enabling $\Gamma$24 and $\Gamma$25 spin alignments under $\Gamma$26 and $\Gamma$27 excitation, respectively (Wurdack et al., 2024).

Janus radiation adds directional asymmetry to the chiral q-BIC toolkit. In bilayer Si$\Gamma$28N$\Gamma$29 photonic crystal slabs, unequal perturbations in the two layers first create a Janus BIC with different net topological charges in the upward and downward channels, and then in-plane perturbations reconstruct the near-$\Gamma$30 polarization topology to produce Janus-chiral BICs and chiral q-BICs. A later bilayer all-dielectric PhC realizes this program with interlayer displacement $\Gamma$31, diagonal in-plane displacement $\Gamma$32, orthogonal displacement $\Gamma$33, and collective antiphase displacement $\Gamma$34, while conductivity $\Gamma$35 supplies a dissipative control knob. In that platform, $\Gamma$36 produces high-CD branches with $\Gamma$37 and resonance blueshift from $\Gamma$38 nm to $\Gamma$39 nm, $\Gamma$40 yields $\Gamma$41 with resonance shift from $\Gamma$42 nm to $\Gamma$43 nm and a sharp sign reversal near $\Gamma$44 nm at $\Gamma$45 nm, and conductivity tuning produces switchable CD exceeding $\Gamma$46 (Kang et al., 2024, Dong et al., 14 May 2026).

Data-driven inverse design has also entered the field, though in a bilayer chiral metasurface rather than a single-layer light-line platform. A multi-head attention network, MuHAN, is reported to achieve forward spectral predictions in approximately $\Gamma$47 ms with $\Gamma$48 forward and $\Gamma$49 inverse prediction accuracy, and inverse-designed chiral q-BICs with $\Gamma$50 up to $\Gamma$51 near $\Gamma$52 THz. This does not redefine the underlying BIC physics; rather, it accelerates navigation of the narrow parameter regions in which symmetry breaking is weak enough to preserve ultrahigh $\Gamma$53 while still producing strong helicity selectivity (Zhang et al., 11 Dec 2025).

6. Figures of merit, design rules, applications, and recurring misconceptions

The central figures of merit are not fully uniform across the literature, and this is important for comparing results. In the planar double-sided scythe metasurface, the transmission-basis circular dichroism is

$\Gamma$54

In the intrinsic visible chiral BIC, a reflection-based quantity is used,

$\Gamma$55

In the grazing-angle chiral-lasing study, transmission CD is written as

$\Gamma$56

while emission chirality is measured by

$\Gamma$57

A direct implication is that “near-unity CD” across different papers is not always the same observable, even when the physical origin is closely related (Shi et al., 2021, Chen et al., 2022, Gromyko et al., 28 Aug 2025).

Several design rules recur. Small geometric asymmetry or small incidence tilt raises $\Gamma$58 but weakens out-coupling; larger asymmetry broadens the linewidth while usually preserving high peak chirality. Strong in-plane birefringence with optical axes near $\Gamma$59, full removal of mirror symmetries when intrinsic chirality is required, and control of the substrate/superstrate environment all appear repeatedly as decisive ingredients. For cone-proximal off-normal designs, the target operating point is slightly inside the regime where the LDOS is strongly enhanced but before $\Gamma$60 collapses as the mode touches the light line. For active or bilayer devices, extra knobs such as interlayer displacement, conductivity, or layer rotation add separate control over upward versus downward radiation, radiative versus absorptive linewidth, and sign reversal of CD (Shi et al., 2021, Gromyko et al., 28 Aug 2025, Dong et al., 14 May 2026).

The application space follows directly from the coexistence of narrow linewidth, large optical chirality, and free-space accessibility. Reported or proposed uses include chiral sensing, enantiomer selection, chiral quantum emitters, polarization control, spin-multiplexed wavefronts, grazing-angle chiral lasing, spin-preserving mirrors and couplers, directional chiral LEDs and perovskite emitters, angle-resolved chiral spectroscopy, nonlinear chiral photonics, valleytronics, asymmetric photocatalysis, and polarization-encrypted imaging (Shi et al., 2021, Gromyko et al., 28 Aug 2025, Chen et al., 2022, Zhang et al., 11 Dec 2025).

Two recurring misconceptions are explicitly corrected by the literature. First, strong chiral BIC response is not confined to complex three-dimensional stereo-nanostructures: planar metasurfaces with controlled symmetry breaking already achieve measured $\Gamma$61 and $\Gamma$62 at optical frequencies, while visible intrinsic chiral BICs achieve $\Gamma$63 and $\Gamma$64 (Shi et al., 2021, Chen et al., 2022). Second, light-cone proximity does not mean only normal incidence. The $\Gamma$65-point tradition remains central, but cone-proximal q-BICs at large oblique angles exploit the $\Gamma$66 LDOS singularity to reach near-unity circular dichroism and fully circularly polarized emission beyond $\Gamma$67 from normal (Gromyko et al., 28 Aug 2025).

In its mature form, the subject is therefore not a single device archetype but a family of BIC-derived chiral resonances governed by a common triad: symmetry-protected suppression of radiation, controlled reopening of a small set of spin-selective channels, and topological organization of the far-field polarization field. That triad is now being extended from planar near-$\Gamma$68 metasurfaces to grazing-angle emitters, Janus radiation topologies, polaritonic strong-coupling systems, magnetically controlled nonreciprocal slabs, and machine-learned ultrahigh-$\Gamma$69 chiral resonators (Shi et al., 2021, Gromyko et al., 28 Aug 2025, Wurdack et al., 2024, Kang et al., 2024, Dong et al., 14 May 2026, Zhao et al., 26 Feb 2026, Zhang et al., 11 Dec 2025).