Radial bound states in the continuum for polarization-invariant nanophotonics

Abstract

All-dielectric nanophotonics underpinned by the physics of bound states in the continuum (BICs) have demonstrated breakthrough applications in nanoscale light manipulation, frequency conversion and optical sensing. Leading BIC implementations range from isolated nanoantennas with localized electromagnetic fields to symmetry-protected metasurfaces with controllable resonance quality (Q) factors. However, they either require structured light illumination with complex beam-shaping optics or large, fabrication-intense arrays of polarization-sensitive unit cells, hindering tailored nanophotonic applications and on-chip integration. Here, we introduce radial quasi-bound states in the continuum (radial BICs) as a new class of radially distributed electromagnetic modes controlled by structural asymmetry in a ring of dielectric rod pair resonators. The radial BIC platform provides polarization-invariant and tunable high-Q resonances with strongly enhanced near fields in an ultracompact footprint as low as 2 µm². We demonstrate radial BIC realizations in the visible for sensitive biomolecular detection and enhanced second-harmonic generation from monolayers of transition metal dichalcogenides, opening new perspectives for compact, spectrally selective, and polarization-invariant metadevices for multi-functional light-matter coupling, multiplexed sensing, and high-density on-chip photonics.

Fig. 1. Conceptual advantages of radial quasi-bound states in the continuum (radial BICs).

Established symmetry-broken quasi-BIC geometries, such as 2D metasurfaces (a), and 1D chains (b), exhibit large footprints, moderately high Q factors, and require polarization-dependent excitation. The radial BIC concept (c) combines a tiny footprint with high-Q factors in the visible. Above all, the radial BIC platform provides the highest Q factor per footprint ratio compared to other 1D and 2D BIC-based platforms, as shown here.

Fig. 2. Versatile radial BIC resonances and polarization invariance.

Electric near fields for a symmetric radial BIC (ΔL = 0 nm) in a and several symmetry-broken radial quasi-BIC (ΔL > 0 nm) geometries in b. c Optical transmittance spectra of the radial BIC structures show a resonance redshift with increasing radii, as illustrated in the gray-scale optical micrograph. d Dependence of the quality factor on the ring radius R and the unit cell asymmetry ΔL. Quality factors exceed 500 for ΔL = 25 nm in the visible wavelength range. e The radial BIC geometry shows polarization invariance as observed by the weak dependence of the resonance quality factor on the polarization angle of the incident light $\phi$.

Fig. 3. Refractive index and molecular biosensing with radial BICs.

a Sketch of the biosensing experiments. The ring structures are functionalized with capture antibodies, and shifts in the spectral position of the resonance are recorded for the binding of different concentrations of biomolecules. b Optical transmittance spectra of three-unit cell asymmetries ΔL with R = 1.5 µm covered with different thicknesses of conformal SiO₂ thin films. c Corresponding figure of merit for bulk refractive index sensing. d Transmittance spectra for a ring with ΔL = 50 nm and R = 1.6 µm after each functionalization and molecular binding step as indicated in the color-coded boxes. e Map of biomolecular sensing performance. Measured resonance shifts normalized to the respective FWHM for three different asymmetries dependent on streptavidin concentrations.

Fig. 4. Radial BIC-enhanced second-harmonic generation in a MoSe₂ monolayer.

a Schematic depiction of a MoSe₂ monolayer covering the ring structures while being illuminated by a pulsed excitation laser. b AFM image showing the coverage of the flake on top of the ring. c Integrated second-harmonic generation (SHG) signal for an excitation wavelength of λ = 744 nm from the MoSe₂ monolayer on a bare substrate showing a quadratic dependence (grey fit line). Inset: Polarization-resolved SHG signal from the MoSe₂ monolayer in either parallel or perpendicular detection (see “Methods”). d SHG maps for the ring displayed in b taken at different excitation wavelengths. SHG enhancement is only present for an excitation wavelength resonant with the radial BIC. In contrast, for the off-BIC excitation, we observe the expected suppression of the SHG signal due to strain. Scale bar: 300 nm. e Transmittance spectrum of the ring structure (ΔL = 50 nm) covered with MoSe₂ monolayer clearly showing the spectral signatures of the radial BIC resonance (744 nm) next to the absorption line of the exciton (785 nm).