Radial bound states in the continuum for polarization-invariant nanophotonics

TL;DR

This paper introduces radial BICs in dielectric nanostructures, offering polarization-invariant, high-Q resonances in ultra-compact footprints, enabling advanced applications in sensing and nonlinear optics.

Contribution

It presents a novel class of radial BICs controlled by structural asymmetry, providing polarization-invariant, tunable high-Q resonances in a compact design.

Findings

Radial BICs exhibit high Q-factors and polarization invariance.

Demonstrated applications include biomolecular sensing and enhanced second-harmonic generation.

Achieved ultracompact resonator footprints as small as 2 μm².

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 $\mu$m$^2$. 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.