  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.
