Polaritonic quasi-bound states in the continuum in individual van der Waals nanoantennas

The study of strong light–matter interaction in van der Waals (vdW) materials is at the forefront of current research in physics and chemistry, and it can be enhanced dramatically by employing optical resonances. Here, we report on the first observation of quasi-bound states in the continuum (qBICs) realized via polaritonic interfering resonances in isolated WS2 nanodisks. We validate experimentally the existence of polaritonic qBICs driven by the intrinsic coupling of Mie resonances and vdW excitons. We observe exceptionally strong light–matter interaction with a measured Rabi splitting approaching 270 meV – the largest reported value among all transition metal dichalcogenide self-hybridized systems to date. The giant coupling strength stems from qBIC-induced in-plane field enhancement, which strongly interacts with in-plane excitonic dipoles while suppressing radiative losses. Polarization-controlled measurements further demonstrate selective excitation of qBIC through switching incident polarization to specific orthogonal configurations. The observed polarization-dependent coupling provides an additional degree of freedom to control the hybrid states’ spectral characteristics and spatial field distributions. Our demonstrations provide a pathway for engineering high-quality light–matter hybrid states in compact nanostructures, with potential applications in on-chip photonics, polaritonics, and quantum optics.