Strong Coupling in Monolayer WS₂ Integrated All-Dielectric Metasurfaces with Electric-Quadrupole Modes

All-dielectric metasurfaces have emerged as promising platforms for studying strong coupling due to their low absorptive losses and diverse resonance modes. However, previous works have primarily focused on all-dielectric metasurfaces with electric/magnetic dipole (ED/MD) modes, while higher-order multipoles, such as electric quadrupole (EQ), have not been fully explored. Here, we report on an EQ-involved strong coupling based on the monolayer WS₂ integrated all-dielectric metasurfaces. By precisely tuning the EQ modes to be spectrally overlapped with the A exciton resonance of WS₂ (at 614 nm), angle-resolved transmission spectra show a clear anticrossing behavior with a 31.7 meV Rabi splitting. EQ modes feature spatially varying field gradients and strong in-plane electric fields at the interface, thus increasing the coupling efficiency with two-dimensional materials. This work presents a novel strategy for developing low-loss, compact polaritonic devices, contributing to a deeper understanding of the strong coupling mechanisms between higher-order multipoles and excitons.