Intrinsic strong self-hybrid coupling empowered by Brillouin zone folding-induced high-Q quasibound modes in van der Waals metasurfaces

Bulk transition metal dichalcogenides (TMDs) provide a prominent platform for self-hybrid light-matter interactions, owing to their strong oscillator strength and large modal overlap. Recently the Brillouin zone folding-induced quasibound states in the continuum (BZF-qBICs) have been attracting significant attention because of their sustainable high quality (Q) factor in momentum space and robustness against disorder. However, studies on strong self-hybrid coupling based on BZF-qBICs are rarely reported. Here, we propose a simple design of a metasurface with holes punched into a standalone bulk TMD flake and demonstrate the strong self-hybrid coupling between the BZF-qBIC resonances and the intrinsic exciton in bulk ${\mathrm{WS}}_{2}$. We show that the BZF-qBICs induced by periodically perturbing the radius of air holes exhibit a folded band structure and reduced radiation channels, leading to tunable high Q factors. We further demonstrate that introducing relative displacements to the perturbed holes to break the in-plane symmetry enables an intriguing transition from electric quadrupole to toroidal dipole dominated BZF-qBICs, with the latter exhibiting robust polarization independence in the associated metasurface. This polarization-independent BZF-qBIC is used to realize self-hybrid coupling with the intrinsic excitons with a large Rabi splitting over 190 meV. Moreover, the coupling strength of the hybrid system can be further modulated by tuning the perturbation factor associated with the radius of the holes. Our findings propose a strategy for self-hybrid strongly coupled platforms, which may serve as a catalyst for the innovation of high-performance optoelectronic devices.