Brillouin Zone Folding-Driven Topological Band Flips Based on BICs via Twisted Photonic Crystals

Brillouin zone folding (BZF) associated with bound states in the continuum (BICs) has provided promising opportunities for light wave manipulation. Despite its significance, achieving and understanding BZF-induced band flips based on BICs in momentum space remain unresolved. Here, we demonstrate that such BZF-driven topological band flips can be realized by using twisted photonic crystals (PhCs) based on BICs. The twist perturbation folds all high-symmetry points of the bands to the Γ point, and each band generates three new bands linked to either a BIC or a guided-mode resonance (GMR) due to the shrinkage of the first Brillouin zone (FBZ). The interaction among these BZF-driven bands can be controlled by tuning the twist angle. At a critical angle, the degeneracy of the p-like and s-like bands at the Γ point creates an accidental-degeneracy-induced Dirac cone (ADC). Breaking this ADC by further twisting directly leads to topological band flips, evidenced by a parity exchange and a transformation between GMR and BIC modes. Experimental observations of this twist-angle-tailored band flipping associated with an ADC show excellent agreement with theoretical simulations. Our findings offer a new perspective on momentum-space topological band engineering and pave the way for advanced light manipulation and tailorable light-matter interactions.