Optical bound states in the continuum (BICs) are polarization singularities with integer topological charges ($\nu$) in momentum space, whose far-field radiation vanishes in the surrounding radiative states. Here, we study the dynamic evolution of topological charges in symmetry-protected BICs within bilayer metasurfaces. Governed by threefold rotation symmetry ($C_3$), the topological charges obey $\nu =3n+1$ ($n\in \mathbb{Z}$). Theoretically and experimentally, we demonstrate that merely tuning the spacer thickness of the bilayer metasurfaces induces a topological transition from a fundamental charge ($\nu =1$, $n=0$) to a high-level charge ($\nu =-2$, $n=-1$) without modifying symmetry. Perturbation theory reveals this transition occurs when the first derivative of the far-field radiation with respect to the Bloch wave number $k$ vanishes at $\mathrm{\Gamma }$, induced by destructive bilayer interference. This interference simultaneously enhances the quality factors of near-$\mathrm{\Gamma }$ resonances and alters their scaling laws from $\mathcal{O}(k^{-2})$ to $\mathcal{O}(k^{-4})$. Our work introduces a symmetry-preserved mechanism to dynamically control BIC topology.

Close

PrevNext

View figure in article

• Metasurfaces
• Photonic crystals
• Topological effects in photonic systems
• Optical materials & elements
• Polarization
• Perturbative methods
• Reflectivity