Tailoring topological nature of merging bound states in the continuum by manipulating structure symmetry of the all-dielectric metasurface

Photonic bound states in the continuum (BIC) promise a versatile platform for construction of optical resonators with highly confined electromagnetic fields and ultralong radiative lifetime. Merging multiple BIC can enable suppression of out-of-plane radiative losses and further boost the quality (Q) factor of the resonance state. However, current studies on merging BIC (M-BIC) are primarily restricted to the elementary BIC with topological charge $\ifmmode\pm\else\textpm\fi{}1$, which hinders further improvement of the performance of optical resonators. Here, an all-silicon terahertz metasurface (THz-MS) is investigated that supports the symmetry-protected BIC (SP-BIC) with topological charge $\ensuremath{-}2$ and accidental BIC (A-BIC) with topological charge $\ifmmode\pm\else\textpm\fi{}1$ simultaneously. Empowered by the topological nature of BIC, we merge twelve A-BIC with a higher-order SP-BIC at the $\mathrm{\ensuremath{\Gamma}}$ point. By tailoring in-plane mirror symmetry of the THz-MS, we achieve M-BIC at almost arbitrary position in the momentum space. Unlike original isolated BIC, M-BIC manifests the dramatic enhancement of the Q factors of nearby resonances and are robust against radiation losses introduced by fabrication imperfections. Our work presents a paradigm for realizing higher-order at-$\mathrm{\ensuremath{\Gamma}}$ M-BIC and momentum-steerable M-BIC that can substantially enhance light-matter interaction and further improve the performance of terahertz optoelectronic devices.