Robust ultrahigh-Q resonances in tetramer metasurfaces through centroid symmetry protection and area conservation

Ultrahigh-Q optical resonances are the cornerstone of next-generation nanophotonic technologies, but their simultaneous realization of robustness and on-chip practicality remains a significant challenge. In this work, we present tetramer composite metasurfaces capable of supporting two distinct classes of ultrahigh-Q resonances: centroid symmetry-protected bound states in the continuum (SP-BICs) and area-conserved guided-mode resonances (GMRs). By employing a four-hole supercell design, we demonstrate that centering each hole within its subcell preserves C_4v symmetry, thereby enabling SP-BICs. Controlled lateral displacement transforms them into quasi-BICs with Q > 10⁶. Independently, enforcing diagonal-hole area conservation within the super unit cell generates degenerate GMRs with Q > 10⁸, which exhibit remarkable stability across a broad wave vector range. Breaking this area conservation splits the GMRs into paired ultrahigh-Q resonances, while adjusting the center-to-center distance of air holes lifts their degeneracy. Experimentally, we validate both resonance types using silicon photonic crystal slabs, achieving measured Q-factors exceeding 10,000, with a maximum value of 43,700. Such ultrahigh-Q composite-metasurfaces provide a versatile platform of enhancing light-matter interactions.