{\sigma}h-Broken Induced Topological quasi-BIC

TL;DR

This paper introduces a novel out-of-plane symmetry-breaking method to induce quasi-BICs, enabling topologically robust high-Q resonances in plasmonic cavities without in-plane asymmetry.

Contribution

It demonstrates that out-of-plane symmetry breaking can induce topological quasi-BICs, expanding the mechanisms for high-Q resonance engineering beyond in-plane asymmetry.

Findings

Out-of-plane symmetry breaking induces topological phase transitions.

Band inversion occurs between quadrupole and dipole modes.

Controlled radiation coupling and defect-immune qBIC regimes are achieved.

Abstract

Transitions from bound states in the continuum (BICs) to quasi-BICs (qBICs) are typically realized by introducing in-plane asymmetry, including permittivity asymmetry ({\epsilon}-qBICs) and geometry asymmetry (g-qBICs). Here, we demonstrate that when the in-plane symmetry is rigorously kept, the transition can also be occurred, provided the out-of-plane asymmetry is designed, which is called {\sigma}h -qBICs in this work. When the {{\sigma}h symmetry is gradually broken, the system undergoes a topological phase transition characterized by a Zak phase inversion, leading to a band inversion between quadrupole and dipole modes. This process not only enables controlled radiation coupling of BICs but also introduces a defect-immune qBIC regime. Our findings establish a general mechanism for engineering high-Q resonances and topologically robust plasmonic cavities.