Terahertz Branch Selection of Friedrich-Wintgen Bound States in the Continuum via Altering the Intra-Action of Meta-Atom

Bound states in the continuum (BICs) in photonics have drawn wide interest due to the infinite quality factors (Q factors) and the topological features of the BICs. BICs can be realized by leveraging the coupling of electromagnetic modes, referred to as Friedrich-Wintgen BICs (FW-BICs). However, the impact of coupling coefficients has not been profoundly explored. In this study, we numerically and experimentally demonstrate that a metasurface composed of two concentric split-ring resonators (SRRs) supports multiple FW-BICs. This can be observed by tuning the intra-action of two meta-atoms of SRRs. We observed that the resonant frequency, radiative line width, and coupling of resonance modes can be tuned by varying the width of an outer SRR or that of the inner SRR. Eigenmode analysis indicates that all coupling-assisted FW-BICs were in the center of the far-field polarization vortex with a certain topological charge. We observed that when a FW-BIC was induced by the coupling of modes with different bandwidths, the narrower mode would evolve into FW-BIC. In the coupled-mode theory (CMT) framework, the radiative damping rate determines at which mode BIC occurs. Our results reveal the branch selection character of FW-BIC, which can facilitate the design of metasurfaces with high-Q factors.