Degeneracy-Broken Toroidal Chain <i>Quasi</i>-Bound States in the Continuum in Photonic Metawires

Optical metawires (MWs) refer to dielectric nanowires with periodic modulation of the effective refractive index, which exhibit unusual meta-photonic properties such as optical bound states in the continuum (BICs). Despite recent advances in the experimental observations of MW BICs, the fundamental understanding of their nature and polarization response still remains limited. In this work, we present degeneracy-broken toroidal chain quasi-BICs in a dielectric MW, which manifest as Fano resonances with drastically disparate line widths under transverse magnetic (TM) and electric (TE) polarizations. We show that the different mirror symmetries of the two toroidal chain modes determine the disparate polarization response. Using temporal coupled-mode theory (TCMT), we also demonstrate that an excessively large excitation strength of the TM mode prevents it from becoming a true BIC, whereas the excitation of the TE mode remains sufficiently small to satisfy the BIC condition through additional geometric tuning. Finally, we show that in-plane polarization rotation effectively controls the azimuthal orientation of the BIC-enhanced electric fields around the MW, enabling nearly 800% absorption modulation of MW-coupled low-dimensional devices.