Polarization-dependent quasi-BIC-induced EIT in a multi-mode terahertz metasurface

Abstract This work presents a C2v-symmetric terahertz metasurface composed of split-ring resonators and a central rectangular ring, which supports multiple symmetry-protected BIC modes coexist within the investigated frequency range. By independently breaking the two orthogonal mirror symmetries, we demonstrate that the symmetry-breaking direction selectively determines the polarization channel. Specifically, breaking the x-axis symmetry activates a quasi-BIC at 0.27 THz and a quasi-BIC-induced EIT-like transparency window at 0.71 THz in the txx channel, whereas breaking the y-axis mirror symmetry activates a quasi-BIC at 0.7 THz and an inverted EIT-like spectral feature at 1.246 THz in the tyy channel. The activated quasi-BIC undergoes destructive interference with the coexisting broadband SRR resonance. To confirm this mechanism, eigenmode band analysis, surface current distributions, and multipole decomposition are employed, which consistently verify the BIC-to-quasi-BIC conversion and reveal the mode coupling pathways. As a representative application, refractive index sensing is demonstrated with stable linear frequency shifts and a maximum sensitivity of 165 GHz/RIU.