Multi-polarization control via symmetry breaking in BIC-based THz metamaterial

Terahertz (THz) metamaterials provide a promising platform for polarization control, which is essential for imaging, sensing, and communication. However, realizing independent manipulation of orthogonal linear polarizations and circular polarization control with high quality ( Q ) factors under normal incidence in a THz platform remains a major challenge. Here, we numerically and experimentally demonstrate a bound state in the continuum (BIC)-based THz metamaterial that achieves multi-polarization control under normal incidence via stepwise symmetry breaking. With in-plane symmetry preserved, the transverse magnetic (TM) resonance is supported by guided mode resonance (GMR). Breaking the in-plane C 2 rotational symmetry excites a high- Q transverse electric (TE) response via quasi-bound states in the continuum (q-BICs), while leaving the TM mode unaffected, thereby realizing independent control of orthogonal linear polarizations. By further breaking the in-plane mirror symmetry, a high- Q , near-unity, and reversible circular dichroism (CD) effect is realized under normal incidence by leveraging tunable topological polarization, allowing selective control of right-handed and left-handed circularly polarized waves (RCP and LCP). This work provides a practical route to integrating linear and circular polarization control within a single platform, opening new avenues for multifunctional THz polarization devices.