Polarization-controlled dual-mode quasi-bound states in the continuum in terahertz all-metallic metasurfaces

Here, polarization-controlled bound states in the continuum (BICs) are demonstrated in all-metallic metasurfaces based on complementary periodic cross-shaped resonators (CPCRs) through varying polarization angles, enabling tunable switching between BICs and quasi-BICs. The CPCR features four slits extending from its center, endowing it with highly flexible structural tunability that can be exploited to break symmetry by adjusting the slit length. A simultaneous adjustment of two slit lengths enables the realization of quasi-BICs in both transverse electric (TE) and transverse magnetic (TM) waves. When two slits have equal length, such quasi-BICs show polarization dependence, emerging at a certain angle, disappearing with increasing angle, and then recurring at π intervals. This cyclic pattern persists for slits of unequal lengths. At a critical polarization angle, the electric field alignment transitions to in-phase, confirming the transformation of quasi-BICs to accidental BICs. The polarization angle regulates the transition between these states, enabling the engineering of quasi-BICs with promising applications in the development of filters, sensors, and detectors operating at terahertz frequencies or other spectral bands.