Magnetically tunable dual ultra-narrowband terahertz filters based on non-Bragg mirror structures

Abstract Narrowband terahertz (THz) filters are essential for high-resolution spectroscopy and communication. However, conventional designs based on photonic crystals, metamaterials, or semiconductor structures typically suffer from limited spectral selectivity and weak tunability. Here, we propose a liquid-crystal-filled, waveguide-type dual ultra-narrowband THz filter based on a non-Bragg mirror-symmetric structure. The upper and lower corrugated plates are laterally staggered to form a non-Bragg configuration, which excites resonances between different transverse modes and gives rise to two distinct transmission peaks within the bandgap. Simulated electric-field distributions and mode analyses confirm that these ultra-narrow transmissions originate from interactions between different modes within the waveguide. By controlling the molecular orientation of the nematic liquid crystal (E7) within the waveguide cavity through an external magnetic field, the transmission frequencies can be dynamically tuned. The proposed structure simultaneously achieves high- Q performance and magnetic tunability, providing an effective pathway toward compact, integrated, and reconfigurable THz filtering devices.