Quasi-BIC empowered asymmetric grating for high-Q angularly tolerant THz filters with switchable transmission-reflection dualities

THz narrowband filters are essential for various applications. Currently, they often suffer from substrate-induced parasitic losses and interference, stringent collimation requirements imposed by angle-dependent resonance excitation, and cumbersome peripheral optical components due to reflection-mode operation. To tackle these issues, geometrically protected BICs are employed in a substrate-free single-layer asymmetric dual grating structure of high-resistance silicon to achieve narrowband polarization-controlled dual transmissive/reflective THz filtering with high-<i>Q</i> factors, broad stopbands, and tolerance to incident angles, simultaneously. Collapse of BICs into Q-BICs is achieved by symmetry breaking via tuning widths of the grating air gaps rather than angular phase-matching conditions. By decreasing the asymmetric factor, theoretical <i>Q</i> factors can reach beyond 10⁴with wide stopbands of ∼2<i>T</i><i>H</i><i>z</i>. The filter exhibits stability in spectral positioning, linewidth preservation, and stopband consistency under oblique incidence up to 10°, demonstrating good angular insensitivity. Dynamic switching between transmissive/reflective filtering is realized by simply turning the polarization angle 90°. This work shows the monolithic integration of high-Q transmissive/reflective filters within a simple unified optical system, facilitating THz applications including trace molecular sensing and imaging with high spectral resolution.