Narrow-band asymmetric THz absorbers and polarizers based on impedance-matched, cloaked quasi-BICs in doped Ge microdisk metasurfaces

We present a novel class of narrow-band, polarization-selective terahertz (THz) absorbers and polarizers based on cloaked quasi-bound states in the continuum (qBICs) supported by doped germanium metasurfaces consisting of tilted microdisk and micropillar arrays that break out-of-plane symmetry, enabling impedance-matched conditions at specific Brewster angles. This configuration yields highly asymmetric absorption: near-complete transparency at one incidence angle and strong coupling to cloaked qBICs at the opposite angle. Microdisk arrays in TE polarization exhibit absorbance approaching 100%, while micropillar arrays in TM polarization reach ∼55% under similar conditions. The metasurfaces also work as efficient polarization filters, suppressing one polarization while transmitting the orthogonal one with near 100% efficiency. Additionally, we discuss practical implementation via obliquely grown Ge disks, along with strategies for active tuning of the asymmetry angle. These findings establish cloaked qBIC-based metasurfaces as a versatile platform for high-Q THz absorption, polarization control, and advanced sensing applications.