Topological phase singularities in terahertz perfect light absorption from Drude–Smith media in the total internal reflection geometry

Topological behaviors of intensity, phase, and polarization of light are increasingly investigated for various optical structures and systems. Despite topological phase singularities (TPS) of perfect absorption (PA) having been realized via Lorentz-resonant systems across mid-infrared to visible spectra, their extension to terahertz (THz) regimes is hindered by the intrinsic limitations of non-resonant conductive materials and the absence of amplitude-phase duality. Here, we demonstrate a tunable and robust TPS in a polarization-selective THz PA via a total internal reflection platform with Drude–Smith (DS) ultrathin films. By exploiting the DS model’s carrier memory effects, we engineer the film’s permittivity to switch the THz PA between lossy dielectric ( s -polarization) and semi-metallic ( p -polarization) states above the critical angle within the entire THz range, without Lorentz resonances. Through parameter-space analysis, we establish a universal criterion linking THz PA to TPS emergence, where TPS exhibits quantized topological charges ( C =±1) and spectral tunability governed by DS parameters. Crucially, TPS robustness persists in disordered regimes, contrasting sharply with resonance-dependent systems. Our platform decouples light–matter interactions from resonant constraints, opening avenues for high-efficiency THz devices and topological photonic devices.