Geometry-Aware Polarization Selection for Brewster-Angle Secrecy in Sub-THz O2I Wiretap Channels

Outdoor-to-indoor sub-terahertz links leak energy through building partitions, enabling penetration-path eavesdropping. This letter studies when polarization-dependent wall penetration can be a physical-layer secrecy resource rather than a favorable-placement effect. Near the pseudo-Brewster angle, TM internal reflection is strongly suppressed, changing Fabry–Pérot fading for the terminal closest to that incidence condition. We formulate a polarimetric stochastic penetration model with co-polar TE/TM slab response, bounded depolarization, roughness loss, and random multipass phase. The closed-form Rician parameters expose a fading-asymmetry term that remains after mean path gain is normalized. We then propose GAPS, a geometry-aware polarization rule using the legitimate incidence angle and an admissible eavesdropper sector, with a minimax fallback when the passive eavesdropper is not localized. At 140 GHz, GAPS achieves 0.741 bps/Hz versus 0.371 bps/Hz for an agnostic baseline in the studied glass case; after gain normalization, it retains 0.0697 bps/Hz versus 0.0446 bps/Hz for a no-Brewster control.