Polarization-free broadband angular selectivity based on uniaxial zero-refractive-index metamaterials

Angle-selective devices that filter electromagnetic (EM) waves from free space are highly desirable for microwave and optical applications. However, most existing designs face challenges in simultaneously achieving a low profile, broad bandwidth, and polarization insensitivity. In this paper, we propose a uniaxial zero-refractive-index metamaterial (UZIM) that offers ultrabroadband transmission under normal incidence while exhibiting narrowband strong reflection for oblique incidence across all polarizations. This unique angular response enables ultrabroadband angular selectivity by cascading UZIM layers with different operating frequencies. By cascading six UZIMs that share the same transmission band under normal incidence but have different reflection bands at oblique incidence, we achieve an angularly selective composite with a 15.56% relative bandwidth. The composite, with a total thickness of $1{\ensuremath{\lambda}}_{0}$, maintains high transmission within 5\ifmmode^\circ\else\textdegree\fi{} of incidence and strong suppression beyond 30\ifmmode^\circ\else\textdegree\fi{} in the whole operating frequency band. The measured results of the fabricated sample show good agreement with full-wave simulations. Our approach addresses the limitations of Brewster-effect-based designs, which suffer from bulky profiles, and overcomes the bandwidth limitations of traditional zero-refractive-index-material-based (ZIM) and frequency-selective-surface-based (FSS) designs. This makes the proposed structure highly suitable for applications such as spatial filtering and antenna sidelobe suppression.