Reflective and Transmissive Off-Axis Sub-THz Beam Shaping Using Passive Complementary Split-Ring Resonators

Terahertz and sub-terahertz links offer wide bandwidths for future wireless communication, but practical deployment is limited by high propagation loss, atmospheric absorption, and sensitivity to blockage in non-line-of-sight environments. Here, we demonstrate ultrathin, passive full-space beam-shaping elements based on complementary split-ring resonators laser-cut into stainless steel foil. By encoding the target phase profile in the spatial arrangement of identical complementary split-ring resonator meta-atoms, which act as a binary amplitude-type zone plate, we shape the wavefront simultaneously in transmission and reflection and form foci on both sides of a single element. We show that off-axis routing (e.g., toward 45∘) is achieved via pattern arrangement without re-optimizing the complementary split-ring resonator meta-atoms for each geometry, providing a straightforward route to dual-channel routing. As a proof of concept, we transform a collimated incident field into Gaussian and Bessel-type beams and compare their propagation characteristics with more classical Fresnel-lens-like reflectors. These results suggest a potential low-profile approach to passive sub-THz beam control, which may support alignment-tolerant operation and could, in future work, be extended toward non-line-of-sight channel conditioning and compact multi-focus beam shaping.