Broadband asymmetric transmission and cross-polarization conversion in an interlayer-coupled bilayer chiral metasurface

Abstract

This paper presents a bilayer 3D-chiral metasurface for broadband asymmetric transmission (AT) and cross-polarization conversion in the microwave regime. The structure achieves a polarization conversion ratio (PCR) above 93% over 10.0–15.3 GHz, peaking at 98%. Simulations further indicate that high PCR is largely retained under moderate oblique incidence around 45°. The structure employs a 90°-rotated and y-mirrored bilayer arrangement, whose interlayer coupling induces effective magnetoelectric interactions and enables efficient cross-polarization conversion. For backward incidence, the polarizer provides y-to-x polarization conversion across the operating band, and the conversion direction reverses under forward incidence. The resulting asymmetric transmission reaches a peak value of 0.93. To provide a physics-oriented interpretation, the effective electric, magnetic, and magnetoelectric surface susceptibilities are retrieved using a complete reciprocity-constrained tangential GSTC model. The retrieved tensors provide a macroscopic description of the coupled anisotropic and bianisotropic response. Surface-current distributions complement this macroscopic description by indicating electric- and magnetic-type current excitations associated with polarization conversion. A fitted Foster equivalent-circuit model provides a compact circuit-level representation of the dispersive CST response. A fabricated prototype is experimentally characterized. The normal-incidence measurements show reasonable overall agreement with the simulated polarization-conversion and AT responses. Taken together, the combined performance characteristics and the physics-oriented interpretation of the conversion mechanism demonstrate its potential for broadband microwave polarization-control applications.