Ultra-broadband and multifunctional polarization conversion enabled by X-shaped metasurfaces

Polarization control is essential for advancing next-generation photonic and mid-infrared (MIR) systems, yet achieving multifunctional polarization conversion across an ultra-broad bandwidth remains challenging for metasurface-based devices. Here, we propose an X-shaped metal-dielectric-metal metasurface that enables ultra-broadband, highly efficient, and multifunctional polarization conversion in the MIR regime. The proposed design offers a versatile metasurface platform capable of two polarization control modes: broadband cross-polarization conversion (Type-1) and polarization state conversion between linear and circular states (Type-2). By configuring the geometric parameters, the Type-1 structure achieves a PCR exceeding 0.98 over 12.9–22 THz, supporting high-efficiency cross-polarization conversion for both linear and circular waves. By tailoring the curvature of the resonator, the Type-2 configuration facilitates reciprocal linear-circular polarization transitions over 12.55–21 THz. The LTC mode exhibits an ellipticity approaching -1 and an axial ratio below 3 dB, confirming high circular polarization purity. Furthermore, the metasurface maintains stable performance under oblique incidence up to 46°, demonstrating remarkable angular robustness. Physical mechanisms are elucidated through electric field vector decomposition, surface current analysis, and multipole expansion, revealing that electric and magnetic dipole resonances dominate different operational modes. This work provides a compact and versatile platform for efficient MIR polarization manipulation, offering significant promise for applications in biological molecule identification, environmental pollutant monitoring, and high-sensitivity infrared gas sensing.