High-performance mid-wave infrared metasurface for broadband linear-to-circular polarization transformation

Metasurface structures have the capability to alter the polarization state of electromagnetic waves. In this work, we designed a metasurface structure composed of three metallic strips of varying lengths through finite element simulation. The results demonstrate effective linear-to-circular (LTC) polarization conversion in the technologically significant mid-infrared spectral region. The simulation results demonstrate that within the mid-infrared wavelength range of 3-5 µm, as defined in the field of atmospheric sciences, the axis ratio of the circularly polarized light obtained through the metasurface polarization converter is less than 3 dB, with an ellipticity greater than 97.77%. The fabricated device was tested, and its maximum circular polarization degree reached 0.9608. Finally, this work discusses the surface current distribution of resonant modes and employs a circuit equivalent model to theoretically analyze the potential physical mechanisms underlying the conversion of linear polarization to circular polarization by the converter. The metasurface structure is more compact and cost-effective compared to traditional polarizing optical elements. This subwavelength-thick metasurface has the potential to replace conventional quarter-wave plates.