Polarization-multiplexed achromatic metasurface for the mid-infrared via inverse design

Metasurfaces composed of micro- and nano-structured artificial media have attracted increasing research attention over the past decades due to their excellent electromagnetic manipulation capabilities and potential applications in ultra-compact optical devices. Specifically, mid-infrared metasurfaces hold significant promise in energy harvesting, chemical and biological sensing. However, simultaneously achieving good achromaticity, wide bandwidth, and optical signal integrity remains a challenge due to the multi-objective optimization constraints of meta-atoms. Here, a simple and efficient inverse design method is used to realize a mid-infrared polarization multiplexed achromatic metasurface composed of dual-layer Si rectangular nanopillars. The structural features of the dual-layer meta-atom allow for a higher degree of design freedom and the ability to modulate the optical field over a wide bandwidth (1600 nm) with a central wavelength at 4µm. Simulation results show that the metasurface focuses linearly polarized light in the x and y directions into a single focal point (L_x = 0) and vortex light (L_y = 2), respectively. The full width at half maximum of the spots is close to the diffraction limit. At the same time, the crosstalk of the optical signal is lower than -13.5 dB, and the signal-to-noise ratio (SNR) reaches as high as 18 dB. The proposed dual-layer cell polarization-multiplexing dispersion optimization method facilitates the design of multiplexed metasurface devices with its simple and effective algorithm while increasing the design freedom. It provides new ideas for the design of complex and reconfigurable metasurface devices.