High-efficiency achromatic metalens in long-wavelength infrared composed of topologically optimized building blocks

Metalenses provide a powerful paradigm for the compact optical system of long-wavelength infrared (LWIR) imaging and detection. However, it remains challenging to design nanopillar-array based LWIR metalenses correcting chromatic aberration and improving the cross-polarization conversion efficiency of circularly polarized light, simultaneously. In this paper, we design an achromatic metalens in the wavelength range from 8 μm to 12 μm by using topology optimization, based on the principle of chromatic aberration elimination. Topology optimization is used to optimize the structural configurations of nanopillars, so as to improve the cross-polarization conversion efficiency of nanopillars and construct the phase library. This approach increases the cross-polarization conversion efficiency of nanopillars, allowing the derived metalens to control a higher proportion of the optical energy and effectively reducing energy losses during the modulation process. Our results indicate that the average cross-polarization conversion efficiency of the entire phase library is 78.4% and the selected nanopillars achieve an average cross-polarization conversion efficiency of 88.5%. The maximum deviation of the focal length of the achromatic metalens is 6.88% and the average focusing efficiency in the entire wavelength range is 76.4%.