Broadband achromatic metalens with polarization insensitivity in the mid-infrared range

Broadband achromatic metalens (BAML) has been successfully demonstrated in the mid-infrared (mid-IR) owing to the importance in numerous applications ranging from night vision to biomolecular sensing. However, these BAMLs only works for specific polarized light, hindering their practical applications. In this paper, we propose an achromatic metalens that works in the transmission mode with polarization insensitivity and continuously provide a near-constant focal length over a broad wavelength region of lambda = 3.7-4.7 mu m. The proposed metalens is comprised of just a single layer of silicon meta-units and has a numerical aperture of 0.84 with a diameter of 77.4 mu m. Moreover, we design an achromatic axilens in the same wavelength region to demonstrate the proposed concept. The performances of the achromatic metalens and axilens are numerically analyzed in detail. Our study represents a major step towards practical implementations of mid-IR devices.