High-NA and broadband achromatic metalens for sub-diffraction focusing of long-wavelength infrared waves

•A broadband achromatic metalens operating in long-wavelength infrared range is proposed.•The multipole decomposition is performed to investigate the fundamental mechanisms of meta-atoms.•The transverse sizes of focal spots from proposed metalens are beyond the Abbe diffraction limit.•The numerical aperture of proposed metalens is up to 0.79.