Broadband achromatic metalens designed for the long-wavelength infrared applications

Long-wavelength infrared (LWIR) thermal imaging technology has been widely used in fields such as the military, medical diagnosis, and aerospace, but the bulky size and high cost of traditional lenses hinder further progress. In recent years, metasurface lenses have emerged as a promising alternative.However they still encounter challenges such as narrow bandwidth, chromatic aberration, and fabrication complexity. Therefore, in this paper, a broadband achromatic LWIR metalens based on the Pancharatnam-Berry(PB) phase and the propagation phase is designed. The finite-difference timedomain(FDTD) method is used to carry out theoretical modeling and simulation analysis of the metalens. The simulated results demonstrate that the chromatic aberration is well corrected within the working wavelength range of 8-11μm, exhibiting a maximum focal length deviation of only 2.48%.Moreover, the actual Numerical Aperture(NA) of the proposed broadband achromatic LWIR metalens attains a value of 0.56. It is worth noting that the aspect ratio of the metalens is just 18:1. The aspect ratio is reduced about 45% compared with the reported metalens using the same achromatic method and the same wavelength band. Above all, the proposed metalens not only retains a broad working bandwidth but also significantly decreases processing difficulty, which provides the foundation for the development of broadband achromatic and low-cost metalenses.