Design and performance analysis of all-dielectric reflective, metalens for LWIR applications

Reflective metalenses with multilayer substrates offer low-loss wavefront control in the LWIR range but remain underexplored and highly dependent on material choice. Here, we report a comprehensive numerical investigation into the design of LWIR reflective metalenses employing dielectric distributed Bragg reflectors (DBRs) substrates composed of high-index semiconductors (Si, Ge, GaAs) and zinc-based dielectric compounds (ZnO, ZnSe, ZnS). We systematically evaluate nine DBR material combinations to assess their impact on the focusing efficiency, reflectivity, and focal spot characteristics. The designed metalens, with an aperture diameter of and a focal length of 0.7, operate at a design wavelength of. All configurations achieve high reflectance over a broad spectral range, with Si/ZnSe and GaAs/ZnO based designs exhibiting the highest focusing efficiencies of and respectively, at Numerical Aperture (NA). All the examined configurations provide nearly complete phase coverage, yielding diffraction-limited focal spot sizes ranging from to. We further analyze the impact of NA and metasurface unit cell periodicity () on the lens performance, demonstrating that smaller unit cell periods improve phase discretization and optical response uniformity, while increasing NA results in tighter focal spots, with diminishing improvements near the diffraction limit.