Design of mid-wave infrared achromatic double-layer metalens with enhanced efficiency

Achromatic metalenses in the mid-wave infrared (3-5 mu m), known for their light weight, CMOS compatibility, and ultra-compactness, offer significant potential in astronomy, security inspections, and health security. However, previous endeavors have been hindered by underdeveloped material technology and relatively low efficiency. To address these challenges, this study introduces an enhanced-efficiency mid-wave infrared achromatic double-layer metalens, featuring a top-layer ZnS nanopillar array and a bottom-layer Si nanopillar array on a Si substrate. Utilizing this approach, we numerically demonstrate both polarization-insensitive and polarization-controlled varifocal broadband achromatic metalenses. For the polarization-insensitive metalens, the double-layer design provides achromatic focusing comparable to the all-Si counterpart, with a focal length of 133 mu m, a focal length shift within +/- 5.5%, and Strehl ratios above 0.8. However, the average focal efficiency improves from 40.8% (all-Si) to 50.2% (double-layer). Additionally, both all-Si and double-layer polarization-controlled varifocal achromatic metalenses show similar focusing abilities, with focal lengths of about 137 and 173 mu m under X and Y linearly polarized light, respectively. Yet, the double-layer varifocal metalens achieves focal efficiencies of 46.4% and 52.7%, an improvement of 13.1% and 17.6% under X and Y linearly polarized light, respectively.