Robust Four-Wavelength Achromatic Metalens Design for Long Wave Infrared Multispectral Focusing

Long wave infrared metalenses provide a promising route toward compact multispectral optical systems, including spaceborne imaging and sensing payloads. However, achromatic focusing at separated LWIR wavelength channels remains challenging because the required phase relation must be maintained together with sufficient dispersion control and optical throughput. Here, we propose a robust four-wavelength achromatic metalens operating at 8, 10, 12, and 14 (Formula presented.) m based on a meta-atom library controlled by nanopillar radius and height. The library feasibility is evaluated before layout optimization to verify whether the required phase and dispersion responses are accessible within the selected unit cell space. By introducing nanopillar height as an additional degree of freedom, the library achieves a phase coverage ratio above 0.92 at all four wavelengths. The optimized metalens achieves a mean absolute focusing efficiency (AFE) of 69.5% with suppressed chromatic focal shift. Monte Carlo perturbation analysis and full device FDTD simulations further confirm the robustness and focusing stability of the design. This work provides a feasibility-driven strategy for robust multispectral LWIR achromatic metalenses.