Objective Traditional long-wave infrared (LWIR) optical systems predominantly rely on a combination of refractive lenses made from specific infrared materials (e.g., Germanium, Zinc Selenide, Zinc Sulfide, Chalcogenide glasses) for aberration correction, including chromatic aberration and athermalization. This approach often results in systems that are complex, bulky, heavy, and costly, in both material and manufacturing costs. Diffractive optical elements can reduce the system size/volume but suffer from complex fabrication. Pure metalenses enable miniaturization but face challenges in broadband chromatic aberration correction, limited by physical trade-offs between numerical aperture, bandwidth, and lens diameter. Existing optical systems combining refractive lenses and metalenses fail to simultaneously achieve simple layout, high performance, and wide field of view (FOV). This study aims to develop a high-performance, wide-FOV LWIR hybrid optical system to address these bottlenecks, realizing compact structure, low cost, and excellent imaging quality.

Methods We propose a hybrid LWIR optical system combining refractive lenses and a metalens to address these limitations. This design combines the fundamental imaging capability of refractive lenses with the phase modulation advantages of metasurfaces, enabling simultaneous correction of wide-field-of-view aberrations and chromatic aberration for the system. A three-element configuration is adopted: two chalcogenide glass refractive lenses and one silicon metalens. The aperture stop is placed at the metalens to reduce lateral chromatic aberration and improve edge illumination (Fig.1(e)). First, a database of meta-units with broadband wavelength-independent phase delay is constructed. Cylindrical silicon meta-units (period 3.5 μm, height 10 μm, diameter 2.04 μm to 3 μm) are selected using Finite-Difference Time-Domain (FDTD) simulations, ensuring 0-2π phase coverage and ≥70% transmittance across 8 to 12 μm (central wavelength 9.6 μm). Crucially, the phase delay versus diameter slope is designed to be nearly identical across the operational bandwidth, achieving wavelength-independent phase delay, which is fundamental for broadband achromatic performance. Then, the incident angle of the metasurface is optimized based on the incident-angle-dependent phase and transmittance of meta-units. The phase and transmittance responses of the meta-units under oblique incidence (0° to 15°) are analyzed via FDTD (Fig.3). Results indicate significant transmittance variation but minimal phase shift for incident angles ≤10°. Consequently, the system is optimized to constrain the chief ray angle of incidence on the metasurface to within 10°. Furthermore, a computationally efficient model is proposed to replace full-wave simulations for large-aperture metalenses. Full-wave simulation of the large-aperture metalens (9.8 mm) is replaced by analyzing a small-aperture counterpart (150 μm) using FDTD. Focal length accuracy is verified by comparing geometric ray-tracing and FDTD results.

Results and Discussions The system achieves an effective focal length of 13 mm, an F-number of 1.03, and a 40° field of view. The root-mean-square (RMS) radius of the spot diagram is less than 6 μm for the central field and below 16 μm for the full field, well within two pixels (Fig.5(a)). The modulation transfer function (MTF) at the Nyquist frequency (42 lp/mm) exceeds 0.3 on average across all fields, reaching approximately 70% of the diffraction limit (Fig.5(b)). Field curvature is below 0.2 mm, and distortion is less than |3.1%| (Fig.5(c)). The fabricated metalens meets tolerances: height error ≤1 μm, straightness ≥86°, eccentricity ≤0.05 mm (Fig.5(e)). The assembled hybrid objective is integrated with a commercial IR detector. Clear thermal images are captured under various scenarios without additional image processing. Details such as facial features and fingers of subjects at 1-2 m, wheels and windows of vehicles at 10 m, and architectural elements of buildings at 60 m are resolvable. Images of clouds against the sky also show clear edges and structures (Fig.6). Quantitative tests confirmed the effective focal length (13.00 mm) and FOV (40.2° diagonal). The measured MTF values at the center field are slightly lower than the design values but follow the same trend, with a maximum difference of 0.11.

Conclusions This study has successfully developed a high-performance, wide-FOV LWIR refractive-metasurface hybrid system that resolves broadband chromatic aberration and balances compactness, performance, and simplicity. The three-element configuration, wavelength-independent meta-unit database, and large-aperture simulation alternative proposed in this study provide a cost-effective solution for LWIR imaging. With excellent MTF, small spot size, and reliable experimental performance, the system is promising for applications in security monitoring, autonomous driving, industrial inspection, and defense.