Optical metasurfaces are artificial structures composed of subwavelength units arranged in specific patterns. By adjusting the size and arrangement of these structural units, unprecedented control over the phase, amplitude, and polarization of electromagnetic waves can be achieved. A metalens is a metasurface-based optical device that manipulates wavefronts using artificially engineered subwavelength unit structures on conventional dielectric substrates. Since Professor Capasso of Harvard University proposed the generalized Snell's law in 2011, metalenses have emerged as a revolutionary flat-lens technology with remarkable advantages, including ultrathin profiles, lightweight design, and ease of integration. This advanced optical component can effectively concentrate incident light, significantly enhancing the effective fill factor of detectors, which in turn improves the performance of infrared detectors. Although metalens structures can efficiently focus incident light and substantially increase the effective fill factor of detectors, achieving sufficient phase accumulation in long-wave infrared (LWIR) bands requires corresponding increases in the height of individual unit structures. This increased height requirement inevitably leads to greater fabrication complexity. Resolving these technical bottlenecks is crucial for the practical application of long-wave infrared focal plane devices.

The selected micro-nano structure in this paper is a cylindrical pillar structure [Fig. 1(c)]. This microstructure serves as the fundamental unit of the metalens, consisting of a GaSb substrate and a GaSb micro-nano pillar. The detector is embedded within the GaSb substrate. The period of the micro-nano pillars is set to 2.4 μm, with radii ranging from 0 to 1.2 μm and heights varying from 1 to 6 μm. The operating wavelength is configured at 10 μm. Taller micro-nano pillars with larger radii are observed to exhibit a broader phase modulation range. To achieve 2π phase coverage, the height of the micro-nano pillars must be at least 5 μm. The metalens is designed using a propagation phase modulation approach. The device is composed of micro-nano pillars with uniform heights arranged in a circular configuration. Assuming the pillar height can be arbitrarily adjusted, simulations are conducted to analyze the focal length and focusing efficiency at the focal point of the metalens under varying heights.

Since fabrication complexity increases with the height of the unit structures, a metalens with a pillar height of 1.6 μm is selected for further investigation. The results show that the focusing efficiency increases for wavelengths below 10 μm and stabilizes between 50% and 51% for wavelengths above 10 μm. Although the focal length decreases from 133 μm to 112 μm within the studied wavelength range, the average focusing efficiency remains 48%. As the incident wavelength increases, the full width at half maximum (FWHM) gradually expands. Notably, the FWHM breaks the diffraction limit at 8 μm and approaches the diffraction limit in the 9?12 μm range, indicating excellent focusing performance. This suggests that the proposed metalens design achieves relatively high focusing efficiency while significantly reducing the fabrication challenges associated with etching micro-nano pillars. Finally, the effects of fabrication tolerances and oblique incident light on the focusing efficiency of the metalens are investigated. Results show that deviations in pillar radii from the designed values (with a standard deviation of less than 50 nm) have a negligible influence on the focusing efficiency and other characteristics. However, the performance of the proposed metalens may be significantly degraded under large-angle oblique incident light, necessitating further research.

Based on propagation phase theory, we design a metalens composed of cylindrical micro-nano pillars directly etched on a GaSb substrate. Using FDTD software, the focal length and focusing efficiency of the metalens are simulated. The results reveal that the metalens exhibits higher focusing efficiency with taller pillars. For instance, when the pillar height ranges from 3.6 to 6 μm, the focusing efficiency reaches 62%?67%. Nevertheless, even when the pillar height is reduced to 1.6 μm, the average focusing efficiency remains 48% across the 8?12 μm wave band, with the focused beam approaching or surpassing the theoretical diffraction limit. The ability to maintain relatively high focusing efficiency across a broad spectral range is due to the extended range along the z-axis, where the field intensity drops to 95% of its peak value at the focal point. This demonstrates that the proposed metalens design achieves a balance between reducing fabrication complexity and retaining competitive focusing performance. Fabrication tolerance simulations demonstrate that deviations in pillar radii (standard deviation <50 nm) have negligible influence on efficiency, confirming the robust tolerance of the designed metalens to manufacturing imperfections. However, the performance of the proposed metalens may be significantly degraded under large-angle oblique incident light, necessitating further research. Such a metalens can enhance infrared detectors and enable direct on-chip integration. In this paper, we introduce a novel approach to lowering fabrication challenges in metalens design and provide a new strategy for realizing high-performance, integrated long-wave infrared detectors.