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Traditional mid-infrared optical devices are characterized by complex structures, large volumes, and high prices, which impede the advancement of future multidimensional, multifunctional, and miniaturized integration. However, metasurfaces comprising planar and ultra-thin nanostructures have emerged as a promising alternative. By manipulating the interaction between light and materials at subwavelength scales, metasurfaces exhibit remarkable control over optical fields and offer multifunctional capabilities. Consequently, they provide new avenues for integrating infrared systems in a miniaturized form. In this paper, an efficient metalens based on Pancharatnam-Berry (PB) phase working in the mid-infrared range(3.7μm-4.8μm) is proposed and numerically demonstrated. The proposed metalens enables precise control of incident light phase, thereby converge the incident light into two focal spots within spectral-band ranges: 3.7μm-4.0μm (with a focal length of 150μm) and 4.5μm-4.8μm (with a focal length of 250μm). The bifocal metalens is space division multiplexing designed using alternately arranged a-Si nanobricks, facilitating a high polarization conversion efficiency exceeding 80% and achieving achromatic behavior within the two spectral-band ranges. This work demonstrates the potential application of metalens for addressing complex tasks in infrared optical detection.
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