Planar metalenses in the mid-infrared

Traditional spherical lenses are bulky and often the limiting factor for the miniaturization of modern smart devices. Metalenses can break the limitations of traditional spherical lenses, allowing for the development of ultra-thin planar lenses. Here, we experimentally demonstrated metalenses in the mid-infrared spectral range by patterning a germanium wafer using standard nanofabrication processes. Three 6 mm × 6 mm planar lenses operating at 3μm, 5μm and 8μm were fabricated and characterized. The results show that the focusing efficiency of the metalenses reaches 80% and the numerical aperture is as high as ∼ 0.8, close to the designed theoretical value. The metalenses are also used to image a lighter fire with a quality comparable to traditional spherical lenses.