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We propose two kinds of high-efficiency free-space wave splitters in the mid-IR band using reverse design. The wavelength divider based on the abnormal reflection principle realizes a beam-splitting angle of 22.00° and 10.92° by controlling the phase distribution, and the reflection efficiency of both wavelengths exceeds 50%. The wavelength divider designed based on the concept of metalens simultaneously accomplishes the functions of focusing and beam splitting. It has a focal length of 100 μm and a relative focal position of 100 μm. Most importantly, the focusing efficiency for the two wavelengths reaches an impressive 48.59% and 72.51%, respectively.

We present a reflective metalens with a hybrid antenna structure, which can realize the beam splitting function of two wavelengths in free space. A key feature of our design is the presence of phase-independent regulation at two different wavelengths in each nanoresonator, which can generate a specific phase or a continuous sweep in the range of 0° to more than 300°. To demonstrate the wavefront manipulation ability of the array, we simulated the characteristics of wavelength beam splitting, especially the free space wavelength beam splitter with a focal length of 300 μm and a relative focal position of 100 μm.