Compound meta-atoms enabling ultra-compact multiband optical manipulation.

Metasurfaces formed by subwavelength meta-atoms have continuously attracted interest owing to their flexible optical modulation, which offers unprecedented capability for the miniaturization of optical devices. Recently, broadband achromatic metalenses have been extensively reported for visible and mid-infrared light. However, it is a huge challenge to simultaneously manipulate the visible and mid-infrared light due to a large wavelength range. Here, a compound meta-atom is proposed to realize the multiband optical manipulation, which comprises two-layer decoupled meta-atoms. The lower-layer meta-atom and upper-layer ones are employed to modulate the mid-infrared and visible light, respectively. As a result, a compound metalens is designed to accomplish optical focusing at wavelengths of 0.65 µm and 3.7-4.8 µm. Numerical results demonstrate that the compound metalens can realize diffraction-limited focusing of visible light and broadband achromatic focusing of the mid-infrared light at the same focal plane. The compound metalens exhibits achromatic focusing for multiband light in a large wavelength range. Additionally, the design methodology of our meta-atoms is also applicable to other multiband optical modulation.