Few-layer metasurfaces for broadband optical antireflection and highly efficient flat lenses

Tailoring the geometry and arrangement of metasurface structures yields a complete control of the reflection/transmission amplitude, phase, and polarization states. Few-layer metasurfaces can overcome the issues of high-loss and fabrication challenge in bulk metamaterials and weak coupling in single-layer metasurfaces. Furthermore, the coupling among layers results in functionalities beyond the individual constituent metasurfaces. Here we show that bi-layer metallic metasurfaces can realize narrow-, dual-, and broadband antireflection in THz and mid-IR spectral ranges, through tailoring the dispersion of the constituent metasurfaces. We also show tri-layer metasurfaces can be used to enable broadband conversion of linear polarization accompanied with a control of phase spanning the full 2p range. Based on this property, we demonstrate highly efficient flat lenses in the THz and microwave frequency ranges.