Fig. 1. Schematic of BAFOV generator, the birefringent meta-atom library, and SEM images of the samples.
(A) Schematic illustration of the BAFOV generation with polarization-dependent functions. The MWIR beam is normally incident on the metasurface from the substrate. The transmitted light is achromatically focused at prescribed focal planes with different topological charge numbers according to the incident polarization state. (Top) The phase profile of l = 0 for x-polarization and (bottom) l = 2 for y-polarization. Blue and green double-headed arrows shown in (A) represent the x- and y-polarization, respectively. (B) The birefringent meta-atoms are made of monocrystalline Si (top). The thickness of the pattern layer of the all-Si metasurface is H = 4500 nm. Bottom: The polarization-dependent magnetic energy density profile along the x-z cross section in the periodic array with Dx = 1160 nm and Dy = 700 nm. Black dotted lines depict the boundaries of the nanoposts. (C) The meta-atom library is composed of the transmittance and phase spectra based on the meta-atom engineering through sweeping wavelength from 3.5 to 5 μm. (D to F) SEM images of the fabricated polarization-controlled BAFOV. (D) Overall SEM image of the metadevice. Scale bars, 27 μm. Top view [scale bar, 2 μm (E)] and oblique view [scale bar, 4 μm] of part of the sample [marking with the green frame shown in (D)].