1 Introduction
2 Pancharatnam–Berry (PB) phase metasurfaces
2.1 Fundamental of PB phase
2.2 Applications of PB phase metasurfaces
Fig.2 (a) PSHE in the plasmon chain. Scanning electron microscope (SEM) images of the surfaces of the plasmons and spin-dependent momentum deviation of a metasurface. (b) Images and schematic illustrations of a refract dipole array, where the broadband anomalous refraction can be observed ranging from visible to near-infrared wavelengths. (c) Schematic diagram of achieving 100% efficiency PSHE on the reflective metasurface: the specular reflection mode disappears completely, and when linearly polarized light is incident, it will be split into two spin-polarized reflected beams that propagate in the directions of two illegal lines. (d) The circularly polarized incident beam is based on the illustration of the computer-generated hologram reflecting the nanorods. The linearly polarized beam passes through the quarter-wave plate and is converted into a circularly polarized incident beam. On the metasurface, the reflected beam forms a holographic image in the far-field. (e) Schematic diagram of 100% efficiency PSHE achieved by PB metasurfaces that meet the conditions in the transmission geometry. (f) Optical image of the metalens designed at the wavelength of 660 nm, the SEM micrograph of the fabricated metalens, and the image formed by the metalens in the transmission geometry. (a) Reprinted with permission [67]. Copyright 2011, Nano Letters. (b) Reprinted with permission [53]. Copyright 2012, Nano Letters. (c) Reprinted with permission [31]. Copyright 2015, Advanced Optical Materials. (d) Reprinted with permission [68]. Copyright 2015, Nature Nanotechnology. (e) Reprinted with permission [69]. Copyright 2017, Physical Review Applied. (f) Reprinted with permission [4]. Copyright 2016, Science |
3 Spin-decoupled metasurfaces based on “merging” concept
Fig.3 Multi-functional devices designed with merged structures. (a) Illustration of the working principle. The state of the beam polarization can be detected without multiple measurements or an interferometric setup. (b) Schematic far-field intensity distribution of wavefronts with positive (red) and negative (blue) helicities emerging from segmented, interleaved, and harmonic response geometric phase metasurfaces composed of gap-plasmon nanoantennas (inset). Here, l denotes the topological charge of the spin-dependent OAM wavefronts. (c) Near-field open channels via disordered gradient metasurfaces. Schematic of directional SPP channels opened by a DGM. The scanning electron microscope image shows the metasurface, wherein and , fabricated using a focused ion beam. The array consists of 80 nm×200 nm nanoantennas etched to a depth of 100 nm into a 200 nm thick gold film, evaporated onto a glass substrate. The diameter and width of the surrounding annular slit are 150 μm and 150 nm, respectively. (d) A metasurface that can generate multiple hologram images as shined by circularly polarized light with different helicity. (e) Design strategy, sample pictures, and experimental characterizations of a multi-functional metasurface that can generate holographic images or a vortex beam depending on the helicity of incident circularly polarized light. (f) Schematic of a metasurface with the interleaved design for helicity-dependent focusing and holograms. (a) Reprinted with permission [70]. Copyright 2015, Optica. (b) Reprinted with permission [36]. Copyright 2016, Science. (c) Reprinted with permission [37]. Copyright 2015, ACS Photonics. (d) Reprinted with permission [71]. Copyright 2015, Nature Communications. (e) Reprinted with permission [72]. Copyright 2017, ACS Photonics. (f) Reprinted with permission [30]. Copyright 2016, Advanced Optical Materials |
4 Spin-decoupled metasurface based on the combined phases
Fig.4 (a) Schematic diagram of a four-port multiplexer using the proposed spin-decoupled meta-atom. (b) Illustration of two completely different wavefronts: oblique planar wavefront and a focusing wavefront. (c) Schematic of a bifunctional reflective metasurface dependent on the helicity. For the LCP incidence, the reflective beams are generated to have OAM modes =2 and =0. For RCP incidence, two vortex beams carrying OAM modes =1 and =-1. (d) Schematic diagram of a meta-device designed to generate independent electromagnetic functions. (e) Left panel: schematics of light scatterings at a meta-device exhibiting vortex wavefront carrying different topological charges under illuminations of LCP and RCP incidences. Right panel: SEM images of the fabricated sample. (a) Reprinted with permission [75]. Copyright 2020, Advanced Materials Technologies. (b) Reprinted with permission [45]. Copyright 2019, ACS Photonics. (c) Reprinted with permission [15]. Copyright 2019, Physical Review Applied. (d) Reprinted with permission [76]. Copyright 2020, Annalen der Physik. (e) Reprinted with permission [84]. Copyright 2020, Nanophotonics |
Fig.5 (a) Schematics of the proposed metasurfaces to achieve helicity-delinked manipulations on both propagating wave and surface waves, which can convert incident CP waves of opposite helicity to surface waves possessing different wavefronts and traveling to opposite directions, both exhibiting extremely high efficiencies. Numerically computed field patterns on the surface of a plasmonic metal supporting a surface wave mode with an eigen wavevector are placed on the top surface of the plasmonic metal. Color maps show the phase distributions. (b) Picture of part of the fabricated sample consisting of a PB meta-device connected with two artificial metals. Simulated and measured near field pattern on a plane 50 µm underneath the sample. (c) Schematic view of the bifunctional meta-coupler that can realize circular polarization controlled unidirectional SSP excitation and anomalous reflection. (d) Left panel: schematic of the spin decoupled multi-functional GSP gradient metasurface for unidirectional SPP coupling and anomalous beam steering under normally incident RCP and LCP light, respectively. Right panel: SEM images of the fabricated spin decoupled GSP gradient metasurface and the calculated reflection phase profiles of the spin decoupled GSP gradient metasurface under RCP (red dots) and LCP (black squares) incidence at 850 nm. (a) Reprinted with permission [85]. Copyright 2020, Nanophotonics. (b) Reprinted with permission [86]. Copyright 2020, Advanced Science. (c) Reprinted with permission [87]. Copyright 2019, Optics Express. (d) Reprinted with permission [88]. Copyright 2020, ACS Photonics |
Fig.6 Spin-decoupled metadevices. (a) Manipulating CP waves in desired multi-prescribed manners, especially in both transmission and reflection schemes, is of particular importance. Specially tailored PB meta-atoms with helicity-dependent transmissions and reflections are used to design high-efficiency CP bifunctional metasurfaces. Two kinds of bifunctional metadevices are designed and characterized at microwave frequencies, and both exhibit remarkably high efficiencies (88%−94%). (b) Experimental demonstration of chiral holograms realized with a single TiO2 metasurface encoding two independent hologram phase profiles for LCP (dog) and RCP (cat) at λ = 532 nm. (c) Experimentally captured optical images of the metasurface nanoprinting illuminated with LCP and RCP light. (d) Spin-decoupled holograms with chiral meta-atoms. (e) Schematic of cylindrical vector beam generation in dielectric metasurfaces. (Left) vector vortex beam generation. (Right) vector Bessel beam generation. Each schematic contains four excitation cases marked by different colors. The color maps illustrate the corresponding phase distributions, where the number represents the value of the topological charge and the carrying OAM. (f) Spin-multiplexed optical imaging system. For a light incident on the device with LCP, the metasurface imprints a masking function on the output beam resulting in a constant phase profile and a Gaussian intensity distribution and flips the handedness of the incident polarization. For a light incident on the same device with RCP, the metasurface imprints another masking function, resulting in a spiral phase profile and a donut-shaped intensity distribution, and again flips the handedness of the polarization. (a) Reprinted with permission [89]. Copyright 2018, Annalen der Physik. (b) Reprinted with permission [25]. Copyright 2017, Physical Review Letters. (c) Reprinted with permission [93]. Copyright 2020, Physical Review Letters. (d) Reprinted with permission [94]. Copyright 2021, Nano Letters. (e) Reprinted with permission [95]. Copyright 2020, Nanophotonics. (f) Reprinted with permission [96]. Copyright 2020, Nano Letters |