Fig. 1 The schematics of fabrication technologies for optical metasurfaces.

Fig. 2 (a) Schematic of the fabrication process for the bottom-up EBL method. A gold (Au) mirror is first deposited on a Si substrate by e-gun evaporator. A silicon dioxide (SiO₂) dielectric spacer is then deposited using plasma-enhanced chemical vapor deposition (PECVD). A resist layer is then spin-coated on the prepared substrate and baked on a hot plate. Subsequently, an Espacer layer is spin-coated on the resist layer. Espacer is an organic polymer with high conductivity to reduce the positional error during the e-beam exposure process. The structural profile of metasurfaces is defined by the e-beam exposure and development process. The final sample will be obtained after the Au deposition and lift-off process. (b) Zoom-in SEM image of the fabricated achromatic converging meta-device which successfully eliminated the chromatic aberration over a continuous wavelength region from 1200 to 1680 nm for circularly-polarized incidence in a reflection scheme. (c) Schematic illustrating the fabrication process of the top-down EBL method. An undoped GaN layer is first grown on a c-plane sapphire substrate by metal−organic chemical vapor deposition (MOCVD). A SiO₂ hard mask layer is then deposited using PECVD. A resist layer is spin-coated on the prepared substrate and then baked on a hot plate. The structural profile of the metasurface is defined by the e-beam exposure and the development process. After that, a Cr layer as an etching hard mask is coated on the substrate by e-gun evaporator and followed by a lift-off process. The patterns are transferred to the SiO₂ layer by reactive ion etching (RIE). The substrate with the patterned SiO₂ hard mask layer is etched by inductively coupled-plasma reactive ion etching (ICP-RIE) using BCl₃/Cl₂ chemistry. The final sample is obtained after the removal of the patterned SiO₂ hard mask layer with the buffered oxide etch (BOE) solution. (d) The SEM image of the on-axis focusing GaN-based metalens, designed with diameter of 100 μm and focal length of 300 μm. The metalens works in a transmission window with extremely high operation efficiency of 91.6% at visible light. (b) Reprinted with permission from [68]. Copyright 2017, Springer Nature. (d) Reprinted with permission from [69]. Copyright 2017, American Chemical Society.

Fig. 3 (a) and (b) are SEM images of the FIB-fabricated metasurfaces. (a) A metasurface hologram of the letter ‘P’ with the 5-μm scale bar. The inset is a zoomed-in view (scale bar, 500 nm). The hologram is designed for the operational wavelength of 676 nm, and the thickness of the sample is only about 1/23 the size of the wavelength. (b) A planar chiral metasurface to produce optical vortex from a circularly polarized light. Optical vortex can enhance data capacity for its extra degree of freedom of angular momentum. The metasurface has the ability to focus the incident light into a tiny point, which greatly increases the power intensity of the generated optical vortex and has potential application in highly integrated optical communication systems. (c) The morphology characterization of the negative refractive index metasurfaces obtained by AFM, which is fabricated with a scanner movement nanolithography on the thin silver film. The double split ring resonators with additional capacitive gaps can compensate for the inertial inductance. (d) Photograph of the fabricated metasurface using LDW in print circuit board (PCB) technology. (e) SEM image of a metasurface fabricated by orthogonal LIL capable of manipulating linear polarization. The near-infrared reflective linear polarization converter is composed of ellipse-shaped plasmonic planar resonators. A polarization conversion ratio in power larger than 91.1% is achieved from 730 to 1870 nm. (f) SEM image of the metasurface hologram film based on bilayered metallic nanowire gratings at different scales, produced by LIL. The bilayered metallic gratings behave as an ideal polarized beam splitter, producing strong negative reflection for transverse-magnetic (TM) light and efficient reflection for transverse-electric (TE) light. (a) Reprinted with permission from [73]. Copyright 2013, Springer Nature. (b) Reprinted with permission from [74]. Copyright 2015, Springer Nature. (c) Reprinted with permission from [75]. Copyright 2006, Elsevier B.V. (d) Reprinted with permission from [76]. Copyright 2016, Springer Nature. (e) Reprinted with permission from [77]. Copyright 2015, AIP Publishing LLC. (f) Reprinted with permission from [78]. Copyright 2014, Springer Nature.

Fig. 4 (a) Schematic of the imaging reflective plasmonic lithography structure with a silver lens. The reflective lens amplifies and compensates evanescent waves, resulting in the production of nano resist patterns. (b) Corresponding SEM image of the anisotropically arrayed nano-slot metasurface using the reflective plasmonic lithography after RIE and ion beam etching processes. Zoomed area has a scale bar of 200 nm. The metasurface can focus a helicity-dependent plane wave into a spot. (c) Schematic of plasmonic cavity lithography system consisting of a Cr mask and a Ag/PR/Ag plasmonic cavity with an air separation layer sandwiched between them to avoid contamination and damage of mask patterns. The cavity can effectively amplify the evanescent waves and modulate the electric field components on imaging plane, resulting in greatly improved resolution and fidelity compared to near field and superlens lithography. (d) SEM image of the corresponding metasurface hologram by hydrogen fluoride (HF) wet etching and ion beam dry etching. The metasurface hologram exhibits a target object in the form of character “E”. (a) and (b) are reprinted with permission from [80]. Copyright 2015, Royal Society of Chemistry. (c) and (d) are reprinted with permission from [81]. Copyright 2017, John Wiley and Sons.

Fig. 5 (a)-(d) SEM images of thermal-NIL-fabricated metasurfaces. (a) Metasurface quarter-wave plate constructed from an orthogonal array of nanorods of two different sizes on a glass substrate. It operates in the wide bandwidth of near-to-mid infrared and provides high polarization conversion efficiency. (b) and (c) Nanostripe and nanohole structures of perovskite metasurfaces, respectively. Insets show cross sections of the metasurfaces (scale bars are 300 nm). The metasurfaces exhibit a significant enhancement of both linear and nonlinear photoluminescence (up to 70 times) combined with advanced stability. It may pave the way toward highly efficient planar optoelectronic metadevices. (d) the heterogeneous all-dielectric metasurfaces as an ultra-broadband reflector. The over etching into the substrate is intentional since any residual a-Si will degrade optical efficiency. (e) and (f) SEM images of UV-NIL-defined metasurfaces. (e) The dual-band metasurface thermal emitters integrated with a resistive membrane heater. The heater pattern is seen on the right-hand side. This is because the bottom of the resist does not reach the substrate after the dry etching in the blank area outside the heater. Therefore, the metasurface is patterned only on the heater. (f) The metasurface with stacked subwavelength gratings. The metasurface has high-contrast and diode-like asymmetric optical transmittance in the visible-to-infrared wavelength range for TM-polarized light. (a) Reprinted with permission from [82]. Copyright 2015, American Chemical Society. (b) and (c) are reprinted with permission from [83]. Copyright 2017, American Chemical Society. (d) Reprinted with permission from [84]. Copyright 2017, John Wiley and Sons. (e) Reprinted with permission from [85]. Copyright 2015, National Institute for Materials Science. (f) Reprinted with permission from [86]. Copyright 2016, Optical Society of America.

Fig. 6 (a) Schematic of a large-scale colloidal self-assembly process. (b) SEM image of a dielectric metasurface acting as a near-perfect mirror in the telecommunications spectral window, based on NL with the PS spheres as a hard mask for the subsequent reactive ion etching of silicon. The optical measurements show an almost perfect reflectivity of 99.7% at 1530 nm with a good spectral tolerance. (c) The SEM images of Si-cylinder metasurface formed with NL and RIE. The inset in (c) is the 60° tilted view of a specially chosen defective area to better illustrate the spatial morphology. Scale bars represent 1 μm. The regularly arrayed Si cylinders with hexagonal lattice fabricated on PET flexible substrate are exploited to detect applied strain and surface dielectric environment by measuring transmission spectra. (d) Schematic of the main fabricating procedure for the flexible, all-dielectric metasurface. A thin layer of Si is deposited on PET substrate by electron beam evaporation. After that, a monolayer of PS spheres is self-assembled at the air/water interface. The size of PS spheres is further reduced with an isotropic oxygen plasma etching. The etched monolayer works as a hard mask for the subsequent etching. Finally, the sample is immersed in chloroform coupled with sonication to remove all remaining PS spheres. (e) Schematic of the multi-angled deposition for metasurface realization with the definition of free parameters relative to the crystal axis. It also shows an example composed of three different types of features: (Case 1) an interconnected line, (Case 2) an asymmetric bar, and (Case 3) a symmetric bar. (f) Left panel: the duplication of feature types from (Case 1) to (Case 3) at intervals of φ = 60 with the composition of six angles of projection. Right panel: images collected with SEM images of the fabricated versions of these patterns (Ag on Si). Inset: The six angles by which the features are reproduced. (g) Top panel: the fabrication schematic of moiré metasurfaces by the NL technique. θ indicates the angle of in-plane rotation between the two layers of self-assembled PS spheres. Bottom panel: SEM images of two representative moiré metasurfaces with θ ∼12° and θ ∼19°. The scale bars are 2 μm. (a) and (b) are reprinted with permission from [87]. Copyright 2015, Springer Nature. (c) and (d) are reprinted with permission from [88]. Copyright 2017, Optical Society of America. (e) and (f) are reprinted with permission from [89]. Copyright 2014, American Chemical Society. (g) Reprinted with permission from [90]. Copyright 2015, Royal Society of Chemistry.

Fig. 7 (a) SEM image of a laser-induced self-organized Si metasurface. Insert: schematic sketch of the laser-induced nanostructuring on the Si film in upper left, and zoom-in of the metasurface in upper right. (b) Schematic of metal nanoparticle ensemble preparation by BCP self-assembly, substrate transfer and pattern shrinkage. (c) SEM images of hexagonal Au nanoparticle arrays as-prepared from BCP self-assembly. The precise manipulation of the distance between block copolymer nanopatterns via pattern shrinkage can increase the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1000 nm wavelength bandwidth. (d) Photo of the Langmuir–Blodgett trough apparatus as a robust and scalable assembly method that is used to form the metasurface constructed of close-packed Ag nano-cube array. The deposition process follows the assembly of Ag nano-cube array at an air–water interface. The array can be subsequently transferred onto substrates of different kinds including those of flexible and non-planar. (e) SEM image showing closely-packed Ag nanocubes after deposition. A measured spacing of 3 nm occurs due to polymer grafts at the Ag surface. Scale bar is 1 μm. (f) Left panel: schematic template-assisted self-assembly process by dip coating the wrinkled templates in a highly concentrated Au nanorod solution. Right panel: the AFM image of the self-assembled array of Au nanorods in the wrinkle template. (a) Reprinted with permission from [91]. Copyright 2016, Royal Society of Chemistry. (b) and (c) are reprinted with permission from [92]. Copyright 2016, Springer Nature. (d) and (e) are reprinted with permission from [93]. Copyright 2015, Springer Nature. (f) Reprinted with permission from [94]. Copyright 2016, Royal Society of Chemistry.

Fig. 8 (a) Schematic diagram of the process flow for the microlens projection lithography. Detailed description can be found in [95] (b) Left panel: a non-periodic metasurface composed of T-shaped nano-patterns that was first patterned in Au on silicon and then etched to yield micro-pillars using dry etching. Right panel: the zoomed-in view of T-shaped nano-patterns. The scale bars of the left and right panels are 20 and 5 μm, respectively. (c) Schematic illustration showing the fabrication process for tilted nano-pillars using hole-mask colloidal lithography and off-normal deposition. (d) SEM images for the fabricated samples with different tilting angles. The scale bar is 500 nm. (a) and (b) are reprinted with permission from [95]. Copyright 2016, American Chemical Society. (c) and (d) are reprinted with permission from [96]. Copyright 2016, American Chemical Society.

Fig. 9 (a) Schematic of a novel method based on femtosecond LIFT for high-throughput and efficient fabrication of nano-structures. With the precise control of laser raster path applied on sputtered multilayer thin films, the laser-ablated materials can be transferred to another substrate leaving a fabricated multilayer structure on the original substrate. SEM images of (b) the fabricated multilayer split-ring resonator arrays on donor and (c) the corresponding transferred structures on receiver. (d) Close-up SEM image of recorded marks showing the void at the center of the mark and a ring surrounding the void. The laser ablation fabrication is realized in a 50-nm-thick phase-change film on a 130-nm-thick ZnS-SiO₂ dielectric layer deposited on a glass substrate. (e) SEM image of a single Au nanoparticle on a SiO₂ substrate fabricated by the laser-induced dewetting process of a 30-nm-thick Au film. Scale bar is 500 nm. (f) Illustration of multiphoton polymerization generated by a focused laser beam. A photopolymer absorbs two near-infrared photons simultaneously in a single quantum event whose collective energy corresponds to the UV region of the spectrum. The rate of two-photon absorption is proportional to the square of the light intensity, so that the near-infrared light is strongly absorbed only at the focal point within the photopolymer. SEM images of 3D microstructures with multi-photon polymerization for (g) a photonic crystal structure and (h) Venus. (a)-(c) are reprinted with permission from [97]. Copyright 2012, John Wiley and Sons. (d) Reprinted with permission from [98]. Copyright 2010, Optical Society of America. (e) Reprinted with permission from [99]. Copyright 2016, John Wiley and Sons. (f) Reprinted with permission from [100]. Copyright 2008, John Wiley and Sons. (g) and (h) are reprinted with permission from [101]. Copyright 2003, Optical Society of America.

Fig. 10 (a) Measured focusing efficiency of the metalenses for incident circularly polarized light as a function of wavelength. The two lenses are designed at two different wavelengths λ_d = 532 and 660 nm. The efficiency defined as the transmitted optical power with opposite helicity divided by the incident circularly polarized light. Insert: SEM micrograph of the fabricated metalens designed at the 660-nm wavelength with the scale bar of 300 nm. (b) Measured focal spot intensity distribution of the metalens designed at the wavelength of 660 nm. (c) Schematic of the imaging principle of the proposed chiral metasurface, which focuses into two spots with different helicity. The inlet in the figure comprises two images of beetle where left and right panels are revealed by focusing the reflected left-circularly polarized (LCP) and the right-circularly polarized (RCP) light from the beetle, respectively. (d) The diagram demonstrates a multiplex color router with the dielectric metalens capable of guiding individual primary colors into different spatial positions. (a) and (b) are reprinted with permission from [109]. Copyright 2016, The American Association for the Advancement of Science. (c) Reprinted with permission from [110]. Copyright 2016, American Chemical Society. (d) Reprinted with permission from [69]. Copyright 2017, American Chemical Society.

Fig. 11 (a) SEM images of fabricated meta-holograms consisting of silicon nanopillars of various sizes. (b) Experimental holographic image obtained through an infrared camera from the sample illuminated with a collimated laser beam at a 1600 nm wavelength. (c) Schematic illustration of the designed multicolor meta-hologram under linearly polarized illumination. The meta-hologram structure is made of a pixel array consisting of aluminum (Al) nanorods that produce images R, G, and B in 405, 532, and 658 nm, respectively. The pixels are patterned on a 30-nm-thick SiO₂ spacer sputtered on an Al mirror. (d) Schematic of the highly dispersive meta-hologram which projects the red image of a flower, the green image of a peduncle and the blue image of a pot. The insert is the partial SEM image of the fabricated meta-device. The white scale bar corresponds to 1 μm. (e) Sketch of the dynamic holographic metasurface which projects the holographic images of the letters ‘P’, ‘K’, ‘U’ at the imaging plane. The middle panel shows the metasurface composed of an array of meta-atoms. Each meta-atom has a pin diode welded between the two metallic loops and independently controlled by a DC voltage through a via (see the unit cell in the upper left corner). (f) Left panel: phase distribution of the metasurface with fork gratings. Right panel: the SEM images of the fabricated plasmonic metasurfaces on an 80-nm-thick Al thin film by FIB. It includes spatially variant nano-slits with a size of ~50 nm by 210 nm. Scale bar is 3 µm. (g) Generated optical vortex beam from metasurface fork gratings with different incident beams. (h) The measured intensity profiles and corresponding interference patterns of the metasurface on the transmission side under the RCP illumination. Three real focal planes under the cross LCP transmission at z = 40, 100, and 160 µm are observed. (a) and (b) are reprinted with permission from [111]. Copyright 2016, Optical Society of America. (c) Reprinted with permission from [112]. Copyright 2015, American Chemical Society. (d) Reprinted with permission from [113]. Copyright 2016, American Chemical Society. (e) Reprinted with permission from [114]. Copyright 2017, Springer Nature. (f) and (g) are reprinted with permission from [115]. Copyright 2017, John Wiley and Sons. (h) Reprinted with permission from [116]. Copyright 2016, John Wiley and Sons.

Fig. 12 (a) The simulated performance of the multi-wavelength achromatic metalens with the focal length of 7.5 mm and the diameter of 600 μm for various wavelengths. Broadband incident light is employed to illuminate the backside of the lens. (b) Upper panel: schematic illustration of scattering element composed of two kinds of a-Si nanoposts. Lower panel: the top view and 30-degree-tilt view of the SEM images for the metalens, respectively. (c) The artist’s view and schematic illustration of the three-layer structure metalens are shown in the left panel and the right panel, respectively. An interlayer distance of 200 nm is designed to prevent the near-field crosstalk between the nano-sized antennas in the different layers. The diameters and the separations of nano-disks are 125 and 185 nm in the Au-based layer, 85 and 195 nm in the Ag-based layer, and 120 and 150 nm in the Al-based layer, respectively. (d) The simulated and experimental results of the focal length versus wavelength for the achromatic metalens. Also shown in the figure is the experimental results for the geometric-phase-based metalens. It shows 1.5% variation in focal length between 490 to 550 nm for the achromatic metalens, which is close to the simulation result of 1.2% variation. The insert is a top-view SEM image of the fabricated sample. (e) Schematic of the achromatic metalens where the focal point becomes a single spot with the optimized phase compensation. (f) Schematic illustration of normal dispersion in refractive prisms and conventional lenses in the left panel. The other right panels are schematics of metalenses with negative, zero, positive and hyper-dispersive in dispersion-controlled metasurfaces. (a) Reprinted with permission from [120]. Copyright 2015, The American Association for the Advancement of Science. (b) Reprinted with permission from [121]. Copyright 2016, Optical Society of America. (c) Reprinted with permission from [122]. Copyright 2017, Springer Nature. (d) Reprinted with permission from [123]. Copyright 2017, American Chemical Society. (e) Reprinted with permission from [68]. Copyright 2017, Springer Nature. (f) are reprinted with permission from [124]. Copyright 2017, Optical Society of America.

Fig. 13 (a) Schematic of the TCO-metasurface serving as the quarter-wave plate in the reflection mode and unit cells of plasmonic resonators with associated geometrical parameters. The material of gallium-doped zinc oxide (Ga:ZnO) is chosen to be TCO. P_x and P_y are the periodicity in x- and y-directions, respectively. (b) Top-view SEM image of the fabricated Ga:ZnO metasurface. (c) The measured polar diagrams of polarization state for the reflected beam at wavelengths of 1.6, 1.9, and 2.0 μm. The wavelength for reflected beam of the bare glass is 1.9 μm. Here both of P_x and P_y, as indicated in the schematic diagram of the unit cell, are designed to be 750 nm. (d) Schematic illustration of the Z-shaped chiral metasurface as the CD waveplate. (e) Top-view SEM image of the fabricated Z-shaped left-handed chiral metasurface. The theoretical and experimental polar diagrams for polarization statesof (f) the linearly-polarized transmission spectra with the RCP incident light and (g) the elliptically-polarized reflection spectra (close to circular polarization) with the LCP incident light at the wavelength of 1.56 μm. (h) Schematic of a monolithically integrated metasurface. The monolithic device is composed of QCLs forming two arms with integrated dielectric waveguides on which antennas are arranged. (i) Optical microscope (OM) image of the fabricated antenna structure arranged as a second-order grating with the white scalar bar of 100 μm. Each antenna has dimensions of 21 μm in length, 3 μm in width, and 0.4 μm in height. Measured polarization state of the active metasurface with pumped current in the left arm (j) equal to 3.49 A or (k) varied from 3.39 to 3.54 A, while keeping the current in the right arm faxed at 3.67 A. The output polarization state can be tuned from linear to near circular with the superposition of polarizations of the emitted light from two arms. (a)-(c) are reprinted with permission from [126]. Copyright 2016, American Chemical Society. (d)-(g) are reprinted with permission from [127]. Copyright 2017, Springer Nature. (h)-(k) are reprinted with permission from [128]. Copyright 2017, American Chemical Society.

Fig. 14 (a) Schematic diagram of the metasurface functioning as the half-wave plate. (b) SEM images of two samples with the optical axis angle of 0° and 45°. The metasurface consists of Au nanorods on a glass substrate. (c) Analytical calculations (the curves) and experimental results (the symbols) of state-of-polarization analysis for the two samples. (d) Optical microscopic photograph of a fabricated sample. Inserted is the schematic of the metasurface. (e) and (f) are simulated and measured cross-polarized transmission spectra, respectively, of the metasurface with different geometric parameters. (g) Schematic of an electrically controllable metasurface operating in the reflection mode. (b) The reflection phase of the active metasurface in relation to various biasing conditions at a specific wavelength of 5.94 μm. Inset shows the diagram describing the complex reflection coefficient for three conditions of bias with the increase in frequency. (a)-(c) are reprinted with permission from [129]. Copyright 2017, American Chemical Society. (d)-(f) are reprinted with permission from [130]. Copyright 2016, AIP Publishing LLC. (g) and (h) are reprinted with permission from [131]. Copyright 2017, American Chemical Society.

Fig. 15 (a) Schematic of the gap-plasmon metasurface for the generation of six polarization states with the incident light of linear polarization. (b) Diagram showing numerical and experimental results of scattering intensity in the left and right panels, respectively, with the 600-nm wavelength of the incident light. The middle panel shows the corresponding SEM images with a white scale bar of 1 μm. (c) The measured polarization extinction ratio for six generated polarization states. The results validate the metasurface preserving a broadband operation in the visible regime. (d) Schematics explaining the unit pixel and off-axis multichannel generation of the OAM metasurface. Each nanorod has dimensions of 220-nm long, 80-nm wide, and 30-nm thickness. (e) Schematic diagram illustratingthe superposition of two kinds of OAM states upon the illumination of linearly-polarized incident light, which can be decomposed into RCP and LCP incident light. Each kind of circularly polarized incident light can generate off-axis reflection light with four OAM states of ℓ = 1 to ℓ = 4. (f) and (g) are SEM image, numerical and experimental results for the fabricated metasurface with two kinds of superpositions of the OAM states. One is for states of ℓ = 1 and ℓ = −1, and the other is for states of ℓ = 3 and ℓ = −3. The white double-headed arrows refer to the angle of linearly-polarized incident light and the axis for the transmission of the polarizer. (a)-(c) are reprinted with permission from [132]. Copyright 2017, American Chemical Society. (d)-(g) are reprinted with permission from [133]. Copyright 2017, John Wiley and Sons.

Fig. 16 (a) SEM image of the metasurface with TiO₂ resonators embedded in PDMS, where blue indicates PDMS and green TiO₂. (b) SEM image of the Si handling wafer for the fabrication of the tunable metasurface after PDMS curing and stripping. Scale bar: 400 nm. Experimental demonstration of the tunable metasurface mounted on four linear translation stages with (c) photographs of an unstretched (left) and a stretched (right) PDMS film. Scale bar: 10 mm. (d) Schematic of the ON state of the LC cell electrically controlled via in-plane potentials. Upper panel: LC ordering has been switched to planar (green) except for a very thin layer at the bottom (blue) with residual twist due to strong surface anchoring. Bottom panel: a hybrid LC cell with nanostructured metasurface. LC switching from twisted to planar state is complete both in the bulk and in the plane of the metasurface. Black arrow indicates the direction of rubbing that sets LC alignment at the top cover. SEM images of (e) the fabricated zig-zag metasurface and (f) a small fragment of the metasurface taken at 52°to the structure’s normal. Dashed box indicates elementary unit cell of zig-zag pattern. (g) The upper-left panel: SEM image of the fabricated prototype of electronically-controlled beam-steering for operation at 100 GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Schematic diagram shows the control of resonance frequency and the phase shift of a transmitted electromagnetic wave through the applied current to the heating electrode of each metasurface unit-cell (red arrow). (h) Schematic illustration of the electrical switching of infrared light for a Fano-resonant metasurface integrated with graphene. (i) SEM image of a metasurface fabricated on top of graphene. Scale bar: 3 μm. (a) Reprinted with permission from [135]. Copyright 2016, American Chemical Society. (b) and (c) are reprinted with permission from [136]. Copyright 2016, American Chemical Society. (d)-(f) are reprinted with permission from [137]. Copyright 2015, John Wiley and Sons. (g) Reprinted with permission from [138]. Copyright 2016, Springer Nature. (h) and (i) are reprinted with permission from [139]. Copyright 2015, American Chemical Society.

Fig. 17 (a) Schematic of the silicon nanodisk metasurface integrated into an LC cell. The cell can be heated by a resistor mounted on the backside of the silicon handle wafer. (b) SEM image of the silicon nanodisk metasurface. (c) Experimentally measured transmittance spectra of the metasurface for linearly polarized light and a systematic variation of the temperature. The resonance positions of the electric resonance are plotted as red dots; the resonance positions of the magnetic resonance are marked as cyan squares. The phase transition is indicated by the white dashed line. (d) Reconfigurable metasurfaces optically written in the phase-change film imaged at λ = 633 nm. Left panel: Fresnel zone-plate pattern. Middle panel: binary super-oscillatory lens pattern. Right panel: the fabricated eight-level greyscale hologram designed to generate a V-shaped five-spot pattern. Inset: computer-generated greyscale hologram with 121 × 121 pixels. Scale bar: 10 μm. (e) Oblique incidence SEM image of the all-dielectric phase-change reconfigurable metasurface with a 750-nm period grating fabricated by FIB milling in a 300-nm-thick amorphous GST film on silica. (a)-(c) are reprinted with permission from [141]. Copyright 2015, American Chemical Society. (d) Reprinted with permission from [142]. Copyright 2015, Springer Nature. (e) Reprinted with permission from [143]. Copyright 2016, AIP Publishing LLC.