Fig. 1. Schematic of the topics in this review

Fig. 2. Process of the design, fabrication, and characterization for meta-devices

Fig. 3. Design principles of broadband achromatic meta-lens^[30]. (a) Schematic of achromatic meta-lens; (b) shifted phase distribution for broadband achromatic meta-lens at different wavelengths; (c) schematic of the Pancharatnam-Berry phase, the rotation angle θ of meta-atom results in a 2θ phase change; (d) IRUs with various phase compensations over continuous and broad bandwidth; (e) optical image of the broadband achromatic meta-lens in the near-infrared region; (f) corresponding zoom-in SEM image

Fig. 4. Characteristics of the achromatic meta-lens^[30]. (a) Schematic of experimental setup for focusing measurement; (b) measured (top) and simulated (bottom) light intensity profiles at different wavelengths when NA=0.268; (c) measured focal length at different wavelengths; (d)(e) measured FWHM and operation efficiency at focal plane

Fig. 5. Process and SEM images of electron-beam lithography. (a) Schematic of fabrication process for bottom-up method^［30］; (b) SEM image of the fabricated achromatic meta-lens^[30]; (c) schematic of fabrication process for top-down method^[48]; (d) SEM image of the fabricated GaN meta-lens^[48]

Fig. 6. Metasurfaces fabricated by direct-write lithography. (a) SEM image of the FIB-fabricated metasurface^[59]; (b) schematic of the fabricated metasurface using LDW in print circuit board (PCB) technology^[61]; (c) schematic of a metasurface fabricated by orthogonal LIL^[49]; (d) SEM image of the anisotropic metasurface produced by LIL^[62]

Fig. 7. Etched structure and SEM images of plasmonic lithography. (a) Schematic of the imaging reflective plasmonic lithography structure with a silver lens^[64]; (b) SEM image of the corresponding Cr mask for reflective plasmonic lithography^[64]; (c) schematic of plasmonic cavity lithography system consisting of a Cr mask and a Ag/PR/Ag plasmonic cavity^[65]; (d) SEM image of the corresponding Cr mask patterns^[65]

Fig. 8. Metasurfaces fabricated by nano-imprint lithography. (a) SEM images of metasurfaces with nanostripe (left) and nanohole (right) structures fabricated by thermal-NIL^[66]; (b) corresponding schematic of imprinting molds for nanostripes (upper) and nanoholes (bottom)^[66]; (c)(d) SEM images of thermal-NIL-fabricated metasurfaces^{[51, 67]}; (e) SEM images of UV-NIL-defined metasurface^[68]; (f) photograph of packaged metasurface emitter constructed by UV-NIL^[69]

Fig. 9. Self-assembly lithography. (a) SEM image of self-assembled PS shperes for NL; (b) SEM images of Si-cylinder metasurface formed with NL and RIE; (c) schematic of the main fabricating procedures for the flexible, all-dielectric metasurface^[70]; (d) schematic of the multi-angled deposition for metasurface realization; (e) schematic and SEM images of the features under different projection angle^[72]; (f) schematic of the fabrication of Moiré metasurfaces by the NL technique; (g) SEM images of two representative Moiré metasurfaces with in-plane rotation angle at θ=12° and θ=19°^[73]

Fig. 10. Process and SEM images of hybrid patterning lithography & photolithography. (a) Schematic of fabrication process flow for micro-sphere projection lithography^［74］; (b) SEM image of a non-periodic metasurface by projection lithography (left) and the corresponding zoom-in image (right),and the scale bars are 20 μm and 5 μm, respectively^[74]; (c) schematic of the fabrication process for tilted nano-pillars using hole-mask colloidal lithography and off-normal deposition^[52]; (d) SEM images for the fabricated samples with various tilting angles,and the scale bar is 500 nm^[52]; (e) fabrication procedures for meta-lens using DUV projection lithography^［75］; (f) photo and SEM image of meta-lens on glass wafer^[75]

Fig. 11. Metasurface-based polarization generation and imaging. (a) Schematic of metasurface for versatile polarization generation and separation (left) and the simulated conversion efficiency for each polarization (right)^[13]; (b) diagram of a background-free metasurface-based QWP (left) and full wave simulations of degree of circular polarization and the extraordinary beam intensity (right)^[76]; (c) left panel: SEM images of the top and side views for multispectral chiral meta-lens; right panel: captured images from color camera of beetle (top) and one-dollar coin (bottom)^[82]; (d) left panel: schematic of corresponding polarization state on metapixels for the polarization camera; right panel: 3D metapixel splitting and focusing of different polarization states to different positions^[87]

Fig. 12. Meta-lens light-field imaging and sensing. (a) Left panel: light-field imaging and rendered images (top), rendered images of rocket at different depths (middle), and estimated depth map (bottom); right panel: rendered image formed by achromatic meta-lens array with incident white light^[23]; (b) schematic of the depth-sensing system composed of meta-lens array^[39]; (c) integral imaging given by achromatic meta-lens array at incident white light; (d) reconstructed images with different depth planes^[93]; (e) schematic of light-field imaging system with multi-dimensional edge detection^[94]; (f) meta-lens for aerial photography and landing assist system (left), experimental (upper-right) and simulated (bottom-right) imaging properties at different height^[40]

Fig. 13. Application of meta-lens in the field of biomedical imaging. (a) Schematic of light-sheet fluorescence microscopy and LSFM image for C. elegans (upper right)^[96]; (b) schematic of generation of AAF beam and the required phase distribution on metasurface (bottom left); (c) bright-field (upper left) and wide-field (upper right) images of the fluorescence-tagged mouse cardiac slice, the corresponding fluorescence intensity profiles for the red box region (bottom)^[97]; (d) schematic of Moiré meta-lens; (e) upper panel: fluorescent image of villi under uniform illumination and HiLo processed image (right) of villi; bottom panel: experimental and simulated focal length and measured efficiency^[98]