Fig. 1. Fabrication techniques of optical metasurface. (a) EBL technique^[21]; (b) FIB technique^[21]; (c) FsLDW technique^[30]; (d) NIL technique^[33]; (e) RIE technique^[35]; (f) two-beam interference maskless lithography^[38]; (g) self-assembly nanosphere lithography^[40]

Fig. 2. Schematic of phase modulation mechanism of metasurface^[44]. (a) Resonant phase; (b) geometric phase; (c) propagation phase

Fig. 3. OLEDs with integrated metasurface for high-resolution full-color manipulation^[51]. (a) Schematic of OLEDs with integrated metasurface (meta-OLEDs); (b) RGB-pixelated meta-OLEDs with ultrahigh density

Fig. 4. Optical metasurface for full-color display. (a) Schematic of plasmonic metasurface for full-color display, and the right shows color palette photographed by a CCD camera^[55]; (b) spectral modulation based on all-dielectric metasurface (the left is schematic of Mie-resonance metasurface, and the right is microprints of butterfly and RGB roses based on all-dielectric metasurface, as well as their scanning electron microscope images)^[56]

Fig. 5. Tunable directional emission based on metasurface. (a) Directional quantum-dot emission based on gradient metasurface (left to right: device schematic illustration, far-field emission pattern, and angle-dependent spectra based on different polarization)^[15]; (b) GaP LEDs with integrated dielectric metasurface for directional emission (left to right: LEDs slab with Lambertian distribution of emission, resonant cavity LEDs with highly directional emission, and metasurface LEDs with beam deflection; upper layer shows the device schematic and lower layer shows far-field emission pattern)^[68]; (c) unidirectional emission based on InGaN/GaN quantum wells metasurface (left to right: schematic of InGaN/GaN quantum-well metasurface, phase and amplitude of the light emitted into the substrate change with the width of the nanopillar, and P-polarized PL spectra in momentum space)^[17]

Fig. 6. Applications of optical metasurface in beam shaping. (a) Light-emitting metalenses and meta-axicons based on metasurface (from up to bottom: schematic illustration of quantum-wells metasurface, collimating emission image of meta-axicons, and light focus of metalenses)^[69]; (b) metasurface-integrated VCSELs for programmable directional laser emissions (left side is the schematic of VCSELs integrated with metasurfaces, right side shows SEM images of metasurfaces)^[72]; (c) multifunctional on-chip VCSELs based on metasurfaces (left side shows the device schematic and concept art of 3D imaging, and right side shows the application of multichannel and wide-range luminescence)^[73]

Fig. 7. Optical metasurface for polarized light emission manipulation. (a) Linearly polarized OLEDs based on periodic corrugated metasurface (left side refers to schematic illustration of OLEDs, right side refers to cross-section SEM image of OLEDs and corresponding electric field distribution of optical modes; WG is the waveguide mode and SPP is the surface plasmon polariton mode)^[49]; (b) linearly polarized white OLEDs based on polarized light selection and conversion (left side is schematic illustration of device structures, and right side is SEM images of nanograting and holography metasurfaces)^[79]; (c) polarization-tunable perovskite light-emitting transistor (upper layer represents schematic illustration of transistor structure and optical microscope images of electroluminescence at different bias voltages, lower layer represents polarized electroluminescence spectra)^[80]; (d) chiral emission based on TiO₂ metasurfaces (top to bottom: schematic illustration of chiral metasurfaces, angle-dependent degree of polarization spectra, and circular polarized light emission spectra in the normal direction)^[81]

Fig. 8. Optical metasurface for improving the efficiency of light extraction. (a) Schematic of broadband light extraction in white light OLEDs by using 2D dual-periodic plasmonic micro-nano structure array^[85]; (b) randomly arranged nanocube morphology and plasmonic device diagram^[86]; (c) SEM image of disordered optical metasurface^[87]