Fig. 1. LSPR of metal nanoparticles^[48]

Fig. 2. Schematic plots of Born-Kuhn models ^[49]. (a) Right (D)- and left (L)-handed coupled-oscillator models; (b) modes excited by right- and left-handed circularly polarized lights for D-enantiomer with rods vertically spaced at an effective quarter-wavelength; (c) hybridization models for chiral plasmonic Born-Kuhn modes

Fig. 3. Schematic plots of Mie resonances. (a) Fundamentals of Mie resonances^[50]. Scattering cross-section of a silicon nanodisk with 200 nm radius and 260 nm height (inset) with contributions from electric-dipole and magnetic-dipole Mie resonances, with their resonance patterns schematically shown below; (b) electric and magnetic field distributions of first and second Mie resonances of a spherical dielectric resonator^[53]. Circular displacement current in x-z plane denotes a magnetic dipole moment along y direction

Fig. 4. Examples of 3D metallic chiral metasurfaces. (a) Schematic plots of helix-type 3D chiral metasurface^[54]; (b) experimental CD spectra and scanning-electron micrograph (SEM) images of chiral and achiral bilayer gold nano-disk structures^[55]; (c) SEM micrograph of corner-stacked gold nanorods and CD spectra, ORD spectra and coupled-oscillator model for right-handed enantiomer ^[49]; (d) top panel: schematic plots and SEM images of left- (LH) and right-handed (RH) corner-stacked gold nanorods structures. Bottom panel: measured (top row), modeled (middle row), and simulated (bottom row) linear and nonlinear CD spectra of LH enantiomer^[59]; (e) top panel: schematics of reflection behavior of meta-mirror (inset is SEM image of enantiomer A), reflection spectra of enantiomer A under LCP and RCP illumination and corresponding reflection spectra of co- and cross-polarized components. Bottom panel: sample images taken with a circular polarization analyzer when pattern is illuminated by both LCP and RCP lights^[60]; (f) top panel: SEM images of a twisted triple Fibonacci spiral, window decoration-type nanobarriers and a deformable spiral.^{Bottom panel: SEM images of LH and RH 3D pinwheel arrays (left panel), measured CD spectra for 2D LH, 3D LH and 3D RH pinwheels, respectively (middle panel), and measured (circular points) and calculated (solid curves) linear polarization rotation angle (θ) versus wavelength for 3D and 2D LH pinwheels, respectively (right panel)[54]}

Fig. 5. Examples of metallic chiral nanoparticles prepared by chemical methods. (a) Schematic of LH and RH helices and their experimental and theoretical CD spectra (inset: transmission electron microscopy images of helices) ^[69]; (b) schematic (left) and SEM images (right) of holicoid^[70]; (c) left panel: schematic and optical images of LH and RH Au chiral trimer. Middle panel: experimental (top) and simulated (bottom) differential scattering spectra of Au chiral trimer.^{Right panel: electric field distribution and differential electric field distribution of Au chiral trimer under RCP and LCP lights illumination at wavelength of 650 nm and 830 nm, respectively[71]}

Fig. 6. Schematic plots of three kinds of two-dimensional chiral metasurfaces. (a) Gammadion nanostructure; (b) “S”-shaped nanostructure; (c) “T”-shaped nanostructure

Fig. 7. Examples of 2D metallic chiral metasurfaces. (a) Schematics of a planar “L”-shaped plasmonic metasurface with a designed asymmetric transmission (top panel) and measured transmission spectra (bottom panel)^[76]; (b) SEM image of chiral Fano oligomer and measured dark-field scattering spectra and optical chirality spectrum^[80]

Fig. 8. Examples of chiral metasurfaces with strong near-field chirality. (a) Chiral near-field distribution (C value) of a planar gammadion nanostructure illuminated by LCP light^[73]; (b) SEM images of “G”-shaped chiral gold nanostructure array enantiomers and their SHG microscopy images^[81]; (c) schematic of a planar “S”-shaped chiral gold nanostructure and its CD spectrum and near-field CD image^[74]

Fig. 9. Examples of 3D dielectric chiral metasurfaces. (a) Top panel: schematic and SEM images of a chiral α-Si split-ring resonator, which is separated from an optically thick silver backplane by a 30-nm-thick SiO₂ spacer. Bottom panel: measured reflection spectra of co- and cross-polarization components of two enantiomers^[89]; (b) top panel: SEM images and schematic plots (insert maps) of fabricated C₂ and C₄ symmetric bilayer Si metasurfaces. Bottom panel: zeroth-order transmission, CD, and absorption spectra of C₂ and C₄ symmetric metasurfaces for circularly polarized illumination. The solid and dashed curves correspond to experimental and simulated results, respectively^[90]

Fig. 10. Examples of 2D dielectric chiral metasurfaces. (a) Left panel shows schematic of a Si disk nanostructure with a hole for Kerker-inspired optical chirality enhancement and right panel shows normalized differential transmission from numerical and analytical studies^[91]; (b) left panel shows schematic of silicon nanodisk metasurface illuminated by CPL and right panel shows distribution of chirality density enhancement -C/C_CPL on cutting plane through disk center (solid black box in left panel)^[46]; (c) left panel shows schematic of metasurface based on a TiO₂ nanocube dimer array and right panel shows distribution of chirality density enhancement C/C₀ on cutting plane through dimer gap center^[93]; (d) left panel shows schematic of biperiodic diamond disk metasurface, middle panel shows distribution of chirality density enhancement C/C_CPL and right panel shows distribution of local (C_local) and plane averaged (C_avg) optical chirality enhancement along z direction^[92]; (e) left panel shows schematic of HfO₂ nanodisk metasurface illuminated by LCP, with structural parameters P=150 nm, h=50 nm, and d=122 nm, middle panel shows transmittance spectrum of metasurface, and right panel shows distribution of chirality density enhancement C/C_CPL on cutting plane (x-y plane) through disk center; (f) left panel shows schematic of HfO₂ nanodisk metasurface with a hole, middle panel shows transmittance spectrum of metasurface, with structural parameters P=200 nm, d₁=120 nm, d₂=22 nm, and height of 50 nm, and right panel shows distribution of chirality density enhancement C/C_CPL on cutting plane through disk center

Fig. 11. Applications of metasurfaces in chiral sensing. (a) CD spectra of LH and RH gammadion arrays in water and in protein solutions^[99]. In lactoglobulin solution, CD peaks are red- and blue-shifted for RH and LH gammadions, respectively; (b) bilayer gold metasurfaces for sensing propanediol^[29]. Top left: SEM image and illustration of bilayer gold nanorods. Top right: CD spectra of LH and RH metasurfaces (±60°) and S-propanediol on metasurfaces. The error bars indicate standard deviation, and middle curve denotes background. ^{Bottom left: CD spectra of metasurfaces and R-propanediol on the metasurfaces.The error bars indicate standard deviation, and middle curve denotes background.Bottom right: CD summation to remove background CD of metasurfaces. The curves show clear opposite signs for R and S propanediol enantiomers; (c) Si nanodisks metasurfaces for sensing phenylalanine[100]. Left panel: a side-view SEM image of Si nanodisks covered with a dense 200 nm thick phenylalanine film. Right panel: CD signals differentiating molecular enantiomers of phenylalanine using Si nanodisks}

Fig. 12. Metasurfaces for separating chiral particles. (a) DDSR nanostructure^[13]. Top left panel: SEM images; top right panel: distribution of chirality density enhancement K/K_CPL; bottom left panel: distribution of chirality density gradient ∇K; bottom right panel: stability of enantiomeric pair; (b) DSRR nanostructure^[14]. Top left panel: schematic diagram, where chiral particles are placed 10 nm above plasmonic metasurface. Top right panel: distribution of chirality density enhancement K/K_CPL. Bottom panel: gradients of chiral intensity ∇K and electric energy density ∇W_E; (c) schematic diagram of sorting of chiral particles with different handedness by optical lateral force at an air–water interface (left panel). Variation of optical lateral force with particle size and incident angle under illumination of s-polarized (middle panel) and p-polarized (right panel) beams^[101]

Fig. 13. Applications of metasurfaces in chirality regulation. (a) Left panel: SEM images of spiral metasurface. Right panel: displacement of spiral structure with respect to applied pressure of N₂ gas. Sign of pressure is positive when pressure is applied from bottom chamber, resulting in left-handed spirals, and vice versa^[102]; (b) top panel: active chiral plasmonic dimer stack consisting of GST-326 with two ZnS/SiO₂ layers sandwiched between gold nanorods embedded in PC403. CD signal shifts when amorphous-to-crystalline phase transition occurs. The CD signal at original wavelength region switches its sign. Bottom panel: simulated (left panel) and measured (right panel) transmittance of active chiral dimer under left- and right-handed circularly polarized light and corresponding CD spectra for amorphous and crystalline state of the phase-change-material layer^[30]; (c) top panel: schematic of reversible tuning of CD of C₃ nano-kirigami metasurface by using a fiber tip. Bottom left panel: measured CD spectra of metasurface in air and in polymer, respectively. Bottom right panel: measured CD spectra of metasurfaces at different compression stages^[103]