Fig. 1. Principle of light modulation with metasurfaces: (a) Light transmission (green arrow) and reflection (red arrow) through a metasurface; (b) the generalized Snell’s law of reflection and refraction. Brown, red and green arrows represent the propagation directions of incident, reflected and refracted light, respectively. Black lines represent the projections of wavevectors of reflected and refracted light onto the surface perpendicular to the plane of incidence.

Fig. 2. Applications of metasurfaces based on modulation of light in amplitude. (a) Efficiency improvement of organic solar cell by exploiting plasmonic metasurfaces^[47]. (b) Liquid concentration sensor based on plasmonic metasurfaces^[51]. Inset: scanning electron microscope (SEM) and optical images of a gold hexagonal-lattice metasurface. (c) Ultra-thin colored textile with simultaneous solar and passive heating abilities^[52]. Left: measured absorptivity of different textiles from visible to near infrared wavelength; Right: optical and thermal images of a toy sheep partly wearing a colored textile. (d) Ultrafast pulsed fiber laser using lithographically-defined plasmonic metasurfaces as saturable absorbers^[53]. Left: experimental excitation power and polarization dependent nonlinear transmittance of a gold nanorod metasurface; Right: sketch of the home-built ultrafast fiber laser integrating lithographical plasmonic metasurfaces as saturable absorbers; laser diode (LD), wavelength-division multiplexing (WDM), erbium-doped fiber (EDF), optical isolator (ISO), polarization controller (PC), collimators (C1, C2) and objectives (O1, O2).

Fig. 3. Applications of metasurfaces based on modulation of light in phase. (a) Broadband achromatic metalens^[54]. Left: SEM image of the metalens (scale bar, 1 μm); Right: measured light intensity profiles for the achromatic metalens at various incident wavelengths. (b) Zoom metalens on a stretchable substrate^[16]. Left: sketch of a metasurface on a stretched PDMS membrane; Right: measured longitudinal beam profiles generated on the transmission side of the zoom metalens with stretch factor s = 100% (top), 115% (middle), and 130% (bottom). (c) Ultrathin quarter-wave meta-plate for circular-linear polarization conversion^[57]. (d) Dielectric-metasurface enabled optical vortex beam converter^[60].

Fig. 4. (a) Sketch of phase transitions between crystalline and amorphous phases of GST-225; (b) dielectric permittivity of crystalline and amorphous GST-255^[65,66].

Fig. 5. Measured refractive index of a 450-nm thick GST-225 film at different temperatures^[23]. As the heating temperature increases from 140 ℃ to 170 ℃, the degree of GST-225 crystallinity increases, and, accordingly, both the real (solid line) and imaginary parts (dashed lines) of the refractive index increase and approach the values of crystalline GST-225.

Fig. 6. (a) Skectch of phase transitions between insulator and metal phases of VO₂^[70]; (b) dielectric permittivity of VO₂ in different phases^[71,72].

Fig. 7. Optical responses of a GST-225 nanorod in the amorphous and crystalline phases. The rod has a radius of 1 μm and a height of 500 nm. (a) Extinction-cross-section spectra of the amorphous (blue area) and crystalline (red area) GST-225 rod illuminated by a plane wave. (b) Eigenfrequencies, , of three dominant resonance modes. Blue and red markers correspond to the GST-225 rod in the amorphous and crystalline phases, respectively. (c) Real part of E_z(z component of electric field) distributions of three modes of the amorphous GST-225 rod.

Fig. 8. Active modulation of light in amplitude by integrating phase-change material GST into plasmonic metasurfaces. (a) Tunable perfect absorber^[27]. Left: sketch of the device; Right: measured reflectance spectra of the metasurface for the GST-326 layer in the amorphous (solid line) and crystalline (dashed line) phases. (b) Plasmonic metasurface with tunable transmittance^[29]. Left: sketch of the device; Right: calculated transmittance spectra of the metasurface for the GST-225 layer in the amorphous (blue line) and crystalline (red line) phases. (c) Tunable chiral metasurfaces^[30]. Left: sketch and SEM image of the device; Right: measured transmittance and CD spectra of the metasurface for the GST-326 layer in the amorphous (lighter curves) and crystalline (darker curves) phases. (d) Optical switching of reflectance spectra using femtosecond laser pulses^[31]. Left: sketch and SEM image of the metasurface; Right: measured reflectance spectra of the metasurface for the GST-326 layer in different phases—as-deposited amorphous phase (AD), thermally crystallized (C) phase, optically reamorphized (MQ) phase, and optically recrystallized (RC) phase

Fig. 9. Active modulation of light in amplitude by integrating phase-change material VO₂ into plasmonic metasurfaces. (a) Plasmonic metasurface with tunable relfectance^[71]. Inset: and distributions at the resonant wavlengthwith VO₂ in the insulator phase. (b) Tunnable plasmonic-metasurface polarizer^[78]. Left: reflectance of the sample as a function of the polarizer angle (put in front of the metasurface) at 3 μm wavelength for VO₂ at 20 ℃ and 80 ℃; Right: sketch of the device. (c) Optical switching of transmittance spectra with UV laser pulses^[80]. Left: sketch of the device; Middle: hysteretic response in the transmttiance during the heating cycle of the phase transition of VO₂; Right: plasmon resonance wavelength of the Au nanodisks of 175-nm diameter on the VO₂ film as a function of the total UV illumination time, while the whole sample was thermally latched at two different temperatures within the phase transition region, 64 ℃ (blacksquares) and 68 ℃ (red circles), and also above (77 ℃, red empty circles).

Fig. 10. Tunable metasurface composed of GST nanodisk arrays^[28]. (a) SEM image of GST disks. The scale bar represents 2 μm. (b) Experimental and (c) simulated extinction spectra of disk arrays with a radius of 1 μm and a height of H = 220 nm. A1 and A2 denotes the 1st- and 2nd-order anapole states, respectively. (d)−(f) Simulated near-field distributions of the ED (electric dipole resonance), and A1 and A2 states.

Fig. 11. Tunable metalens and beam switcher based on phase-change materials. (a) Flat polaritonic lenses optically written in a GST-326 film below a hBN film^[36]. Left: optical images of the laser-written metalens; Right: s-SNOM image of the metalens showing focusing of polaritons at 1445 cm^–1. (b) Beam switcher composed of a plasmonic metasurface above a GST-226 layer^[34]. Left: sketch of the device; Top right: SEM image of the device; Bottom right: infrared camera images and intensity plots of the beam transmitted through the device for the GST-226 layer in the amorphous (left) and crystalline (right) phases.

Fig. 12. Tunable thermal emitters based on phase-change materials. (a) An ultrathin meta-insulator-metal plasmonic thermal emitter incorporating GST-225^[26]. Left: sketch and SEM image of the device; Right: experimental results of continuously tuning emissivities of the thermal emitter at different baking temperatures. (b) Tunable thermal emitter composed of a GST-225 film on top of a gold film^[23]. Top: sketch and SEM image of the thermal emitter; Bottom: visible and infrared photographs of the black soot (left), the amorphous-GST-Au emitter (middle) and the crystalline-GST-Au emitter (right) at 100 ℃. (c) Thermal camouflage and thermal image sharpening based on GT-225^[82]. Left: optical images of the checkerboard and ZJU patterns of GST-based devices; Right: thermal infrared images recorded after different annealing times. (d) Tunable thermal emitter optically written in a VO₂ film^[72]. Left: sketch of the spatially-resolved thermal-emission control platform; Right: writing (top panels) and erasing (bottom panels) of the bilevel thermal images (Zhejiang University logo).