Fig. 1. Typical working mechanisms of dynamically tunable metasurfaces

Fig. 2. Electrically tunable metasurfaces with different active materials. (a) Liquid crystal integration^[48]; (b) Electrochromic material integration^[57]; (c) Graphene material integration^[50]; (d) Integration of phase change material as the sandwich layer^[61]

Fig. 3. Thermally tunable metasurfaces with different active materials. (a) Schematic diagram of an electrothermally tunable silicon-based nanophotonic phased array^[63]; (b) Thermally tunable reflectance spectrum based on a heavily doped InSb substrate and InSb nanostructures^[66]; (c) VO₂-based electrothermally tunable metasurface: (i) schematic diagram and (ii) the shift of its resonant peak^[68]; (d) GST-based thermally tunable metasurface: (i) the reflection modulation caused by the phase change and (ii) schematic diagram of the device^[70]; (e) LC-based thermally tunable metasurface: (i) schematic diagram of the device and (ii) the transmittance modulation at different temperatures^[74]

Fig. 4. Optically tunable metasurfaces with different active materials. (a) Schematic diagram of the optically controlled THz device based on Si and Al SRR structure^[78]; (b) Schematic diagram of a III-V semiconductor device with reflection modulated by ultrafast laser pump^[79]; (c) Schematic diagram of a CdO:In device with optically controlled fast reflection modulation^[82]; (d) The transmittance spectrum modulation of a VO₂-based metasuface modulated by the THz wave, and its schematic diagram^[83]; (e) Schematic diagram of the erasable metasurface modulated by the femotosecond laser direct writing^[84]; (f) Schematic diagram of the polarization modulation of an optically controlled metasurface based on azo ethyl red^[87]

Fig. 5. Mechanically tunable metasurfaces with different active materials. (a) Tunable metasurfaces for achieving dynamic polarization control and holography: (i) structural configuration, (ii) simulated amplitude and (iii) radiation phase spectra at varied cantilever angles^[89]; (b) Birefringent reconfigurable metasurfaces in the visible range: (i) device structure and (ii) the modulation of delay and transmittance for TM and TE waves at 633 nm wavelength under different voltages^[90]; (c) Optical metasurface holograms based on a flexible substrate: (i) schematic configuration, optical holograms at (ii) unstretched and (iii) stretched states^[95]

Fig. 6. Chemically tunable metasurfaces with different active materials. (a) Reconfigurable metasurface holograms based on Mg nanobrick: (i) dynamic modulation principle and (ii) different holographic images after hydrogenation and dehydrogenation reaction^[110]; (b) Optical metasurface holograms based on liquid crystals that are used for volatile gas detection: (i) different images are produced when the left and right circularly polarized light is incident on the metasurface hologram and (ii) the schematic diagram showing the change LC molecular orientation upon contacting volatile gas^[115]

Fig. 7. Imaging applications of several typical dynamically tunable metalenses. (a) Electrically tunable molecular orientation of liquid crystals ^[38]; (b) Microfluidically controlled ratio between metal and dielectric of the material ^[117]; (c) Optically controlled deformation of thin films with the nanostructures ^[118]; (d) Temperature controlled phase change of GST ^[119]; (e) MEMS-controllable gap between two substrates with metasurface nanostructures ^[58]; (f) Mechanically controlled geometries of nanostructures on the PDMS substrate ^[106]

Fig. 8. Dynamically tunable displays. (a) Electrcially reprogrammable metasurface holograms^[128]. The metasurface in the middle is formed by an array of meta-atoms, with each having a diode welded between the two metallic loops and independently controlled by a DC voltage; (b) Liquid crystal tunable metasurface holograms and captured images in the far field at different applied voltages^[129]; (c) Schematic diagram of a hologram on a stretchable substrate^[95]. Holograms are switched and enlarged when the substrate is stretched; (d) Thermally tunable meta-holograms using a vanadium dioxide integrated metasurface^[135]; (e) Polymer-dispersed liquid crystal-based metasurfaces for optical encryption^[139]; (f) Principle of dynamic bifunctional metasurfaces^[140]

Fig. 9. Dynamically tunable beam shaping. (a) Schematic diagram of a tunable metasurface with adjustable gate electrodes^[141]. The structure consists of a quartz substrate, a gold back plane, a thin ITO film covered by a thin alumina film, and a gold stripe nanoantenna array on the top. Appling voltages between the stripe antenna and the bottom gold will result in charge accumulation at the transparent oxide near the aluminum oxide; (b) Illustration of the active metasurface array composed of electrically tunable channels, with each channel composed of 11 individually addressable plasmonic nanoresonators^[142]. The incident beam from the right side is reflected by the metasurface array, and the direction of the reflected beam is steered by adjusting the top and bottom gates, V_t and V_b, respectively; (c) 3D depth image produced using the metasurface SLM^[142]; (d) Working principle diagram of dynamic beam switching by liquid crystal tunable dielectric metasurfaces^[75]; (e) Schematic diagram of mirror-like light reflection of MEMS-metasurfaces under three driving conditions: pre-drive, anomalous reflection, and focusing^[59]; (f) Experimental results of beam deflection using the phase-change metasurface and its SEM image^[72]