Fig. 1. Examples of absorbing or directional scattering cloaking methods: The unique appearance of U.S. Air Force F117A stealth fighter enables it to be effectively cloaked under the single-based station radar system

Fig. 2. Examples of absorption cloaking. (a) A mid-infrared absorption cloaking sheet^[32]. Most of the reflected wave will be absorbed without entering any detector. (b) Effect of infrared cloaking/illusion on thermal images^[33].

Fig. 3. “A perfect cloak”^[37]. The electromagnetic wave still propagates in the original direction after passing through the designed anisotropic cloaking sphere.

Fig. 4. The working principle and effect of “self-cloaked material”^[39]: (a) Effective parameters of the solid slab composed of closely arranged corrugated wires with different layer thickness; (b) steady-state electric field distribution under an oblique plane-wave incidence at 10 GHz upon such self-cloaked material.

Fig. 5. Schematic of the scattering cancellation cloaking method^[41].

Fig. 6. Photo of cloak based on transformation optics^[61].

Fig. 7. Invisibility cloak designed by homogeneous transformation optics^[64]: (a) The original virtual space of the transform; the red line represents a selected ray; (b) real space after transformation; the red line represents the transformed ray; (c) layered system for modeling the one-directional cloak.

Fig. 8. A TM wave visible light invisibility cloak composed of calcite under natural light designed by homogeneous transformation optics^[68].

Fig. 9. Schematics of a nature light cloak for large objects when the phase consistency is ignored^[71]: (a), (b) Dynamic monitoring of a fish swimming through the aquatic ray cloak; (c), (d) experimental observation of a cat in the terrestrial ray cloak.

Fig. 10. Carpet invisibility cloak: (a) The design of carpet invisibility cloak^[74], through the compression of virtual space, the blue region of right sub-figure is transformed to the blue region in the left sub-figure, thus concealing the green objects existing in the real space in figure (a); (b) experimentally realized carpet invisibility cloak and I-shaped unit structure by Liu et al^[75].

Fig. 11. (a) A schematic of carpet cloak^[84]; (b) schematic view of a visible spectrum invisibility cloak^[86]; (c) cloaking devices designed at terahertz and millimeter wave frequencies by applying ring resonators to metasurfaces^[88]; (d) a full-polarization carpet cloak by applying closed-loop resonators^[92].

Fig. 12. (a) Cloaking by designing a “complementary media” ^[97]; (b) remote cloaking based on multi-folded transformation optics^[100]

Fig. 13. Experimental realization of active cloaking: (a) Active invisibility cloak at microwave frequency^[110]; (b) active direct current field invisibility cloak^[111]; (c) active heat conduction field invisibility cloak^[112].

Fig. 14. Examples of imitation cloaking methods: (a) The protective color of a frog (Rana dalmatina) makes it difficult to be distinguished from the grasslands environment; (b) the scattering wave of soldiers in ghillie suit is almost the same as that of grasslands background, which is difficult to be spotted by enemy observers.

Fig. 15. Tunable invisibility cloak: (a) Temperature tunable one-dimensional transformation optics invisibility cloak^[137]; (b) carpet cloaking/illusion device based on tunable metasurface^[140]; (c) the time-domain digital-coding metasurface which is able to create the analogue of Doppler shift, or velocity illusion^[141]; (d) deep learning-enabled self-adaptive microwave cloak^[143].