Fig. 1. Information hiding based on phase modulation within MIM nanostructures. (a) Hiding information by filling particular liquid^[30]; (b) information hiding based on water-absorbing materials^[34]; (c) nano-steganography with near-perfect information hiding and countersurveillance^[15]; (d) steganography combining visible and infrared bands^[39]

Fig. 2. Information security techniques based on the design of structural anisotropy. (a) Image information hiding based on cross-polarization detection^[45]; (b) image information hiding based on non-cross-polarization detection^[46]; (c) multichannel information encryption based multiple polarization combinations^[47]; (d) information hiding combining polarization-dependence and electrical modulation^[48]

Fig. 3. Diversity of working principles for surface structural color image information hiding based on optical nanostructures. (a) Information hiding based on angle-dependent optical response^[14]; (b) information hiding based on reversible chemical reaction^[13]; (c) information hiding based on up-conversion nanoparticles^[51]

Fig. 4. Matesurface holography based on modulation of single physical parameter. (a) Metasurface holography based on resonant phase^[16]; (b) metasurface holography based on Pancharatnam-Berry phase^[17]; (c) metasurface holography based on propagation phase^[18]; (d) metasurface holography based on nanopore diffraction^[19]

Fig. 5. Multidimensional metasurface holography. (a) Design of metasurface vector holography field^[26]; (b) metasurface vector holography with continuous polarization^[76]; (c) full-color metasurface holographic imaging^[81]; (d) design of circular polarization-multiplexed color metasurface holography^[25]; (e) metasurface vector holography with projection encoding full color image^[82]; (f) metasurface hologram with simultaneous modulation of amplitude, phase and polarization^[83]

Fig. 6. Encryption based on tunable metasurfaces. (a) Cascade metasurfaces to realize information hiding^[88]; (b) elastic substrate to realize information encryption^[90]; (c) changing chemical environment to control metasurface holography^[91]; (d) laser-induced graphene metasurface-based holographic encryption^[92]

Fig. 7. Information encryption combining hologram image and surface structural color image. (a) Metasurfaces using wavelength as decoding key^[98]; (b) wavelength-multiplexed metasurfaces^[99]; (c) metasurfaces using polarization as decoding key^[101]; (d) polarization-multiplexed metasurfaces^[105]; (e) polarization-multiplexed metasurfaces with nine information channels^[107]; (f) metasurfaces with integrated images of full-color and hologram^[24]; (g) metasurfaces integrating seven images^[110]; (h) dynamic and adjustable metasurfaces^[111]

Fig. 8. Encryption utilizing orbital angular momentum light. (a) Enabling dual-channel OAM information transmission and encryption based on OAM^[121]; (b) multi-channel information encryption realized by using OAM, polarization, wavelength, and spatial coordinates^[23]; (c) multi-channel information encryption based on incoherent white light^[122]

Fig. 9. Encryption system of orbital angular momentum light based on Dirac comb. (a) Schematic diagram of the OAM integrated encryption algorithm^[123]; (b) SEM image of GaN nano-pillars metasurface^[124]; (c) OAM multiplexing with multi-channel encryption in SHG^[126]; (d) schematic diagram of multi-channel information encryption using entangled photons^[128]