Metasurface‐Assisted Optical Encryption Carrying Camouflaged Information

Abstract

to protect information from attacking and tampering. Among them, optical encryption, which scrambles and encodes the information of plaintext through optical transformations such as interference, diffraction, and imaging, opens a new path for information security. Owing to the abundant degrees of freedom (DOF) of light (e.g., amplitude, phase, polarization, and frequency), optical encryption technology has significant advantages of multi-dimensionality, large capacity, high design freedom, and high security.Metasurface is an artificially planarstructure material that has the superior capability of controlling DOF of light at subwavelength scale, [1][2][3][4][5][6][7][8] leading to a series of novel optical encryption technologies. [9][10][11][12][13][14][15][16][17][18][19][20] For nanoprinting, [21][22][23][24] these DOFs offer the possibility of encoding multiple meta-images into different optical dimensions, thus improving the security and capacity of optical encryption. By modulating geometric dimension of single-celled nanostructures in two orthogonal directions, two color meta-images can be encoded with different polarization light. [25][26][27] By designing super-celled nanostructures consisting of multiplexing pixel or coherent pixel, multiple meta-images can be encoded with different incident wavelengths, angles or/and polarization states. [28][29][30][31][32][33] In addition, some researchers have proposed multilayer nanostructure to encode multiple meta-images into different incident directions. [34][35][36] To decrease the complexity of fabrication processing while ensuring the information capacity and density, multi-channel Malus metasurfaces inspired by the orientation degeneracy recently have been proposed, which consist of single-sized nanostructures and can encode multiple meta-images into different polarization states only by arranging the orientations of nanobricks. [37,38] The multi-channel information can be decoded by simultaneously inserting a polarizer and an analyzer in the optical path, and the difference is only the different polarization directions, which demonstrates the encryption levels of the multiple channels are consistent. Once one is observed, the other is also cracked.In the field of information encryption, to ensure the security of real information, camouflage information is sometimes preset to confuse the public. In this case, it is necessary to store camouflage information and real information in different decryption levels with different decryption difficulties, which can ensure that even if the camouflage information is The superior capability of light manipulation makes optical metasurfaces capable of high-quality multichannel displays, demonstrating great potential in high-security optical encryption. Different from the previous multichannel nanoprinting metasurfaces based on the sophisticated nanostructure design, Malus metasurface can obtain multichannel meta-images only by arranging the orientations of single-sized nanostructures and thus have gained great attention....

Metasurface-based nanoprinting: principle, design and advances

“…Apart from polarization dependence, for Malus metas- urfaces, there exists a unique feature that an intensity value corresponds to multiple orientation angles on account of Malus law, so called orientation degeneracy (OD). In 2020, Zhang et al applied the concept of OD and presented nanoprinting-image display and encryption method by employing anisotropic silver (Ag) nanopolarizers considered as pixels 22 . For each pixel, there are two orientation angle options corresponding to an equal grayscale value.…”

Multiplexing Optical Images for Steganography by Single Metasurfaces

“…Metasurfaces, a class of artificial flat optical elements with feature cell size less than half wavelength, exhibit unprecedented abilities to manipulate the polarization, amplitude, and phase of incident light at the subwavelength level of the individual unit cell. [14][15][16] A variety of functionalities have been demonstrated such as flat metalenses, [17][18][19][20] holograms, [21][22][23] nanoprinting images, [24][25][26] and vortex generations. [27,28] In recent years, researchers have devoted more attention to multifunctional applications and integrated meta-devices.…”

Full Complex‐Amplitude Engineering by Orientation‐Assisted Bilayer Metasurfaces

“…[9][10][11][12][13] There are abundant achievements in the study of phase [14][15][16][17][18][19][20] and amplitude [21][22][23][24][25] modulation by metasurfaces. Compared with phase-or amplitude-only meta-elements, [26][27][28][29][30][31][32][33] complex-amplitude meta-elements have attracted extensive interest due to their ability to independently and flexibly manipulate the two important optical parameters, i.e., amplitude and phase. In the current state of knowledge, there are several methods to realize complex-amplitude modulation by metasurfaces: changing the angle between two arms of the V-shaped nanostructure with different lengths, [34] changing the size and orientation angle of a single nanobrick, [35][36][37][38][39] changing the opening size and orientation angle of C-shaped nanostructure, [40][41][42][43][44] changing the orientation angle between two arms of X-shaped nanostructure, [45] changing the length/width of arm and orientation angle of cross-shaped nanostructure, [46][47][48][49] and changing the orientation angles of multi-nanostructure metaatom, ...…”