Metasurfaces have become a rapidly growing field of research in recent years due to their exceptional abilities in light manipulation and versatility in ultrathin optical applications. They also significantly benefit from their simplified fabrication process compared to metamaterials and are promising for integration with on-chip nanophotonic devices owing to their planar profiles. The recent progress in metasurfaces is reviewed and they are classified into six categories according to their underlying physics for realizing full 2π phase manipulation. Starting from multi-resonance and gap-plasmon metasurfaces that rely on the geometric effect of plasmonic nanoantennas, Pancharatnam–Berry-phase metasurfaces, on the other hand, use identical nanoantennas with varying rotation angles. The recent development of Huygens' metasurfaces and all-dielectric metasurfaces especially benefit from highly efficient transmission applications. An overview of state-of-the-art fabrication technologies is introduced, ranging from the commonly used processes such as electron beam and focused-ion-beam lithography to some emerging techniques, such as self-assembly and nanoimprint lithography. A variety of functional materials incorporated to reconfigurable or tunable metasurfaces is also presented. Finally, a few of the current intriguing metasurface-based applications are discussed, and opinions on future prospects are provided.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/smtd.201600064

Fundamentals and Applications of Metasurfaces

Information security with optical means, such as double random phase encoding, has been investigated by various researchers. It has been demonstrated that optical technology possesses several unique characteristics for securing information compared with its electronic counterpart, such as many degrees of freedom. In this paper, we present a review of optical technologies for information security. Optical security systems are reviewed, and theoretical principles and implementation examples are presented to illustrate each optical security system. In addition, advantages and potential weaknesses of each optical security system are analyzed and discussed. It is expected that this review not only will provide a clear picture about current developments in optical security systems but also may shed some light on future developments.

Advances in optical security systems

Metasurfaces are planar metamaterials exhibiting certain inhomogeneous phase distributions for transmitted or reflected waves, which can efficiently reshape the wave‐fronts of incident beams in desired manners based on the Huygens principle. Due to their exotic abilities to freely manipulate electromagnetic (EM) waves on a flat and ultrathin platform, metasurfaces have attracted intensive attention recently, resulting in numerous new concepts and effects that could possibly find applications in many different aspects. In this article, the key achievements in this field are briefly summarized, with focus put on the efficiency issue of metasurfaces. After introducing the basic concept of metasurfaces and their early realizations, successively the mechanisms and realizations of high‐efficiency metasurfaces are described: for manipulating linearly polarized EM waves in reflection and transmission geometries, and for manipulating circularly polarized EM waves based on the geometric‐phase concept, using plasmonic materials as well as dielectric ones. Then, several important applications of high‐efficiency metasurfaces are summarized, including polarization control, metaholograms, metalenses, and surface wave couplers. Finally, this review is concluded with personal perspectives on the future directions of this rapidly growing research field.

High-Efficiency Metasurfaces: Principles, Realizations, and Applications

Color image encryption by using Arnold transform and color-blend operation in discrete cosine transform domains

Holography has numerous applications because of its capability of arbitrary wavefront modulation. Computer-generated holograms (CGHs) take it a big step forward. Conventional holography engineers the wavefront via phase accumulation, suffering from large size, low resolution, and small viewing angle. Metasurfaces, ultrathin two-dimensional metamaterials with subwavelength features, can manipulate the amplitude, phase, and polarization of the light, solving the above issues. In this review, advances of holography, CGH algorithms, and the principles of various metasurfaces are presented. Metasurface holography, realized by encoding the hologram in the metasurface, is investigated. Information multiplexing methods of metasurface holograms, including wavelength-multiplexed, polarization-multiplexed, complex amplitude modulated, nonlinear, and dynamic metasurfaces, are presented. The challenges and outlook of metasurface holograms are discussed.

When metasurface meets hologram: principle and advances

We proposed a novel architecture for optical image encryption based on interference. The encryption algorithm for this new method is quite simple and does not need iterative encoding. The parameters of the configuration can also serve as additional keys for encryption. Numerical simulation results demonstrate the flexibility of this new proposed method.

Optical image encryption based on interference.

A wavelength de-multiplexing metasurface hologram composed of subwavelength metallic antennas is designed and demonstrated experimentally in the terahertz (THz) regime. Different character patterns are generated at the separated working frequencies 0.50 THz and 0.63 THz which determine a narrow frequency bandwidth of 130 GHz. The two working frequencies are around the central resonance frequency of the antennas where antennas behave strong wavefront modulation. Each antenna is fully utilized to control the wavefront of the metasurface at different frequencies by an optimization algorithm. The results demonstrate a candidate way to design multi-colors optical display elements.

/pdf/wavelength-de-multiplexing-metasurface-hologram-2l0q2tewns.pdf

Wavelength de-multiplexing metasurface hologram.

Dispersion characteristic of ultrathin terahertz planar lenses based on metasurface

Recently, a new image hiding method based on optical interference was proposed. The image is hidden in two pure phase masks. However, the silhouette of the encrypted image can be obtained by using just one of these two masks, which reduces the secrecy of the algorithm. A method for secrecy enhancement based on exchanging the same parts of two masks is proposed. This method can effectively remove the silhouette of the encrypted image constructed using just one mask. Simulations are carried out to demonstrate the validity of this method.

https://iopscience.iop.org/article/10.1088/1464-4258/11/12/125406/pdf

Enhancement of image hiding by exchanging two phase masks

Double images hiding based on optical interference

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