In a classical metasurface-based holographic display system, a clear holographic reconstruction image can be obtained only in the back-focal-plane of the lens. However, when the receiving plane deviates from the focal plane, the reconstructed holographic image suffers from degradations that limit the quality of the images. Hence, we propose a novel metasurface-based holographic display to realize clearly continuous imaging within the specified position range by superimposing the meta-axilens phase. It can effectively relieve the alignment requirements of the imaging system. Firstly, the parameters and properties of alldielectric meta-atom is analyzed. Then we compare and demonstrate two sets of metasurface-based optical elements (metaaxicon, meta-lens, and meta-axilens), made by silicon meta-atoms working at 610 nm. Furthermore, we show that the metasurface hologram with the axilens phase can form a series of the relatively clear holographic images away from the focal plane. Our proposal suggests a method to reconstruct holographic wavefront in different planes simultaneously, and one can find their application domain, such as 3D biological imaging, spectroscopy, optical information storage, and encryption.
As artificially designed two-dimensional (2D) arrays of subwavelength nanostructures, metasurfaces provide a new avenue for the design of static planar optics. However, one would prefer a dynamic modulation of the metasurface in the event of practical applications. As a potentially important technique for information security, meta-holographic encryption has the characteristics of subwavelength pixels, precise control, and high safety factor. In this paper, by integrating birefringent liquid crystals (LCs) with all-dielectric metasurfaces in combination with visual cryptography (VC), we demonstrate a tunable metasurface with an information encryption function in the visible range. In the encryption process, the secret image is hidden in a set of unidentifiable and orthogonal arranged phase-only meta-holograms with high security of concealment. In the decryption process, the generated meta-hologram is illuminated by a plane wave and an electric field across the LC layer is applied to superimpose the reconstructed holographic patterns, thus we could acquire the secret image. Therefore, the decryption method can be flexibly and sufficiently adjusted with applied field under monitorable condition. For this reason, it is useful in enhancing the diversity of information hiding methods and improving the security of information. Our method can be extended to other similar applications.
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