The metasurface is composed of a large number of sub-wavelength unit structures, which has received wide attention in the fields of holographic display and functional encryption due to its smaller structure size and more flexible modulation capability. And it has been extended to a more elaborate design for the increasingly demanded information-carrying capability. In this work, a dual-band four-channel reflective metasurface that operates simultaneously in the visible and near-infrared light is designed using heterogeneous meta-atoms and polarization-multiplexed holography. Hydrogenated amorphous silicon (a-Si: H) and Au are interleaved in the same plane, and the properties of the composite phase modulation determine the dimensional parameters of the two materials, which modulate the polarization distribution of the holograms using an optimized Gerchberg-Saxton algorithm based on quadratic phases. The designed dual-band multi-channel metasurface may provide a new avenue for device integration and enhancement of anti-counterfeiting technology.
Compared with monochromatic holographic displays, color holographic displays have rich depth and color information, which can give observers a more pleasant visual effect. Inspiring by the adaptive weighted GS method, we introduced an optimized iterative method based on exponential weighted constraint to calculate phase-only holograms of color images. After channel separation of the target image to obtain each component image, the input complex amplitude is formed with quadratic phase and iterated. During the iteration, subtract the amplitude of the reconstructed image from the amplitude of the target image to construct an exponential constraint. This exponential constraint is applied to the signal area of the image plane. After calculating the three-channel phase hologram, a color holographic reconstruction can be obtained using the time multiplexing method. Simulated and optical experiments verify the effectiveness of this method.
Holographic stereogram, as a marriage of holography and multi-view display technique, benefits of these two 3D display technologies. Recent research about the dual-view holographic stereogram 3D display technology shows an attractive way to present the visual information. However, due to the utilization of the random phase to smooth the spatial frequency spectrum, the troublesome of the quality of reconstructed images from holographic stereograms are those images containing speckle noise. In this paper, inspiring by the works about the computer-generated holograms, we introduced an optimized phase for the generation of dual-view holographic stereogram based on integral imaging to get a better suppression effect of speckle noise. The key goal is to determine the complex amplitude employed to calculate the holographic stereogram. Firstly, the pickup process of multiple parallax images corresponding to the left-view field and right-view field are finished by the virtual cameras. Then, the optimized phase is generated by the full-support iterative process, whose phase range is limited in the first time. Next, instead of the random phase, it is combined with corresponding parallax image to yield the complex amplitude. Holographic stereogram plane is segmented into many element holograms and each of them is generated by the Fourier transforming of the above complex amplitude. The phase of each complex element holograms is retained to generate the phase-only holographic stereogram, which have higher diffraction efficiency and no conjugate image than amplitude type holographic stereogram. Finally, the two set of holograms are combined together to reconstruct the corresponding dual-view. The optimized phase as the initial phase is demonstrated by simulations, the comparison of the simulated reconstructions shows that the method of this paper could improve the reproduction quality of the dual-view holographic stereogram, that extended its applicability and practicality.
A multiple-image encryption method based on a computer-generated phase-only hologram (POH) algorithm and chaotic systems is proposed. In the proposed method, first, a modified Gerchberg–Saxton (GS) algorithm is applied to transform the multiple-image into corresponding sub-sampled POHs. Then, the multiple POHs are combined using spatial division multiplexing (SDM). The combined hologram is then mapped into a digital image, and each pixel of the digital image is transformed by a chaotic system to improve security and form the final ciphertext. The modified GS algorithm is employed to generate a sub-sampled POH, which is a prerequisite for SDM. The adoption of SDM eliminates issues such as information leakage due to inter-image crosstalk and ensures the quality of decrypted images. The transformation based on a chaotic system leads to nonlinearity and unpredictability in the encryption process, which further increases the complexity of the encryption system. Numerical simulation demonstrates the security and feasibility of the proposed multiple-image encryption method.
Metalenses have drawn a lot of interest for use in near-infrared (NIR) bioimaging due to their benefits, such as flexible design, light weight, and simple integration. However, chromatic aberration is inevitable in optical systems, which substantially reduces their imaging quality. A broadband achromatic metalens for linearly polarized light in the range 1200 to 1550 nm is constructed to address this challenge. The phase and dispersion of the incident light are modulated by altering the ratio between the long and short axes of the elliptical nanopillars. Through the application of the particle swarm optimization algorithm, appropriate compensating phases can be added to the phase profiles at various wavelengths to align the real phase with the target phase of the broadband achromatic metalens. The structure of the nanopillar at any position in the metalens can be found from a relative phase library. Our numerical experimental results demonstrate that the proposed metalens has relatively consistent focusing characteristics and robustness in the 1200 to 1550 nm band, with an average focusing efficiency of 83.14%. The proposed high-efficiency robust NIR achromatic metalens can increase the quality of NIR biological imaging and the efficiency of a detector.
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