Single-pixel imaging (SPI) enables an invisible target to be imaged onto a photosensitive surface without a lens, emerging as a promising way for indirect optical encryption. However, due to its linear and broadcast imaging principles, SPI encryption has been confined to a single-user framework for the long term. We propose a multi-image SPI encryption method and combine it with orthogonal frequency division multiplexing-assisted key management, to achieve a multiuser SPI encryption and authentication framework. Multiple images are first encrypted as a composite intensity sequence containing the plaintexts and authentication information, simultaneously generating different sets of keys for users. Then, the SPI keys for encryption and authentication are asymmetrically isolated into independent frequency carriers and encapsulated into a Malus metasurface, so as to establish an individually private and content-independent channel for each user. Users can receive different plaintexts privately and verify the authenticity, eliminating the broadcast transparency of SPI encryption. The improved linear security is also verified by simulating attacks. By the combination of direct key management and indirect image encryption, our work achieves the encryption and authentication functionality under a multiuser computational imaging framework, facilitating its application in optical communication, imaging, and security.
Optical metasurfaces are optical elements with artificial subwavelength nanostructures, and it can achieve functions that cannot be realized by traditional optical elements. Combining holography and metasurface boosts performance of light controlling and expanding the application scenario. Here, we will report our works on the methods of holographic multiplexing and realized plenty of channels for information reconstruction, including code division multiplexing method (CDM) for metasurface holography for optical information projection, and a multiplexing optimization method for independent light field manipulation in diffraction orders of metasurface holography. All these methods have bright potential to be applied in fields of optics.
Metasurfaces have been widely studied for arbitrary manipulation of the amplitude, phase and polarization of a field at the sub-wavelength scale. However, realizing a high efficiency metasurface with simultaneous and independent control of the amplitude and phase in visible remains a challenge. In this work, an ultrathin single-cell dielectric metasurface which can modulate complex amplitude in transmission mode is proposed. The amplitude is controlled by adjusting the dipoles and quadrupoles by tuning the geometric size, while the phase is manipulated based on the Pancharatnam-Berry phase (also called geometric phase) by rotating the meta-atom. It has been experimentally demonstrated that the quality of holographic image of complex-amplitude hologram encoded on the proposed metasurface is better than that of phase-only holograms (generated by angular spectrum method and Gerchberg–Saxton algorithm). The proposed metasurface expands the superior limits of various applications, including arbitrary beam shaping, 3D biological imaging, optical computing, and optics-on-chip devices.
Metasurfaces are known as a powerful tool for complex wavefront shaping. However, two-dimensional metasurface systems of nanoparticles exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly reciprocal optical transmission features. To influence this reciprocity, we present a metasurface design principle for nonreciprocal polarization encryption of holograms. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and allows full phase and amplitude control of the transmitted light. We experimentally show that our pixel-by-pixel encoded Fourier-hologram appears in a particular linear cross-polarization channel, while it is disappearing in the reverse propagation direction.
Optical holography became a powerful tool for arbitrarily manipulating the wavefronts of light. With the recent development of metasurface holography it became possible to tailor all the fundamental properties of light (amplitude, phase, polarization, wave vector and frequency) within a thin slab of material. However, for exploring the full capability of the information storage of metasurface holograms and enhance the encryption security, smart multiplexing techniques together with suitable metasurface designs are required.
Here, we demonstrate a novel method for achieving multichannel vectorial holography and show its potential for obtaining dynamic displays and high-security applications. We explore birefringent metasurfaces for the complete control of polarization channels with the freedom of designing both the polarization dependent phase shift and polarization rotation matrix. We show that although the target holographic phase profiles have quantified phase relations they can process very different information within different polarization manipulation channels. For our metasurface holograms, we demonstrate high fidelity, large efficiency, broadband operation, and a total of twelve polarization channels. Such multichannel polarization multiplexing can be used for dynamic vectorial holographic display and provide triple protection to the optical security devices. The concept is appealing for applications of arbitrary spin to angular momentum conversion and various phase modulation/beam shaping elements.
Metallic nanorod metamaterials, arrays of vertically aligned nanorods embedded in an alumina matrix (diameter ~80 nm, length 100-250 nm, period ~113 nm), have recently emerged as a flexible platform for applications in photonics, opto-electronics and sensing. The optical constants for these nanostructured materials are directly associated with their crystallinity. Controlling the crystallinity of these metamaterials in a fast manner has presented a new challenge. Here we show a laser annealing with a pulsed Nd:YAG laser (λ = 532 nm, FWHM 15 ns) to rapidly change the crystallinity of the metallic nanorods. The small column X-Ray diffraction characterization shows that not only the crystallinity of the metallic nanorods is changed, but also that evaporation of the metal occurs with laser annealing.
Metasurfaces provide great feasibilities for tailoring both propagation waves and surface plasmon polaritons (SPPs). Manipulation of SPPs with arbitrary complex field distribution is an important issue in integrated nanophotonics due to their capability of guiding waves with subwavelength footprint. Here, with metasurface composed of nano aperture arrays, a novel approach is proposed and experimentally demonstrated which can effectively manipulate complex amplitude of SPPs in the near-field regime. Positioning the azimuthal angles of nano aperture arrays and simultaneously tuning their geometric parameters, the phase and amplitude are controlled based on Pancharatnam-Berry phases and their individual transmission coefficients. For the verification of the proposed design, Airy plasmons and axisymmetric Airy beams are generated. The results of numerical simulations and near-field imaging are well consistent with each other. Besides, 2D dipole analysis is also applied for efficient simulations. This strategy of complex amplitude manipulation with metasurface can be used for potential applications in plasmonic beam shaping, integrated optoelectronic systems and surface wave holography.
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