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.
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