Janus monolayers, a class of two-faced 2D materials, have received significant attention in electronics, due to their unusual conduction properties stemming from their inherent out-of-plane asymmetry. Their photonic counterparts recently allowed for the control of hydrogenation/dehydrogenation processes, yielding drastically different responses for opposite light excitation spins. A passive Janus metasurface composed of cascaded subwavelength anisotropic impedance sheets is demonstrated. By introducing a rotational twist in their geometry, asymmetric transmission with the desired phase function is realized. Their broken out-of-plane symmetry realizes different functions for opposite propagation directions, enabling direction-dependent versatile functionalities. A series of passive Janus metasurfaces that enable functionalities including one-way anomalous refraction, one-way focusing, asymmetric focusing, and direction-controlled holograms are experimentally demonstrated.
Keywords: Janus metasurfaces; asymmetric transmission; chiral materials; holograms; lenses.
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