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  1. Engineered noise can increase the channel capacity for polarization multiplexing with a metasurface hologram. 
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  2. Abstract

    On the basis of the Jones matrix, independent control over the amplitude and phase of light has been demonstrated by combining several meta‐atoms into the supercell of a metasurface. However, due to the intrinsic limitation of a planar achiral structure, the maximum number of independent, complex elements in one Jones matrix is three, giving rise to up to three‐channel amplitude and phase control. In this work, more Jones matrices corresponding to different angles of incidence are proposed to add, so that the degrees of freedom in the amplitude and phase control can be further increased. The supercell of the designed metasurfaces consists of three dielectric nanoblocks with predefined rotation angles and displacements in the 2D space, which can be inversely determined with the help of the genetic algorithm. Empowered by the ability to realize four‐ or even eight‐channel amplitude and phase control, the generation of multiple structured light, including two independent perfect Poincaré beams, two double‐ring perfect Poincaré beams, two perfect Poincaré beam arrays, and four vector vortex beam arrays, is numerically demonstrated. Such novel designs are expected to benefit the development of modern optical applications, including but not limited to optical communications, quantum information, and signal encryption.

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  3. Abstract

    In this work, mode conversion and wavefront shaping by integrating a metallic metasurface on top of a planar waveguide are proposed and demonstrated. The metasurface consists of C‐shaped nanoantennas. By controlling the orientation of each nanoantenna, mode conversion and focusing effect for the cross‐polarized electric fields inside the waveguide are achieved. The design and simulation results of 16 scenarios of wideband transverse‐magnetic to transverse‐electric mode converters with the mode purity up to 98%, and on‐chip lenses at the wavelength of 1550 nm are reported. It is worth noting that the dimension of the devices along the propagation direction is only 9.6 µm. This work manifests the potential application of mode division multiplexing systems and on‐chip optical interconnections based on metasurfaces.

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  4. This review focuses on the control over the degrees of freedom (DOF) in metasurfaces, which include the input DOF (the polarization, wavelength and incident angle of the input light and some dynamic controls), parameter DOF (the complex geometric design of metasurfaces) and output DOF (the phase, polarization and amplitude of the output light). This framework could clearly show us the development process of metasurfaces, from single-functional to multi-functional ones. Advantages of the multi-functional metasurfaces are discussed in the context of various applications, including 3D holography, broadband achromatic metalenses and multi-dimensional encoded information. By combining all the input and output DOF together, we can realize ideal optical meta-devices with deep subwavelength thickness and striking functions beyond the reach of traditional optical components. Moreover, new research directions may emerge when merging different DOF in metasurfaces with other important concepts, such as parity-time symmetry and topology, so that we can have the complete control of light waves in a prescribed manner. 
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  5. Abstract

    As 2D metamaterials, metasurfaces provide an unprecedented means to manipulate light with the ability to multiplex different functionalities in a single planar device. Currently, most pursuits of multifunctional metasurfaces resort to empirically accommodating more functionalities at the cost of increasing structural complexity, with little effort to investigate the intrinsic restrictions of given meta‐atoms and thus the ultimate limits in the design. In this work, it is proposed to embed machine‐learning models in both gradient‐based and nongradient optimization loops for the automatic implementation of multifunctional metasurfaces. Fundamentally different from the traditional two‐step approach that separates phase retrieval and meta‐atom structural design, the proposed end‐to‐end framework facilitates full exploitation of the prescribed design space and pushes the multifunctional design capacity to its physical limit. With a single‐layer structure that can be readily fabricated, metasurface focusing lenses and holograms are experimentally demonstrated in the near‐infrared region. They show up to eight controllable responses subjected to different combinations of working frequencies and linear polarization states, which are unachievable by the conventional physics‐guided approaches. These results manifest the superior capability of the data‐driven scheme for photonic design, and will accelerate the development of complex devices and systems for optical display, communication, and computing.

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