Electromagnetic Manipulation Evolution from Stacked Meta-Atoms to Spatially Cascaded Metasurfaces

Metasurfaces, known as planar two-dimensional (2D) metamaterials, are proposed to overcome obstacles like high loss and bulky volume occurring with three-dimensional (3D)metamaterials. Single-layer structures face limited degrees of freedom, and cannot satisfy the growing functional demands for meta-devices. To simplify the design process and gain more controllability, quasi-2D structures are introduced into metasurfaces in the form of stacked meta-atoms design or spatially cascaded metasurfaces. These configurations greatly expand the manipulation capability of metasurfaces and spawn a variety of functions and applications. In this review, the progress of metasurfaces with multi-layer stacked meta-atoms and spatially cascaded metasurfaces is presented. Progress is presented from metasurfaces with multi-layer stacked meta-atom configurations to spatially cascaded metasurfaces, focusing on the development of versatile applications for these quasi-2D configurations. Special attentions are paid to the diffractive deep neural networks(D²NNs), and a category of recently developed cascaded metasurfaces introduces a brand-new method into metasurface inverse designing as well as paves paths to all-optical computing. Finally, the promising avenues for such metasurfaces are discussed.