Metamaterials are artificial structures with the ability to efficiently control light‐field, attracting intensive attention in the past few decades. People have studied the working principles, design strategies, and fabrication methods of metamaterials, making this field cross and combine with many disciplines, including physics, material science, electronics, and chemistry. In recent years, with the rapid development of high‐efficiency and multifunctional metasurfaces, which are a two‐dimensional version of metamaterials, great efforts have been made to push this material to practical applications. In particular, the introduction of artificial intelligent (AI) algorithms enables metamaterials‐based photonic devices that exhibit excellent performances and intelligent functionalities. In this review, we first introduce the basic concepts, working principles, design methods, and applications of metamaterials, and then focus on the rapidly developing metamaterials research combined with AI algorithms. Finally, we conclude this review with personal perspectives on the current problems and future directions of metamaterials research and developments.

The refractive-lens technique has been well developed over a long period of evolution, offering powerful imaging functionalities, such as microscopes, telescopes, and spectroscopes. Nevertheless, the ever-growing requirements continue to urge further enhanced imaging capabilities and upgraded devices that are more compact for convenience. Metamaterial as a fascinating concept has inspired unprecedented new explorations in physics, material science, and optics, not only in fundamental researches but also novel applications. Along with the imaging topic, this paper reviews the progress of the flat lens as an important branch of metamaterials, covering the early superlens with super-diffraction capability and current hot topics of metalenses including a paralleled strategy of multilevel diffractive lenses. Numerous efforts and approaches have been dedicated to areas ranging from the new fascinating physics to feasible applications. This review provides a clear picture of the flat-lens evolution from the perspective of metamaterial design, elucidating the relation and comparison between a superlens and metalens, and addressing derivative designs. Finally, application scenarios that favor the ultrathin lens technique are emphasized with respect to possible revolutionary imaging devices, followed by conclusive remarks and prospects.

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