Metamaterials are artificially engineered electromagnetic (EM) materials achieved by structuring subwavelength metallic and/or dielectric composites (e.g., “meta-atoms”) with pre-designed EM responses[1–4]. The concept of artificial media was initially proposed to construct the so-called left-handed metamaterials with simultaneously negative permittivity and permeability for the purpose of achieving negative reflection[5–7] and super imaging[8–11]. Later, transformation optics was developed, and significant efforts have been devoted to the design of metamaterial-based devices for engineering EM spaces[12–16]. Metamaterials have shown unique prospects in the manipulation of EM waves that leads to a series of new types of metadevices[17–21]. However, many metamaterials realized so far have complex structure, large sizes, and relatively high losses, which hinder their practical use.
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