Concepts, Working Principles, and Applications of Coding and Programmable Metamaterials

As a digital version of metamaterials, coding and programmable metamaterials have experienced rapid development since they were initially proposed in 2014. Unlike conventional metamaterials that are characterized by the sophisticated effective medium theory, coding metamaterials are described in a much simpler manner with binary codes, which builds up a bridge between the physical world and the digital world. In this article, the development of coding and programmable metamaterials in the past three years is reviewed, focusing primarily on the basic concept, working principle, design method, fabrication, and experimental validation. First, reflection-type and refraction-type coding metamaterials, along with two bifunctional coding metamaterials, are presented in the microwave, terahertz, and acoustic regimes. Second, the digital convolution theorem and information entropy of coding metamaterials are introduced to demonstrate the strong connection between metamaterials and information science. Then, recent progresses on engineering realization of field-programmable metamaterials are demonstrated, including the compensation technique of plane waves under point source illumination, and applications in single-sensor single-frequency imaging systems. Finally, future directions and potential applications are summarized, followed by discussions on major challenges encountered in the design and fabrication of programmable metamaterials at higher frequencies.