Research progress of information metamaterials

<sec>Metamaterials are artificial structures composed of subwavelength unit cells in periodic or non-periodic arrays, which are regarded as one of the most important tops in today’s physics and information engineering. Traditional metamaterials are characterized by effective medium theory, in which the array of differently-shaped subwavelength particles can be described as an effective medium with effective permittivity and permeability. The metamaterials allow us to engineer the medium parameters with unusual values, such as negative permittivity and permeability, zero index of refraction, etc. In 2014, Cui et al. (Cui T J, Qi M Q, Wan X, Zhao J, Cheng Q 2014 <i>Light-Sci. Appl.</i> <b>3</b> e218) proposed the concept of information metamaterial, which is a digital version of the metamaterial with each unit cell described by digital codes representing different reflection/refraction phases. The direct connection between information metamaterials and digital logic devices allows the dynamic controlling of the electromagnetic (EM) waves by real-time programming the digital states of each unit cell in the information metamaterials with preloaded digital coding sequences. As information metamaterials build up a digital world (digital coding information) directly in the physical world, digital information can be processed on the information metamaterials directly without any intermediate conversion process, thus realizing the unification of microwave engineering and digital processing.</sec><sec>In this paper, we review the recent developments of digital coding metamaterials, programmable metamaterials, and information metamaterials, mainly focusing on their basic concepts, working mechanisms, experimental realizations, and system-level applications. Firstly, we introduce the concepts of digital coding and programmable metamaterials and present their advantages to realize the dynamic controlling of EM waves at low cost. The working mechanisms of isotropic, anisotropic, and tensor digital coding metamaterials are described, following the first prototype of the programmable metamaterial. Then we introduce the concept of information entropy for the information metamaterial and reveal the connection between the amount of information carried by the coding pattern and the radiated field of the information metamaterial. Convolution operation and addition theorem are further presented to show their powerful manipulations of EM wave in generating arbitrary beam patterns pointing to arbitrary directions. Finally, we introduce three representative system-level applications of information metamaterials, including a reprogrammable hologram imaging system which can dynamically project different microwave images at the imaging plane through the preloaded coding sequences stored in field programmable gate array (FPGA), a machine-learning reprogrammable metasurface imager that can be trained <i>in-situ</i> to produce high-quality images and high-accuracy object recognition in the real case at low cost, and directly digital wireless communication systems, in which the digital information is directly processed and radiated to free space by using the information metamaterial and FPGA. The information metamaterials are currently advancing towards higher frequencies (millimeter waves, terahertz, and infrared) to have higher capacity of information, and are becoming more “intelligent” with the combination of many advanced algorithms in computer science. We believe that the future information metamaterials possess signatures of self-sensing, self-learning, self-adaptive, and self-decision.</sec>