Electromagnetic metamaterials—from effective media to field programmable systems

Electromagnetic metamaterials are periodic or nonperiodic artificial structures comprising basic elements in subwavelength scales. They can be used to control electromagnetic waves by designing the basic elements and their spatial arrangements. Metamaterial has been a research hotspot in physical and information sciences. In this review, I present the history and main progresses achieved in metamaterial studies conducted in the State Key Laboratory of Millimeter Waves at Southeast University, which were performed in three stages. In the first stage, we improved the developments in effective-medium metamaterials. In particular, we proposed a general theory to accurately describe the effective permittivity and permeability of metamaterials. Based on the general theory, we designed and realized a three-dimensional (3D) wideband ground-plane invisibility cloak, free-space electrostatic invisibility cloak, electromagnetic black hole, electromagnetic illusions, and radially anisotropic zero-index metamaterial for omni-directional radiations and nearly perfect power combinations of source arrays. Moreover, we undertook significant efforts in engineering applications of microwave metamaterials, such as broadband and low-loss 3D transformation-optic lens for wide-angle scanning, 3D planar gradient-index lens for high-gain radiations, and random metasurfaces to reduce radar cross sections. In the second stage, we opened up a new direction of plasmonic metamaterials from the viewpoint of microwave transmission lines. We proposed an ultrathin, narrow, and flexible corrugated metallic strip to guide spoof surface plasmon polaritons (SPPs) with small bending and radiation losses, based on which we designed and realized a series of passive SPP devices (such as power divider, coupler, filter, and resonator), active SPP devices (such as amplifier and duplexer) and SPP antennas in the microwave bands. We demonstrated a significant feature of the ultrathin SPP transmission line in overcoming challenges of signal integrity existing in the traditional integrated devices and circuits. We built up a full-SPP wireless communication system with unique properties to remotely transmit two movies in two deep-subwavelength-distance channels exhibiting excellent performance. In the third stage, we established a new area of metamaterial-information metamaterial. We proposed to describe the metamaterial using digital coding from the information science viewpoint, which directly resulted in a new kind of metamaterial-digital coding metamaterial. We theoretically and experimentally illustrated that the digital coding metamaterials comprising digital units can be controlled using different coding sequences to achieve different functions. When the digital state of the coding unit was controlled using a field programmable gate array (FPGA), we realized a field programmable metamaterial that can manipulate electromagnetic waves in real time and generate numerous different functions using a single piece of hardware. The digital coding metamaterial provides a platform to establish a digital space on the physical space and enriches the methodology and theory of metamaterials. We proposed measuring the information capacity of metamaterials using Shannon entropy and presented the convolution and addition theorems on metamaterial platforms, which can be used to simultaneously control the physical properties and digital information. Based on information metamaterials, we established new-architecture microwave imaging systems, wireless communication systems, and other intelligent information systems.