Metamaterial has gradually cemented its place in the past 20 years as an area of exciting research and drawn broad attention from the physics and engineering communities, owing to its exotic electromagnetic (EM) behaviors. However, metamaterials have conventionally been described by effective medium parameters and governed by physical principles. They can be compared to, in a sense, analog circuits from the circuit perspective, which obviously have some drawbacks such as worse noise-resistant performance, lower signal precision, and sophisticated design.
Digital circuits, on the contrary, take only two levels of signal and are insensitive to background noise, thus could process and deliver signals without loss of the precision. To realize a digital version of metamaterials, Prof. Tie Jun Cui and co-workers at Southeast University, China, proposed concepts of coding and programmable metamaterials in 2014 (see Figure 1a), which could manipulate the EM waves through pre-designed digital coding sequences.
With digital coding metamaterials, engineers no longer need to care the complicated physics behind the complex array structures, but simply design the functionality with a series of binary numbers based on logic operations. The functionality of a given coding pattern holds valid for any type of coding particles, any operational frequency spectra, and even any different kinds of waves (e.g., EM wave and acoustic wave). This is similar to the digital circuit design, where researchers only focus on the code design using hardware description languages (VHDL, verilog) but do not need to worry about what type of semiconductor technology should be used to realize the constituent logic gate.
In addition to the exotic functionalities of coding metamaterials such as beam splitting, beam redirection, random diffusion, with or without polarization-dependent and frequency- dependent behaviors, the digital quantization of coding metamaterials allows us to study them from the information perspective. For example, Shannon entropy, a very famous theorem in the field of digital communication, was employed to estimate the average information carried by the coding metamaterial (see Figure 1c), which is helpful in new information systems (e.g. communication, radar, and imaging). More interestingly, the convolution operation was also performed on coding metamaterials to realize unusual physical phenomena of perfect beam steering and continuous beam scanning (see Figure 1d), which can be hardly accomplished with conventional reflect/transmit array antennas.
Another obvious benefit of coding metamaterials is that we could readily design an active coding particle by biasing a pin-diode at “ON” and “OFF” states, leading to the programmable metasurface that could make real-time control of EM waves. By independently controlling the digital state of every coding particle with a field programmable gate array (FPGA), the functionality can be switched in real time by simply changing the input coding sequences. With the programmable metamaterial, one could implement many new concept systems such as single-sensor and single-frequency microwave imager which scans an object without any movements, and a reprogrammable hologram that could generate dynamically changing microwave images on the imaging plane.
It is believed that the future coding and programmable metamaterials may have more combinations with many other digital signal processing algorithms to enable more freedoms in controlling the EM waves, and should advance along the road of intelligent metamaterials featuring self-sensing, self-learning, self-adaptive, and self-decision.
Please refer to review articles (Cui et al., Journal of Materials Chemistry C 5, 3644-3668, 2017; Liu et al., Advanced Optical Materials, DOI: 10.1002/adom.201700624, 2017) for details.
This study, Information Metamaterials and Metasurfaces was recently published in Journal of Materials Chemistry C; Concepts, Working Principles, and Applications of Coding and Programmable Metamaterials was recently published by Tie Jun Cui and Shuo Liu in Advanced Optical Materials.