Abstract
In this chapter, the authors introduce the concept of MMA through reviewing the development of conventional absorbers. To illustrate the important operating mechanism, design process, and functionalization of the metamaterial absorber, they show the absorption relation of the absorber, impedance matching condition, equivalent circuit model, the evaluation of thickness, and oblique incidence condition in the first part. Then, they show the basic design method of the absorber, including parameter sweep method, equivalent circuit method, and deep learning method. Next, the planar integration strategy, vertical stacking strategy, and dispersive material strategy for broadband absorption bandwidth are discussed. In addition, they illustrate the tunable design of absorber, involving lumped element design method, thermal tunable design method, electrical tunable design method, mechanical tunable method, and reconfigurable tunable method. Finally, they talk about the basic engineering application of the metamaterial absorber in practical engineering application.
Top1. Introduction
The advance of modern electronic science and technology comes with a dramatic increase in the number of electronic devices, such as communication equipment, mobile phones and base stations. However, in order to achieve higher transmission efficiency, communication equipment needs to cover a wider range of electromagnetic wave spectra, which consequently results in a higher amount of electromagnetic radiation energy (Jeong, Nguyen, & Lim, 2018). Excessive electromagnetic radiation has negative effects in everyday life, industrial production and military defence. When the energy exceeds a certain range, the biological level interferes with the body’s DNA and brain waves, which may cause fetal malformation or miscarriage in pregnant women, and may have adverse effects such as headaches in normal individuals. In industrial production, strong electromagnetic radiation energy may interfere with electronic equipment and prevent it from functioning properly, thus affecting normal industrial production activities. Besides, some electronic functional devices that require precise measurements cannot be measured accurately in environments with high levels of electromagnetic radiation interference due to the presence of large interference (Shen, Zhai, & Zheng, 2014). For example, the complex electromagnetic radiation environment that prevails requires artificial “electromagnetic clear zones” by means of electromagnetic absorbing materials. The locations with “electromagnetic clear zones” are often found in remote and mountainous areas, which is inconvenient for experimental functional tests. On the defence and military front, military radars emit spatial electromagnetic waves at certain frequencies and then analyze the echoes by measuring them to construct a radar reflection target. Radar detection stealth by absorbing space electromagnetic waves emitted by radar is an effective method to keep military targets out of detection and surveillance (Z. Li, Li, Zhao, & Zhou, 2020). In summary, realizing electromagnetic energy absorption in the microwave regime is of great importance in everyday life, industrial production and defence and military areas. Therefore, we mainly discuss the MMA in the microwave regime in this chapter to the scope of practical engineering application for wireless systems.
In order to address absorption issues in engineering, traditional absorption materials are limited by the preparation conditions and parameter characteristics of the materials themselves, making it difficult to achieve perfect absorption. Therefore, achieving perfect impedance matching conditions usually requires exploring improvements in materials composition, making it difficult to achieve effective perfect absorption design (Jones & Wooding, 1964). Compared to traditional absorption materials, MMA can accurately control the equivalent dielectric constant and magnetic permeability of the material surface, achieve perfect impedance matching with free space, reduce surface reflectivity, and combine with loss media in the structure to achieve perfect absorption (Wen, Ren, Peng, & Zhao, 2021). In 2008, Landy (Landy, Sajuyigbe, Mock, Smith, & Padilla, 2008) first introduced the metamaterial method into the field of absorption to achieve perfect absorption in a microwave band using a super surface. Figure 1(a) shows the structure of this absorbing super surface. The top is the (b) classic aperture resonant ring structure, the middle is the (c) dielectric layer, and the bottom is the (d) metal wire. When the incident wave enters the super surface, the aperture resonant ring above dominates the magnetic resonance mode that affects the equivalent magnetic permeability, the middle dielectric layer serves as the loss structure, and the metal wire at the bottom dominates the electric resonance mode that affects the equivalent dielectric constant. By optimizing the structural parameters separately, impedance matching conditions that satisfy both the equivalent dielectric constant and the equivalent magnetic permeability can be obtained.
Figure 1. The basic element of the perfect MMA: (a) Perspective view, (b) metallic split ring, (c) dielectric layer, and (d) metallic bar
Key Terms in this Chapter
fr: The resonant frequency.
R(??), A(??), and T(??): Reflectivity, absorptivity, and transmissivity.
: The electric field of the incident, reflected, and transmitted waves.
S12(??): Transmission parameter.
OTMMA: Optically transparent broadband MMA.
S11(??): Reflection parameter that returns from the incident port.
??r and ??r: Relative permittivity and permeability.
DL: Deep learning.
and Ef: Reduced Planck’s constant and Fermi level.
MMA: Metamaterial absorber.
TE and TM: Transverse electric and transverse magnetic.
: The magnetic field of the incident, reflected, and transmitted waves.
LC: Inductance-capacitance.
neff and Zeff: Equivalent reflection coefficient and impedance.
D: The thickness of the absorbing layer.
EM: Electromagnetic.
RCS: Radar cross section.
k and k’: The magnitude of the wave vector and equivalent wave vector.
Z0, zr, Zin, and Zeff: The impedance in vacuum, relative impedance, input impedance, and effective impedance.
VAE: Automatic variational encoders.
PEC: Perfect electric conductor.
GANs: Generative adversarial networks.