Metamaterial-Based Microwave Absorbers: The Current State of the Art: Microwave Absorbers

Metamaterials [1] are artificially engineered structures that, unlike other conventional materials, are capable of manipulating the properties of an electromagnetic (EM) wave. There are three types of metamaterials: 1) single negative, where the material has either a negative permittivity value or a negative permeability value; 2) double negative, the most common type, in which both permeability and permittivity have negative values; and 3) zero index, where either ${\epsilon}$ or ${\mu}$ is zero, which leads to a zero-refractive index. In 1968, Veselago [2] studied these types of materials theoretically and discovered various fascinating properties, including the Doppler effect reversal, reversal of Snell's law, change in the reflection properties of a concave and convex lens, and reversal of the boundary conditions used to analyze an EM wave interaction with a medium. An important characteristic of a double-negative metamaterial is its ability to support wave transmission in the reverse direction, where the phase and group velocities exhibit opposing orientations, (i.e., the incident wave direction is opposite to that of the maximum power). By employing a repetitive arrangement of thin wires with periodicity $p$ and diameter $a$, as shown in Figure 1(a), Pendry et al. [3] have obtained negative permittivity.