I. Introduction
Metamaterials (MMs) with their outstanding properties have attracted great research interests in recent years. Their splendid capabilities beyond nature in term of the engineerable permittivity, permeability, and index of refraction advanced many versatile application areas such as filtering, sensors, electronics, antennas, solar cells, photodetectors, and absorbers [1]–[8]. With the benefits of metamaterials, scientists and engineers are now able to control the transmission, reflection, and absorption of the electromagnetic waves in different structures. Nonetheless, metamaterial-based absorbers are promising and widely used in broad applications, for example, stealth technologies in the military, electromagnetic interference shielding, specific absorption rate (SAR) attenuation, and energy harvesting applications [9]–[13].