Metamaterial Based Broadband Absorber Design

Abstract

Metamaterial absorbers (MA) have exhibited great interest from researchers due to the near-perfect absorptive efficiency, as well as wide-angle incidence properties and an almost complete absence of polarization sensitivity [1], [2]. These materials aim to absorb the incident wave to generate a greater intensity of the electric field in the receiver. It implies that all the incident radiation is absorbed in a specific frequency range. These materials can produce resonances for the electrical and magnetic polarizations and autonomously adjust the effective parameters of the medium [1].Although great efforts have been concentrated in the search for metamaterials with good performance in the THz range, research is still necessary to develop and improve MAs [3], [4]. The design presented in this work combines the materials used in [5] with the design suggested in [6], to produce a simpler design without noble metals. To choose the dimensions and thickness of each layer of the MA, several simulations were carried out to determine the parameters that would result in the maximum absorption of the incident waves for the frequency range under analysis. The MA is presented in Fig.1-a and Fig.1-b, and it is composed of Nickel (Ni) disks resonators arranged in a 4x4 array and a nickel ground plane separated by silicon dioxide (SiO2) substrate. The choice of materials, such as nickel rather than gold (Au), for the construction of the MA structure, aims to obtain cost-effective and easily producible metamaterials. The size and thicknesses of metals and the dielectric layers are exhibited in the top view at Fig.1-a and the side view at Fig.1-b.Boundary conditions are set as electric on the x-axis, magnetic on the y-axis, and open on the z-axis. The propagation has the direction along the positive z-axis from the open add space wave port with a separation distance to prevent near field effect between EM wave and the MA, as demonstrated in [5]. The study selected visible frequency range, but also investigated infrared and ultraviolet frequency. The dielectric parameters of nickel and silicon dioxide are complex and dependent on frequency. Nickel was chosen because it presents better absorption efficiency than other metals in the visible region and enables high-temperature applications [5]. The results are obtained using the CST Microwave Studio software.The absorption and dispersion of the incident electromagnetic waves are related to the reflected and transmitted radiation at the materials media interfaces through the Scattering Parameters [2], [7]. These parameters can be determined by the absorption capacity (A) through the relation: A = 1 - R - T. To reach the maximum absorption level, the reflection coefficient (R), as well as the transmission coefficient (T) should be minimized by using a continuous metallic plane [5]. Then, the MA response will depend only on the reflection values.Sweep simulations were performed varying the unit cell's dimensions, and the best results were presented in Fig.2-a. The proposed design has a high absorption capacity, which is above 96.14% at all frequencies in the visible light region, and the average absorption in all spectrum analyzed is 99.24%. It means that the presented MA can absorb almost all the incident radiation. Hence, the total incident energy can penetrate the structure and be converted into electrical energy. Therefore, the MA can be used in the entire frequency range of the visible light spectrum with better performance compared to the other metals proposed in the literature, such as copper, gold, and aluminium [4], [5], [6].Furthermore, the MA's absorption behaviour is also examined at infrared and UV frequency regions and are shown in Fig.2-b. For the IR-region, the average absorption percentage is 99.79% between 800 and 1000 THz (300-375 nm). That is a good performance for an infrared detector operating in the mentioned frequency band. For the UV region, the average absorption percentage is 90.11% from 100 up to 350 THz (850-3000 nm). Additionally, the proposed MA has 99.22% of average absorption in all analyzed spectrum. The structure has an absorption value greater than 90% in the entire range of the analyzed spectrum. The reason is that the imaginary part of nickel's dielectric constant is higher than other metals in this frequency range, as shown in [5].It should be noted that the proposed MA has an extremely wide-bandwidth absorption in a frequency range close to those reported in the current literature. Furthermore, the presented design has the thinnest thickness compared with the other structures presented in [5], [6], with a simpler design and without noble metals. The MA shown can be considered compatible with several applications, including solar energy harvesting, IR detector, cloaking, shielding for EMC, and sensing. A detailed analysis of the results will be discussed in the full paper.Acknowledgment(s): This work was supported by the Brazilian institutions CAPES, CNPq, and FAPEMIG. **