# A Polarization-Dependent Frequency-Selective Metamaterial Absorber with Multiple Absorption Peaks
## 3 Simulated Results and Discussion  
The simulations were conducted in CST Microwave Studio 2014 (Darmstadt, Germany), using the finite-element frequency-domain method, and the unit cell boundary conditions were utilized in both \(x\) and \(y\) directions. Meanwhile, the Floquet port condition was employed in the \(z\)-direction. Since the proposed absorber has similar absorption characteristics for TE-polarized and TM-polarized  
Figure 1: (**a**) The proposed metamaterial absorber (MA) unit cell structure; (**b**) layout of the unit cell structure and its optimal values given in micrometers: \(a=820\), \(d=60\), \(r_{1}=112.5\), \(r_{2}=161\), and \(r_{3}=367.5\). normal incident waves, only the electromagnetic responses for the TE-polarized incident wave, whose electric field is parallel to the \(x\)-axis, were calculated for the normal incidence angle. Three distinct absorption peaks, which are at 92.4 GHz, 102.0 GHz, and 130.1 GHz with absorptivities of 88.1%, 98.2%, and 94.6%, respectively, are presented in Figure 2. In addition, three resonance peaks were labeled as mode\(f_{1}\), mode\(f_{2}\), and mode\(f_{3}\) starting from low to high frequencies. From Figure 2, the separation of the first two adjacent resonant peaks of the MA is not high enough, which may hinder its applications in the multiband detection. This can be improved by reconstructing the geometries of the proposed structure. In order to show the intrinsic mechanism of the proposed metamaterial absorber, the surface currents distributed on the metal pattern and the metal ground plane that correspond to three absorption peaks were calculated and the obtained results are presented in Figure 3.  
As shown in Figure 3a, for mode\(f_{1}\), the surface currents on the metal pattern diverge from inner circular patch to two arms of the closed ring, and then converge to the lower left corner of the ring, which means that mode\(f_{1}\) represents a typical dipole resonance. Meanwhile, the surface currents on the metal ground plane, shown in Figure 3b, are reversed, and circulating current loops are formed because the current on two metal layers are anti-parallel, so the magnetic response is excited. Similarly, for mode\(f_{2}\), the electric response is excited by surface currents on the metal pattern that flow along two arms of the closed ring, forming a dipole resonance (Figure 3c). However, compare with mode\(f_{1}\), the surface currents on the closed ring in mode\(f_{2}\) diverge from the lower right corner and converge to the upper left corner. According to results presented in Figure 3d, the magnetic response of mode\(f_{2}\) is also excited by circulating currents between two metal layers. In the case of mode\(f_{3}\), the surface currents are mainly concentrated on the connecting bar between a circular patch and a closed ring (Figure 3e), and mode\(f_{3}\) represents a dipole resonance that mainly depends on the connecting bar dimensions. In order to demonstrate the absorption mechanism, the power loss density in the dielectric layer and the ohmic losses at resonator surface for mode\(f_{1}\)\(-f_{3}\) were determined, and the obtained results are illustrated in Figures 4 and 5, respectively. Dielectric losses of the absorber are mainly concentrated in areas with a strong electric field, while the ohmic losses at resonator surface are determined by surface currents. In addition, it has been observed that the power losses in the dielectric layer are more than dissipated on the resonator surface, and the polyimide film has a very important role in the absorption of the incident waves. Figure 2: Absorption spectra of the proposed metamaterial absorber for the TE-polarized normal incident wave. Figure 4: Distribution of dielectric power losses on polyimide film at **(a)** 92.4 GHz; **(b)** 102.0 GHz, and **(c)** 130.1 GHz.