## 2 Absorber Design
### Current Distribution and Surface Power Loss  
The incoming EM wave decays with the loss of the ITO film after it transmits through the absorber surface. For this reason, the reflectionless transmission characteristic is indispensable, and it can only be obtained from perfect impedance matching between the air and absorbing surface, provided that the absorber presents in free space. The surface impedance can be determined by the proper combination of the electrical and magnetic  
Figure 1: Proposed unit cell design for the MTM absorber. (**a**) Top view, and (**b**) Side view. Design parameters: \(w=15\), \(r_{1}=1.5\), \(r_{2}=3.5\), \(r_{3}=5.5\), \(r_{4}=7.5\), \(g=1\), \(g_{d}=1\), \(t=0.175\), and \(t_{d}=3\). All units are in millimeters. Figure 2: Simulated results of the proposed absorber. resonances, as they are strongly related to effective permittivity (\(\varepsilon_{eff}\)) and permeability (\(\mu_{eff}\)), such as \(Z(w)=\sqrt{\frac{\mu_{0}\mu_{eff}}{\varepsilon_{eff}}}\). In Figure 3, the surface current distributions at 8.35 GHz, 18.65 GHz, and 21.65 GHz--where an absorbance near 100% is observed--are plotted. At all frequencies, it is commonly seen that the current density on the top layer is relatively higher than that on the bottom layer. It is also found that the current from the top and bottom layers flows in the opposite direction at 8.35 GHz and 18.65 GHz, whereas it flows in the about same direction at 21.65 GHz. The current distribution on the top layer itself causes electrical resonance, and the current distributions at the top and bottom layers together create a current loop that results in a magnetic resonance [23]. Thus, it can be said that the surface impedance is adjusted by both the electrical and magnetic resonances at 8.35 GHz and 18.65 GHz, whereas it is predominantly controlled by an electrical resonance at 21.65 GHz. This explains why a stronger current density is formed at the top at 21.65 GHz when compared with the other two frequencies. Next, the surface power loss density both at the top and bottom layer is simulated and plotted at the same frequencies as those shown in Figure 4. It is clearly observed that most losses occur in the top layer of the structure. From Figures 3 and 4, it is seen that the top  
Figure 3: Surface current distributions at the top and bottom layers at (**a**) 8.35 GHz, (**b**) 18.65 GHz, and (**c**) 21.65 GHz. layer of the absorber plays a core role in impedance matching, as well as the decaying of the wave at the absorber. As such, the parts in the SRRs where the current distribution for the resonance occurs and higher power loss density is found generally agree with each other. Since the current distribution differs on the SRR by frequency, the operating frequency could be simply tuned by its dimension. We also study the current distribution and the surface power loss at 2 GHz, where a very low absorbance of around 10% is observed. First, it is observed from the low surface current density plot at the top layer that neither electrical resonance nor magnetic resonance occur at this frequency, as shown by Figure 5a. This means impedance matching cannot be completed at all at this frequency. This naturally leads to a very low surface power ohmic loss at the surface, as shown in Figure 5b. This study supports the idea that MTM-EM absorbers can be categorized as a band-pass frequency selective surface filter. In other words, the frequency range matched to the free space impedance of 377 \(\Omega\) allows the incoming wave to pass through the absorbing surface, while the waves at all other frequencies are rejected. The proposed absorber operates as a function of the reflection coefficient with nearly zero transmittance, because the bottom layer acts like a ground  
Figure 4: Surface power loss at the top and bottom layers at (**a**) 8.35 GHz, (**b**) 18.65 GHz, and (**c**) 21.65 GHz. plane. For this reason, a frequency with a low absorbance means that most of the incoming wave to the absorber is highly reflected.