## 2 Single-Polarized 3D Absorber
### Operating Principle  
The proposed absorber evolves from the original lossy slot resonator in [16]. Instead of directly loading the resistor at the open end of the notch slot, a microstrip line on the other side of the substrate is utilized to couple the energy from the notch slot to the microstrip line. When the incident TE-polarized EM wave strikes the proposed absorber, the energy is first coupled into the notch-slot resonator, exciting the \(y\)-oriented electric field in the slot. The current along the \(y\)-oriented microstrip line across the slot is then generated and absorbed by the loaded resistor. It is defined that the TE- and TM-polarized waves are with the E-field along the directions of the \(y\)- and \(x\)-axes, respectively. It is worth mentioning that the coupled microstrip line is inspired by the typical wideband slot antennas [17; 18] fed by a microstrip line, and therefore it has a similar operating principle. Figure 1: (a) Perspective and (b) 2D view of the proposed single-polarized 3D absorber. The proposed single-polarized 3D absorber is simulated using a high frequency structure simulator (HFSS) 15.0. It is noted that the absorption band is defined with \(\mid\)\(S_{11}\)\(\mid\)\(\leq\)\(-\)10 dB or absorption \(\geq\) 90%. Figure 2a shows the \(\mid\)\(S_{11}\)\(\mid\) of the proposed 3D absorber, and the \(\mid\)\(S_{11}\)\(\mid\) of the original lossy notch-slot absorber with a resistor loaded at the open end (see the inset of Figure 2a) is also shown for comparison. It can be seen that the original lossy notch slot exhibits two absorption bands around \(f_{a1}\) = 3.05 GHz and \(f_{a3}\) = 9.05 GHz, and a reflection band around \(f_{a2}\) = 6.50 GHz, which correspond to the fundamental, third-order and second-order resonate modes. In other words, the electric length of the notch slot is 1/4 and 3/4 wavelength at \(f_{a1}\) and \(f_{a3}\), and 1/2 wavelength at \(f_{a2}\). Owing to the coupled microstrip line, the proposed absorber exhibits a wide absorption band from 2.65 GHz to 11.00 GHz (122.34% FBW) because the reflection band is eliminated, and the two absorption bands are merged. Figure 2b shows the simulated input impedance of the proposed and original lossy notch-slot absorbers. It can be seen that for the original lossy notch-slot absorber, there are two resonances around \(f_{a1}\) and \(f_{a3}\). However, the resistance around \(f_{a2}\) is about 0 \(\Omega\), leading to a reflection band (\(\mid\)\(S_{11}\)\(\mid\) = 0 dB, see Figure 2a). For the proposed absorber, the resistance around \(f_{a2}\) is lifted up to about 300 \(\Omega\), resulting in a flat resistance around 377 \(\Omega\) (the wave impedance of free-space) from \(f_{a1}\) to \(f_{a3}\). Moreover, the reactance is also improved, which is quite flat and close to 0 \(\Omega\). Hence, an ultrawide absorption band from \(f_{a1}\) to \(f_{a3}\) is obtained. Two key parameters that affect the resonate frequencies are shown in Figure 3. It is readily seen that the length of the notch slot (\(L_{s}\)) determines the absorption band. All the resonances decrease to lower frequencies as \(L_{s}\) increases. On the other hand, the length of the \(z\)-oriented open microstrip line (\(L_{m1}\)) mainly affects the two higher resonances (\(f_{b2}\) and \(f_{b3}\)). As \(L_{m1}\) decreases, \(f_{b2}\) and \(f_{b3}\) get closer to each other. To further illustrate the operating principle of the wideband absorption of the proposed absorber utilizing the coupled microstrip line, the current distributions on the proposed and original absorber at \(f_{a1}\),\(f_{a2}\) and \(f_{a3}\) are plotted in Figure 4. It can be seen that no current flows through the resistor at \(f_{a2}\), resulting in a reflection band. However, for the proposed absorber, the coupled current on the U-shaped microstrip line flows through the resistor at all of the three frequencies, resulting in a wide absorption band from \(f_{a1}\) to \(f_{a3}\).