# A Wideband Terahertz Transmissive Polarization Manipulator Based on Metasurfaces
## 3 Results and Discussion
### Physical Mechanism for LCPC Operation  
The proposed structure is an anisotropic structure which has dual diagonal symmetry. In such a case, the incident x-polarized wave generates transmitted \(x\) and \(y\) polarized waves and the incident \(y\) polarized wave generates transmitted \(x\) and \(y\) waves. To explain the physical phenomenon behind the proposed LCPC, we considered the surface current vectors at any two frequency points within the LCPC operation, \(f_{1}\) and \(f_{2}\): \(f_{1}=1.085\) THz and \(f_{2}=1.454\) THz. Figure 7 shows the surface current distribution with the incident x-polarized wave at different frequencies. Figure 7a shows surface current vectors in the top metasurfaces, whereas Figure 7b shows surface current vectors in bottom (ground) surface at \(f_{1}\). There are two parts of the metasurfaces that contribute to the linear to circular polarization conversion. One is the outer square ring and the other is the inner metallic tri-square eliminated square patch. Figure 7a shows that the current in the unit cell's metal square ring travels antiparallel to the current induced in the ground surface. This will generate circulating current loops inside the substrate. It will give rise to magnetic resonance and equivalent magnetic dipoles will be created under x-polarized incident waves [69]. Figure 7c,d show surface current vectors on the top metasurfaces and ground surface, respectively, at \(f_{2}\). It is clear that besides contribution of outer square ring, the innermost eliminated square patch contributes to the wide bandwidth and the currents are parallel in the top and bottom surfaces. The outer square ring and tri-square eliminated square patch both give rise to electric resonance due to their parallel currents [42]. To support our work, the electric field distribution between the four adjacent cells of the array structure was investigated. Figure 8a,b show the electric field distribution at 1.085 THz and 1.454 THz, respectively. It is clear from Figure 8a that the electric field concentrates on the outer layer of the tri-square eliminated patch, whereas, for 1.454 THz, the electric field is very strong and is concentrated on the whole tri-squares eliminated patch with the innermost square having high electric field strength. This multiresonance structure validates the wider bandwidth of the polarization converter. Figure 6: Ellipticity for the transmitted wave with incident \(x\) polarized wave. Figure 7: Surface current distributions at 1.085 THz: (**a**) top surface; (**b**) ground surface; and at 1.454 THz: (**c**) top surface; (**d**) ground surface. In order to enhance the idea of wide-band LCPC conversion, we calculate the frequency-dependent impedance for the unit circuit shown in Figure 2a under x-polarized incident wave:  
\[\mathrm{Z_{x}(w)=\frac{1}{\frac{2}{jwC2}+\frac{1}{jwC1}+\frac{2}{jwL3}+\frac{1}{ jwL2}}+\frac{1}{jwC3}} \tag{13}\]  
Due to the complex and un-matched frequency-dependent impedance of the unit cell in Equation (13), the proposed structure behaves differently at different frequencies. It has been shown in Figure 7a,b that at \(f_{1}=\)1.085 THz, circulating currents flow inside the substrate which shows that adjacent unit cells in the \(x\)-direction are playing the role of a parallel circuit at \(f_{1}\). At \(f_{2}=\)1.45 THz, surface currents in the top and bottom surface are parallel, which predicts the role of the series circuit for adjacent unit cells in the \(x\)-direction. Thus, two different modes exist at two frequencies, which also supports the unit's wide-band operation.