## 5 Physical Explanation and Equivalent Circuit  
Since the unit cell is based on an anisotropic structure having dual diagonal symmetry, the incident X polarized wave will generate transmitted X and Y polarized wave components and the incident Y polarized wave will generate transmitted X and Y components. To explain the physical phenomenon behind the proposed LP-to-CP, we considered the surface current vectors within two frequency bands; let these be f\({}_{1}\) and f\({}_{2}\): f\({}_{1}\) = 1.398 THz and f\({}_{2}\) = 4.82 THz. Figure 6 shows the surface current distribution with the incident X polarized wave at the output surface of the proposed converter for t = 0, T/4, T/2, 3T/4 at f\({}_{1}\). The orientation of the electric field vectors shows that with every T/4 cycle, it rotates by 90\({}^{\circ}\). Further, it can be seen that the surface current at f\({}_{1}\) is concentrated in the inner tri-square conducting patches with an anti-clockwise rotation. Thus, the transmitted wave is LHCP at f\({}_{1}\). Figure 7 shows the surface current vectors at the output surface at f\({}_{2}\). It can be clearly seen that with every quarter cycle, surface currents are rotated 90\({}^{\circ}\) in a clockwise rotation. Unlike in Figure 6, this time surface current vectors are concentrated in the outer square ring. The opposite direction of rotation for surface current vectors in time cycle T validates our proposed opposite handedness of circular polarization for the same structure at two different frequencies. Figures 8 and 9 indicate the response of the proposed structure to the incident X polarized electromagnetic field. Figure 8a,b show electric field distribution at 1.398 THz and 4.82 THz, respectively. It is clear from Figure 8a that the electric field concentrates on the outer two conducting patches of the tri-square patch with a minor contribution from corners of an outer square ring, whereas for 4.82 THz, the electric field is concentrated on the whole tri-squares patch and outer square ring. This multi-resonance structure validates the dual-wide-band performance of polarization conversion. Figure 6: Surface current distribution of the proposed LP-to-CP converter at 1.398 THz at (a) t = 0, (b) t = T/4 (c) t = T/2 (d) t = 3T/4. Figure 7: Surface current distribution of the proposed LP-to-CP converter at 4.82 THz at (a) t = 0 (b) t = T/4 (c) t = T/2 (d) t = 3T/4. Figure 9a shows the magnetic field strength at f\({}_{1}\) = 1.398 THz. It shows that the magnetic field is concentrated in the intersected corners of the tri-squares conductor patch. Figure 9b shows that for the second frequency band, at f\({}_{2}\), the magnetic field is predominantly attributed to the outer square ring and vertical sides of the squares in the tri-square patch. Figure 10a,b show the equivalent circuit for the unit cell of the proposed dual-wide-band LP-to-CP converter with incident X and Y polarizations, respectively. With incident X polarizations, upper and lower arms interact with the incident waves, whereas the left and right arms of the outer square ring will have no interaction. Thus, inductors L\({}_{1}\) representing the outer square ring will appear as shown in Figure 10a. C\({}_{1}\) represents the value of capacitance induced between the corner of the square ring and the diagonal conducting patch with incident X polarization. L\({}_{2}\) shows the combined inductive effect of the two outer squares (area: C \(\times\) C) and inner square (area: C\({}_{1}\)\(\times\) C\({}_{1}\)). It is interesting to note that due to the discontinuity between the three square inductors in a diagonal position, there will also be a capacitive effect, but the overall effect for three inductors will be inductive. Thus, it is represented as L\({}_{2}\). Z\({}_{0}\) and Z\({}_{4}\) represent transmission line models for free space layers and the substrate. In the lower frequency band of operation, the impedance corresponding to L\({}_{1}\), Z\(\_\)L\({}_{1}\) will be lower compared to its value in a higher frequency band of operation.