## 3 Results and Analysis
### Simulation Results
in the HFSS model were much more complicated than those in the equivalent circuit, which caused the slight difference between the results of the two models. To analyze the polarization insensitivity feature, the surface current of the metasurface unit cell was explored. Figure 6a shows the current distribution of the TE wave at the normal incidence, and Figure 6b-d show the surface current with the polarization plane being rotated by 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) along the xoy-plane, respectively. For any polarization angle, the current always passed through the circle in the center, and the harvesting load was placed at the concentrated point of the current, which was the criterion of determining the harvesting load position. What is more, the current distribution density and pattern stayed relatively unchanged, due to the approximate symmetrical pattern of the unit. Therefore, the unit cell could effectively harvest the energy and maintain a high efficiency for arbitrary polarization waves at the normal incidence. Figure 4: Equivalent circuit model of the proposed metasurface. The parameters of the elements are as follows: L\({}_{1}\) = 0.2 nH, C\({}_{1}\) = 1.6 pF, L\({}_{2}\) = 0.1 nH, C\({}_{2}\) = 2.6 pF, C\({}_{3}\) = 1.2 pF, and R = 50 \(\Omega\). Figure 5: (**a**) Reflection coefficient of the equivalent circuit in Advanced Design System (ADS), compared with that of the metasurface in HFSS. (**b**) Reflection coefficient, changing with the value of R in ADS. (**c**) Reflection coefficient, changing with the value of R in HFSS. Figure 6: Surface current on metal of the unit cell at the normal incidence for different polarization angles. (**a**) \(\varphi\) = 0\({}^{\circ}\), (**b**) \(\varphi\) = 30\({}^{\circ}\), (**c**) \(\varphi\) = 60\({}^{\circ}\), and (**d**) \(\varphi\) = 90\({}^{\circ}\). The harvesting efficiency of the metasurface under an oblique incidence for TE and TM polarizations was studied. Figure 7a shows the efficiency for TE polarization. As the incidence angle increased from 0\({}^{\circ}\) to 75\({}^{\circ}\), the operation frequency in the broadside direction was nearly unchanged, and the harvesting efficiency reduced gradually. When the incident angle increased to 60\({}^{\circ}\), the harvesting efficiency was 72%. For TM polarization, as the incidence angle increased from 0\({}^{\circ}\) to 75\({}^{\circ}\), the operation frequency in the broadside direction shifted by only 11 MHz. When the incidence angle was 60\({}^{\circ}\), the harvesting efficiency was 91%, as shown in Figure 7b. The simulated results show that the metasurface can maintain a high efficiency at a fixed frequency over a wide range of incidence angles, and it remains insensitive to polarization, even in the case of an oblique incidence under 45\({}^{\circ}\). In this design, a square-shaped metal via ring around the unit cells is proposed to decrease the mutual coupling and improve the frequency stability. Figure 8 shows the impedance for TM polarization versus the frequency with and without the square rings when the incident angles are different. It can be observed that the relative impedance shifts with the frequency as the incident angle increasing. When the incidence angle increases from 0\({}^{\circ}\) to 75\({}^{\circ}\), the frequencies shift 15 MHz and 48 MHz with and without the square rings, respectively. The variation tendency of the harvesting efficiency is similar to that of the impedance for TM polarization as shown in Figure 9. Without the square rings, the operation frequency at the broadside direction shifts 48 MHz when the incidence angle increase from 0\({}^{\circ}\) to 75\({}^{\circ}\). With the square rings,  
Figure 8: The impedence (**a**) without, (**b**) with square-shaped metal via rings. Figure 7: Simulated harvesting efficiency of the infinite metasurface under different incidence angles for (**a**) TE and (**b**) TM polarized incident waves.