## 3 Experimental Verification for the Proposed Plasmonic Metasurface  
The proposed plasmonic metasurface, which was analyzed above, was fabricated as shown in Figure 6. The metasurface consisted of a 10 \(\times\) 10 array of unit cells, as described here and in Figure 1, with the following parameters: \(D\) = 187 mm, \(L\) = 18.7 mm, \(R_{out}\) = 7 mm, \(W\) = 0.1 mm, and \(S\) = 0.1 mm. The plasmonic metasurface was fabricated by using the standard PCB technology on an FR-4 substrate with a thickness of 3.2 mm. The copper layers on both the top and bottom surfaces of the structure had a thickness of 0.035 mm and conductivity of 5.96 \(\times\) 10\({}^{7}\) S/m. FR-4 is a highly durable material resistant to environmental factors such as mechanical, temperature, and chemical factors, while the copper layer, manufactured by using the standard PCB technology, exhibits excellent adhesion to the FR-4 substrate. This method is widely employed in the fabrication of electronic circuits and antennas owing to its high durability [38]. The structure comprised only three layers, and with the dimensions of the unit cell as specified above, it was relatively straightforward to fabricate. This simplicity is one of the advantages of the proposed plasmonic metasurface by utilizing the SSPP effect, compared to multilayered or 3D structures commonly employed in MPA designs. The overall size of the fabricated plasmonic metasurface was 187 \(\times\) 187 mm. The detailed geometrical parameters of the proposed plasmonic metasurface are presented in Table 1.  
The plasmonic metasurface was characterized by using a Rohde & Schwarz ZNB20 Vector Network Analyzer (VNA) and a pair of wideband antennas operating in a frequency range of 12 to 18 GHz, as shown in Figure 7. The two antennas were connected to the two ports of the VNA, enabling the transmission of EM waves toward the structure and the reception of reflected waves. After the calibration process of the VNA, the reflection coefficient (\(S_{11}\)) of the plasmonic metasurface was determined through the transmission coefficient (\(S_{21}\)) from the transmitting antenna to the receiving one. Since the back side of the structure consisted of a continuous copper layer, EM waves could not transmit through the structure, and the absorption of the plasmonic metasurface was calculated by employing formula \(A=1-\left.\right|S_{11}\left|{}^{2}\right.\). To measure the incident angle dependence of the plasmonic metasurface, the two antennas were rotated and moved along an arc-shaped support, ensuring that the incident angle \(\theta_{i}\) equals the reflection angle \(\theta_{r}\). Figure 8a presents the measured absorption performance of the plasmonic metasurface for the TE-polarized waves at incidence angles ranging from 0 to 45\({}^{\circ}\). The results demonstrate that the plasmonic metasurface realized an absorption over 90% of the incident wave within a frequency range of 12.40 to 14.75 GHz. At larger incidence angles, the absorption bandwidth becomes broader (e.g., 12.10 to 14.90 GHz at 45\({}^{\circ}\)) compared to smaller incidence angles. This could be attributed to the parallel component of the EM waves interacting \begin{table}
\begin{tabular}{c c} \hline
**Structure Parameters** & **Value** \\ \hline Unit-cell periodicity (\(L\)) & 18.7 mm \\ Dielectric thickness (\(t_{d}\)) & 3.2 mm \\ Dielectric constant (\(\varepsilon\)) & 4.3 \\ Loss tangent (tan \(\delta\)) & 0.025 \\ Metal thickness (\(t_{m}\)) & 0.035 mm \\ Width of metal trace (\(W\)) & 0.1 mm \\ Spacing between turns (\(S\)) & 0.1 mm \\ Outer radius (\(R_{\text{out}}\)) & 7 mm \\ Number of rings (\(n\)) & 10 \\ Total size of plasmonic metasurface & 187 \(\times\) 187 mm \\ \hline \end{tabular}
\end{table} 
Table 1: Geometrical parameters of the plasmonic metasurface. Figure 6: Fabricated plasmonic metasurface and its unit cell. Figure 7: Experiment setup for measuring the absorption of the proposed plasmonic metasurface.