## 2 Design and Analysis  
The proposed plasmonic metasurface in this paper was composed of ten concentric split rings with varying diameters (Ring-1 to Ring-10), as illustrated in Figure 1a. The entire structure was fabricated as a three-layer configuration, where the top layer consisted of a structure comprising ten concentric split rings to generate the SSPP effect. However, this particular concentric ring structure was specially divided into four separate segments, and the rings were interconnected by four metallic bars. Thanks to these connections, a portion of the rings could simultaneously contribute to the generation of multiple resonance frequencies when linked with other split rings of different diameters. The multiple resonance frequencies, generated by these pairs of split rings, induced superposition, forming a broad resonance band for the structure. Similar to the conventional MPA structures, this design adopted a sandwich configuration, with a dielectric layer of FR-4 in the middle, having a thickness of 3.2 mm, a dielectric constant of \(\varepsilon\) = 4.3, and a loss tangent of tan \(\delta\) = 0.025. The bottom layer was a continuous copper sheet that prevented EM waves from transmitting through the structure [33]. Figure 1b presents the simulated absorption spectrum of the proposed plasmonic metasurface at some incident angles of EM waves for the TE mode, ranging from 0 to 45\({}^{\circ}\). The simulations were conducted using CST Studio Suite 2023 software with periodic boundary conditions applied along the \(Ox\) and \(Oy\) directions to model the 2D metamaterial  
Figure 1: (**a**) Schematic illustration of the plasmonic metasurface and its unit cell, and (**b**) high absorption at various incident angles. structure [34]. In the CST Studio Suite 2023 software, this boundary condition can be selected by using the \"unit cell\" mode for the \(Ox\) and \(Oy\) dimensions, while \"Et = 0\" was chosen for the \(Oz\) axis (\(Z_{\text{min}}\)) at the bottom of the unit cell. The opposite side (\(Z_{\text{max}}\)) was configured in the \"open\" mode, serving as the wave source for the incoming wave. The results indicated that the plasmonic metasurface exhibited an absorption exceeding 90% within a broadband absorption range from 12.30 to 14.80 GHz. The plasmonic metasurface, as illustrated in Figure 1, can be described in terms of an equivalent circuit model based on the transmission line theory [35]. The plasmonic metasurface is represented by a combination of circuit elements connected together, as shown in the circuit diagram in Figure 2. The impedance of free space is represented by \(Z_{s}\). Subsequently, the circular structure is modeled by the \(RLC\) circuits. The dielectric substrate is represented by impedance \(Z_{d}\). Finally, the continuous copper layer on the backside of the dielectric layer is considered a short-circuit component in the circuit model [36]. From the equivalent circuit, the input impedance of plasmonic metasurface can be calculated by using the following formula:  
\[Z_{IN}=\frac{Z_{PM}Z_{d}}{Z_{PM}+Z_{d}}. \tag{1}\]  
\(Z_{PM}\) is the impedance of the circular structure of the plasmonic metasurface and is calculated from the parallel RLC circuit components, as shown below. \[\frac{1}{Z_{PM}}=\frac{1}{Z_{1}}+\frac{1}{Z_{2}}+\dots\frac{1}{Z_{n}} \tag{2}\]  
where  
\[Z_{i}=R_{i}+j\omega L_{i}+\frac{1}{j\omega C_{i}} \tag{3}\]  
with \(R_{i}\), \(L_{i}\), and \(C_{i}\) as the resistance, inductance, and capacitance of the \(i\)th branch, respectively. In addition, \(n\) is the total number of branches in the parallel circuit. Consequently, the reflection coefficient of the plasmonic metasurface under the incidence of an EM wave can be determined with the following expression:  
\[\Gamma=\frac{Z_{IN}-Z_{s}}{Z_{IN}+Z_{s}}. \tag{4}\]  
Figure 2: Equivalent circuit model for the plasmonic metasurface.