# Broadband Plasmonic Metamaterial Optical Absorber for the Visible to Near-Infrared Region
## 2 Geometric Structure of the Absorber Unit Cell
Decreased absorption in the visible range results from reflecting light from the second metal resonating layer. Another coating of SiO\({}_{2}\) was added to the final nanoarchitecture unit cell design to prevent the incident light from reflecting from the Tu layer to increase the absorption in the visible spectrum. After adding SiO\({}_{2}\) coating, the MA showed a 92% average absorption from 400 nm to 2400 nm wavelength with perfect peak absorption at 600 nm wavelength. The geometrical shape of the resonating patch plays an essential role in achieving broader and higher absorption. Figure 3 shows the effect of the patch shape. When the patch had a triangular shape with three segments, it had weak absorption in 850 nm to 1700 nm wavelengths. The absorption pattern gets more consistent as the number of segments rises. However, the fabrication process becomes more challenging if the nanoarchitecture unit cell is designed with extreme precision. This study uses a patch with five segments as it has comparatively broad bandwidth and a high average absorption of 92% throughout 400 nm to 2400 nm wavelength with relatively fewer segments. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter Symbol** & **Value (nm)** & **Parameter Symbol** & **Value (nm)** \\ \hline h1 & 105 & h5 & 200 \\ h2 & 50 & h6 & 50 \\ h3 & 25 & w & 200 \\ h4 & 12.5 & r & 85 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural parameters of the MA unit cell. Figure 3: The design of the internal structure of the layers of the unit cell. Figure 2: The design evaluation and its respective absorption. The absorption property of the unit cell is related to the geometry of the layers. The MA unit cell's ground layer thickness is shown in Figure 4a by the notation h1. Absorption bandwidth and average absorption increase as h1's value increases from 5 nm to 105 nm. The MA's peak absorption of 93% was at 600 nm, its average absorption was 85%, and its bandwidth was 700 nm for h1 = 5 nm. As h1 increases, the peak absorption, average absorption, and bandwidth increase. In the final design, h1 is 105 nm, and it shows 92% average absorption with 2000 nm bandwidth. So, the absorption of this MA is proportional to the thickness of the ground layer. Similarly, the thickness of the first resonating Tu layer also has the same effect on the average absorption. But it does not show significant changes as the bottom layer shows in the figure. In Figure 4b, h2 represents the thickness of the ground layer of the MA unit cell. The figure shows that for a lower value of h2 average absorption decreases, and it increases as the value of h2 increases. But overall average bandwidth remains the same for different values of h2. Figure 4: The absorption property of the unit cell for different thicknesses of (**a**) ground layer, (**b**) first resonating Tu layer, (**c**) second Tu layer, (**d**) relation between MA absorption property and SiO\({}_{2}\) antireflection coating, (**e**) The influence of the Tu resonator size to the absorption property of the MA. In Figure 4c, the thickness of the second Tu layer has the opposite effect on the average absorption. Here h3 is the thickness of the second Tu layer. If the thickness of the second Tu layer (h3) increases, the average absorption decreases. When h3 decreases, the opposite occurs. However, the overall average bandwidth for h3 is unchanged. The relationship between the MA absorption property and the SiO\({}_{2}\) antireflection layer is depicted in Figure 4d. The SiO\({}_{2}\) anti-reflection layer primarily changes the absorption bandwidth. There is a 99.6%-peck absorption in all three cases of no SiO\({}_{2}\) coating, with a SiO\({}_{2}\) support layer in between two Tu resonating layers and the final design with full SiO2 anti-reflection coating. But full SiO\({}_{2}\) anti-reflection coating structure has achieved a wide bandwidth of 2000 nm.