## 2 Materials, Structure, and Methods  
A schematic diagram of planar multilayer films' designed structure is displayed in Figure 1, which has some important novelties in this designed absorber. The first novelty is that the top MgF  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **Ag** & **Au** & **Fe** & **Ti** & **Cr** \\ \hline Melting point (K) & 1234 & 1337 & 1811 & 1941 & 2180 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The melting point of metals.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & \begin{tabular}{c} **Bandwidth** \\ **(nm)** \\ \end{tabular} & **Structure** & \
\begin{tabular}{c} **Polarization** \\ **Independent** \\ \end{tabular} & **Materials** &
\begin{tabular}{c} **Average** \\ **Absorption** \\ \end{tabular} \\ \hline
[19] & 400–2000 & Patterned structure & NO & non-noble material & \textasciitilde{-}96.4\% \\
[13] & 400–800 & Planar film structure & YES & non-noble material & \textasciitilde{-}99.5\% \\
[9] & 300–2500 & Patterned structure & YES & graphene & \textasciitilde{-}85\% \\
[15] & 400–1520 & Patterned structure & YES & noble material & \textasciitilde{-}99\% \\ this work & 300–2500 & Planar film structure & YES & non-noble material & \textasciitilde{96.6\%} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparisons between solar absorbers.  
layer is not only designed as an anti-corrosion layer to prevent iron from being oxidized, but it is also designed as an anti-reflective film. The second novelty is that the bottom Fe layer is designed as a backside reflector. The third novelty is that the addition of SiO\({}_{2}\) can be used as a separator of different metals. The related geometric parameters of the designed absorber are labeled in Figure 1. MgF\({}_{2}\) and SiO\({}_{2}\) can be considered to be the lossless dielectric materials; their refractive indexes are 1.38 and 1.45, respectively. The electromagnetic parameters of the Fe, Cr, and Ti are taken from the CRC _Handbook of Chemistry and Physics_[23].  
In order to design a solar absorber with the optimal absorption rate, our proposed device's structure is numerically demonstrated and optimized by using the FDTD simulation [24]. The FDTD method, as well as Maxwell's Solver, is one of the most frequently used numerical techniques for computing optical properties, and it can construct a gold-standard for modeling nanophotonic devices, processes, and materials. Specifically, we used a commercially available FDTD simulation software package of Lucer. The reliability and accuracy of the numerical results were ensured by many reports [11; 12; 13; 25; 26; 27; 28]. The period boundary conditions are used in the \(x\)-axis and \(y\)-axis directions, and an ideally matched layer boundary condition is set in the \(z\)-axis direction. A plane wave with TM polarization with the electric field and magnetic field respectively polarized in the directions of \(x\)-axis and \(y\)-axis is normally incident onto the proposed absorber. The absorption rate (A) can be defined as follows: \(\mathrm{A}=1-\mathrm{R}-\mathrm{T}\), where the R and T indicate reflection and transmission rates, respectively. Because the thickness of the backside Fe layer is smaller than the skin depth in the infrared light spectrum, the T is set as zero. For that, the absorption rate is reduced to: \(\mathrm{A}=1-\mathrm{R}\).