## 3 Results and Discussions  
Since our proposed absorber has a structure composed of multilayer planar films, we analyzed the structure with Fe-SiO\({}_{2}\)-Fe (structure A), firstly so as to further analyze the ideal broadband absorption. The simple Fe-SiO\({}_{2}\)-Fe multilayer films can be recognized as a structure of asymmetric F-P nano-cavity, which consists of a sandwich structure. The core is lossless dielectric layer, the top layer is a lossy metal layer, and the backside layer is a reflector, as the schematic diagram in Figure 2a shows. As the results in Figure 2a,b show, since an ordinary F-P nano-cavity absorber is usually characterized by high wavelength selectivity in absorption spectra, the absorption of Structure A maintains a high level in the wavelength of visible spectrum, but declines in the wavelength of infrared spectrum. In order to obtain the better insights of the physical mechanism for the high absorption rate in the visible spectrum, we calculated the spatial distribution of magnetic field (Hy) and absorbed power distributions over the cross-section at the absorption peaks \(\lambda=620\) nm, as labeled in Figure 2c,d, separately. The absorbed-power distribution can be defined as \(\mathrm{P}_{abs}=\
abla\cdot\overset{\rightarrow}{s}=\frac{1}{2}a\omega^{ \omega}|E|^{2}\) for nonmagnetic material, where \(\varepsilon^{\omega}\) is the imaginary part of the permittivity, \(\omega\) is the angular frequency, and \(|E|\) is the total electric field distribution [29]. The distribution of magnetic field (Hy) is shown to be highly  
Figure 1: Schematic view of the proposed solar absorber with multilayer film structures. The light of incidence is vertical to the absorber, where t1 = 80 nm, t2 = 3 nm, t3 = 100 nm, and t4 = 5 nm. limited by the boundary opaque metals (Fe layers) and trapped in the dielectric core (SiO\({}_{2}\) layer), as shown in Figure 2c shows. The resonance is considered to be the PSP resonance between the interface of Fe and SiO\({}_{2}\) films. According to the calculated absorbed-power distribution, the reason to reach the high absorption rate is the loss of top thin Fe layer, as well as that of backside Fe reflector. However, the main reason to cause the absorbed-power distribution is due to the top Fe layer, accounting for much larger than the dissipation inside the backside Fe layer. Thus, the distribution of magnetic field and absorbed-power distribution are illustrated in Figure 2c,d, respectively, which indicate a combination of the PSP and F-P resonances. One can see from Figure 2b that, although Structure A can form an absorber with a high absorption rate, the absorption bandwidth is narrow and the absorption efficiency is low, and these things will limit its practical application. In order to overcome the above shortcomings, a simple method can be investigated to improve these problems, using vertical superposition of the multilayer plane films, to enable the enhancement of spectral absorption. As shown in Figure 3a, the absorption rate of Cr-SiO\({}_{2}\)-Fe-SiO\({}_{2}\)-Fe (Structure B) is higher than that of Structure A; however, it still has a problem of having an absorption bandwidth that is too narrow and a discontinuous absorption rate. Furthermore, the absorption rate of Ti-Cr-SiO\({}_{2}\)-Fe-SiO\({}_{2}\)-Fe (Structure C) is shown in Figure 3b. One can see that the high absorption rate does not cover the entire solar spectrum, especially in the range of 2000-2500 nm. Nevertheless, we can find that the absorption bandwidth and efficiency is limited in the above-proposed solar absorbers. As a matter of fact, the absorption bandwidth and efficiency play a key role in many actual applications, e.g., thermal emitters, solar energy utilizations, photodetector applications, and so on.