## 3 Results and Discussions
In order to achieve an ideal absorption in the entire solar spectrum shown in Figure 3d, the structure of the designed multilayer absorber is determined to be MgF\({}_{2}\) (anti-reflection layer)-Ti-SiO\({}_{2}\)-Cr-SiO\({}_{2}\)-Fe-SiO\({}_{2}\)-Fe (Structure D), as Figure 3c shows, which can obtain an ideal absorption throughout the entire solar spectrum (300-2500 nm). Figure 2: (**a**) Absorption spectra of Structure A over the range of 400 to 1000 nm. (**b**) Absorption spectra of Structure A over the range of 300 to 3000 nm. (**c**) The magnetic field distribution (Hy) and (**d**) the absorbed power distribution along in the x–z plane, at the wavelength \(\lambda\) = 620 nm. Incident light on a metal surface will produce a reflection. Therefore, for reducing the reflection of incident light of an absorber, a multilayer design is usually carried out. As we all know, the higher the refractive index, the stronger the transmittance of the incident light. The refractive index of MgF\({}_{2}\) is 1.38, which is much lower than that of ordinary dielectric medium. Therefore, we choose MgF\({}_{2}\) film as the antireflection layer. The average absorption is defined as follows:  
\[A=\int_{\lambda 1}^{\lambda 2}A(\lambda)d\lambda/(\lambda_{1}-\lambda_{2}) \tag{2}\]  
where \(\lambda_{1}\) and \(\lambda_{2}\) respectively are 2500 and 300 nm. From our numerical simulation results, the average absorption will reach 96.6% over the wavelength region of 300-2500 nm. It can be seen clearly in Figure 3c that the high absorption rate is really obtained in the wavelength range of 300-2500 nm. The gray curve in Figure 3d is the air mass 1.5 solar irradiance spectrum, i.e., the entire solar spectrum (300-2500 nm). Figure 3d can be used to prove that the designed absorber can have a high absorption rate in the wavelength range of sunlight. The high absorption rate is due to the resonance absorption peak generated in the F-P cavity, and the broadband high absorption rate results from the overlapping of resonant frequencies of multiple cavity. As Figure 2 shows, there is only one resonance absorption peak in absorption spectrum when only one F-P cavity is designed. Meanwhile, there are multiple resonance absorption peaks in the absorption spectrum when multiple cavities are investigated, as Figure 3 shows. Each resonance peak occurs in a different position, to broaden the absorption range. Obviously, there is a resonance absorption valley between two adjacent resonance absorption peaks, due to the presence of multiple F-P cavities. Thus, reflection spectra of Structures B, C, and D are used to smooth the resonance intensity of F-P cavities and enhance the problem of the resonance absorption valley. In order to further clarify its physical mechanisms of the high absorption rate in the entire solar spectrum, we calculate the distribution spectra of absorption power at the wavelengths of \(\lambda=\) 500, 1000, 1500, 2000, and 2500 nm, respectively. It can be clearly seen that the most absorbed power distribution occurs in the top Ti layer, and the wavelength and the bottom Fe layer always contribute the least amount of absorbed power distribution, in Figure 4. However, there is no doubt that the high absorption rate of each wavelength is the superposition effect of four metal layers. In the  
Figure 3: Absorption spectra of (**a**) Structure B, (**b**) Structure C, and (**c**) Structure D. (**d**) AM1.5 solar irradiance spectrum. abovementioned reports, Fabry-Perot (F-P) nano-cavity consists of a sandwich structure, in which the core is lossless dielectric layer and the top layer is a lossy metal layer and backside layer is a reflector, i.e., metal-dielectric-metal planar film structure. Thus, our proposed structure of multilayer planar films can be regarded as multiple F-P cavities, which can overcome the shortcoming of single F-P cavity, such as narrow bandwidth [30].