## 1 Introduction
proposed a 90-nm-thick graphene sheet absorber with approximately 85% absorption of unpolarized visible and near-infrared light covering almost the entire solar spectrum (300-2500 nm) [9]. However, some of reported solar absorbers are typically fabricated by using noble metals (e.g., gold and silver), because they have excellent surface plasmon polarization [16; 17], but they have the problem of high cost, which limits the practical applications in the solar absorbers. Moreover, graphene sheets (2-nm-thick carbon layers), femtosecond laser, and other lithographic processes limited the wide application of this absorber and increased its manufacturing cost. Therefore, designing a solar absorber with the advantages of ideal broadband absorption, and fabrication simplicity still represents great challenges. Since the solar-thermal applications need to work at extremely high temperatures, the use of refractory metals is necessary. For example, bulk Chromium (Cr) has a high melting temperature of 2180 K, which is relative to the blackbody radiation at a peak position of 1.33 \(\upmu\)m. However, the melting temperature of bulk gold (Au) is about 1337 K relative to a peak position of 2.17 \(\upmu\)m for the blackbody radiation. The spectral density \(B_{\lambda}(\lambda,T)\) of thermal radiation from a black body absorber can be defined by [PERSON]'s law:  
\[B_{\lambda}(\lambda,T)=\frac{2 hc^{2}}{\lambda^{5}}\frac{1}{e^{\frac{hc}{kT}-1}} \tag{1}\]  
where \(\lambda\) is the wavelength, \(T\) is the absolute temperature of the black body absorber, \(h\) is [PERSON] constant, and \(k\) is the Boltzmann constant [18]. Maximum radiation for absolute temperatures, \(T=1337\) and 2180 K, will occur at 1.33 and 2.17 \(\upmu\)m, respectively. Although the melting temperatures for metals at the nanoscale will be below their bulk values due to small size effects, refractory metals still have higher melting temperatures than those of noble metals [13]. For example, the melting temperatures of Titanium (Ti), iron (Fe), and Cr are 1941, 1811, and 2180 K, while the melting temperatures of Au and Argentum (Ag) are 1337 and 1234 K, respectively, as shown in Table 1. Besides, [PERSON] et al. proved that Fe arrays had the advantageous impedance matching effect between the structure of multilayer films and the free space could be achieved over an ultra-broad band of optical frequency (from visible to near-infrared region), which could play an extremely important role to construct an ideal absorption device in solar bandwidth [19]. Moreover, [PERSON] et al. showed that Cr has optical lossy properties and less optical dispersive properties, and SiO\({}_{2}\) has a high transparency broadband window with non-dispersive optical characteristics in the range of visible light [13]. Therefore, the selection of appropriate materials for the design of a high-efficiency absorber is very important. Here, we propose a high-efficiency absorber with simple a multilayer structure, which could enable omnidirectional, polarization-independent, and broadband absorption across the entire solar bandwidth. The efficiency of proposed absorber could be proven by the Finite-Difference Time Domain (FDTD) simulation. To the best of our knowledge, numerical analysis describe that the designed absorber has a high average absorption rate of 96.6% in the range of 300-2500 nm, which is higher than all previously reported solar absorbers listed in Table 2[9; 13; 15; 19]. The high absorption is not only completely independent of polarization direction but also exhibits angle-independent absorption behavior for oblique incidences up to \(\pm\)60 degree. Such a designed absorber is simpler as compared with those of patterning structures. This investigated structure signifies that the solar absorber can be designed to have the advantages of non-noble-metal manufacturing convenience because of its extremely simple structure and higher performance for practical industrial production.