A three-dimensional cross-shaped fractal metamaterial absorber with ultra-wide wavelength band, polarization-independence and wide-angle, is numerically investigated by the finite-difference time-domain method. In this absorber, the solar energy is trapped by the cross-shaped fractal of the upper layer, and the Si-ring filled with iron in the middle layer and the wavelength band can be broadened by the self-similarity of fractal structure. The absorber exhibits absorptivity higher than 91% for the wavelengths from 400 to 2000 nm and an absorption bandwidth of about 133%. Furthermore, the proposed absorber realizes polarization independence, and the maximum incident angle is 76°. However, as the iron material applied in the nano-metamaterial absorber (NMA) can be easily oxidized and rusted, it is replaced by nickel with characteristics such as corrosion resistance and high-temperature resistance; thus, an improved NMA is obtained. The improved absorber not only eliminates the corrosion-prone defects of the above proposed structure but also maintains polarization independence and high absorption and widens the angle of incidence up to 79° and thereby can be applied in many areas, such as solar energy harvesting.

中文翻译：

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超宽带和偏振独立超材料交叉形分形吸收体的数值研究

利用时域有限差分法，对具有超宽波段，偏振无关和广角的三维十字形分形超材料吸收体进行了数值研究。在该吸收体中，太阳能被上层的十字形分形捕获，并且通过分形结构的自相似性，在中间层中填充铁的Si环和波长带可以变宽。对于400至2000nm的波长，吸收剂表现出高于91％的吸收率，并且吸收带宽约为133％。此外，所提出的吸收体实现偏振无关，并且最大入射角为76°。但是，由于应用于纳米金属吸收剂（NMA）的铁材料容易被氧化和生锈，用镍代替，具有耐腐蚀性和耐高温性。因此，获得了改进的NMA。改进的吸收器不仅消除了上述结构的易腐蚀缺陷，而且保持了偏振无关性和高吸收率，并将入射角扩大到79°，因此可用于许多领域，例如太阳能收集。