# Combined Mie Resonance Metasurface for Wideband Terahertz Absorber
## 1 Introduction  
The use of absorbers can reduce or eliminate the interference of complex electromagnetic environment on wireless communications. Furthermore, the research on absorbers also plays an important role in solar cells [1; 2], photodetectors [3], military stealth, and radar detection [4; 5]. There have been a number of designs for absorbers, such as multilayer gratings [6] and double-cavity photonic crystal [7], among others.  
Many applications based on metasurfaces [8; 9; 10; 11] have been realized, such as polarization conversion [12], antireflection coating [13], ultralens [14], etc. Absorbers based on metasurface are also an interesting application and has attracted the attention of many scholars [15; 16; 17]. The working range of the metamaterial absorbers can be designed in the visible [15; 16], near-infrared [18; 19], mid-infrared [20], terahertz [21], and microwave bands [19; 22]. A good absorber not only requires high absorption but also a relatively wide operating bandwidth. The insensitivities of absorbers to the polarization and incident angle of incident light are desired characteristics as well. Traditional metamaterial absorbers are based on the electromagnetic dipole of the resonator, which limits the working frequency band of the absorber. Current metamaterial absorbers mostly use a multilayer structure to achieve a wide absorption bandwidth or multiband tunable absorption [23]. Metasurface absorbers based on Salisbury Screen [22] have also been reported. However, these thick and bulky multilayered structures make it difficult for the absorbers to integrate with commercialtechnologies, and the manufacturing process is usually expensive. Synthetic fractals inherently carry spatially encoded frequency information, thus becoming ideal candidates for broadband optical structures. Many metasurface absorbers based on fractals have been proposed recently [24; 25]. [PERSON] et al. [24] proposed a structure where self-similar shapes were scaled down in size and added onto the original shape, thus increasing the effective perimeter of the element and achieving a resonance frequency shift. Similarly, [PERSON] et al. [26] proposed a leaf vein shaped structure. In this case, only the filling factor had changed and the overall size changed very little. Therefore, fractal provides a new degree of freedom--independent of the traditional resonator scale rule--to realize the change of resonant frequency. However, the increase in fractal order will inevitably lead to complexity and the need for precision of the cell structure, which requires advanced manufacturing process. The major disadvantages of fractals are the complicated design and fabrication processes.  
As far as we know, although the idea of combining different structures into one unit cell has appeared in other fields, we rarely see reports that widen the absorption bandwidth based on this scheme. In this paper, we propose a wideband terahertz metasurface absorber based on a simple structure that combines resonators of the same shape and different sizes in one unit cell to broaden the absorption bandwidth of the metamaterial absorber. Our metasurface effectively broadened the absorption bandwidth of conventional Mie resonance absorbers. The absorption bandwidth could still be widened when the polarization of the incident light changed; when the incident angle was 45\({}^{\circ}\), the absorption rate could still reach about 75%.