## 1 Introduction  
Electromagnetic (EM) absorbers, as the components that allow for EM wave absorption, has aroused significant attention in academia and industry for a long time and is widely used in different areas, such as stealth technology [1; 2], EM compatibility and shielding [3], sensors [4; 5], energy harvesting [6; 7], and passive cooling technologies [8; 9; 10]. In this context, numerous structures and different materials have been introduced in the design of EM absorbers. The operating frequencies of absorbers have covered the microwave band [11; 12; 13; 14; 15], Terahertz band [16; 17; 18; 19; 20], and the infrared and visible light band [21]. Different features of EM absorbers, such as their low profile, easy fabrication process and light weight, have been deeply investigated to meet various requirements. Among these features, the low cost and broad operating bandwidth are two pressing issues, as they are highly desired in different applications. In the design of the absorber with a wide operating bandwidth, multilayer structures [1] and taper rod, utilizing high lossy materials [22], are two commonly used methods, which may lead to high cost and bulk volume. In 2008, [PERSON] et al. [23] proposed a new method to obtain ultra-thin microwave absorbers based on metamaterials. The so-called perfect metamaterial absorber (PMA) attracted much attention due to its advantageous low profile and easy fabrication [24; 25; 26; 27]. Due to the resonance property, the operating bandwidth of PMA is usually narrow, which is not enough for certain applications where a wide operating bandwidth is desired. To address this issue, different structures have been introduced to PMA design, such as a tapered helical structure [28], multi-sized tapered waveguide array [29], multiple dielectric layers [30] and a resistance frequency selective surface [31]. Most of these methods utilize a combination of multi-operating bands of PMA. At the same time, it has also been proved that the high absorption can be obtained using materials with high dielectric loss [32; 33], indicating that the high-dielectric-loss materials have a large potential to extend the bandwidth of the EM absorbers. Inspired by this fact, materials with different nanoparticles [34] have been investigated to obtain low-cost absorbers with a broad operating bandwidth. In [35], the carbon-coated nickel nanocapsule is proposed for microwave absorption and a maximum reflection loss of 32 dB can be obtained at 13 GHz with 2 mm thickness. ZnO-coated Fe nanocapsules are investigated in [36] for microwave absorption. The reflection loss higher than 20 dB can be obtained in the frequency range of 6.1-15.7 GHz with different thicknesses of the material. In addition, the CaCoTi ferrite composites [37], (Fe, Ni)/C nanocapsules [38], carbon-coated iron nanocapsules [39], Fe\({}_{3}\)O\({}_{4}\)/TiO\({}_{2}\) Core/Shell Nanotubes [40] and \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)-filled carbon nanorods [41] are also investigated for EM wave absorption at different operating bands. Most recently, composite materials, such as graphene composite [42], \(\alpha\)-Fe/Fe\({}_{3}\)C/Woodceramic nanocomposite [43] and Fe\({}_{3}\)O\({}_{4}\)/ZnO core-shell nanocomposite [44], are also studied. Besides, in recent years, the planar metasurface [45], materials with near-zero refractive index [46], emerging graphene [47], spoof surface plasmon polariton [48; 49; 50] and water [51; 52; 53; 54] are also introduced in the design of a low-cost absorber with a broad operating bandwidth. At the same time, the emerging additive manufacturing (AM) technology [55; 56], often referred to as 3D printing technology, has become a hot area in recent years due to its advantageous low cost, environmental friendliness, and ease of forming. Compared with traditional manufacturing technologies, AM technology can complete irregular profile structures and significantly increase the degrees of freedom for optimal design.