Wide-Angle, Polarization-Independent Broadband Metamaterial Absorber by Using Plasmonic Metasurface-Based Split-Circular Structure  
[PERSON]  
1 School of Electrical and Electronic Engineering, Hanoi University of Industry, Hanoi 100000, Vietnam; [EMAIL_ADDRESS]  
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with EM waves, all or most of the incident waves are trapped and dissipated within the structure [7; 8]. Thanks to its unique properties, the MPA can be applied in various fields. For instance, in sensing applications, MPAs can detect changes in the refractive index of the surrounding environment [9]. In radar applications, they can absorb EM waves in the K\({}_{\text{u}}\)-band wavelength range, rendering objects invisible to military radar systems [10]. In energy harvesting, they enhance the absorption and conversion of incident EM waves into usable electrical energy [11]. Additionally, MPAs find applications in many other areas. Research on different MPA structures is highly diverse, depending on specific applications. The most fundamental and earliest studies focused on MPAs capable of fully absorbing EM waves at a single designed frequency [12]. Subsequently, those capable of absorbing waves at multiple frequency peaks were introduced as an extension of the single-peak absorption type [13]. As the demand for practical applications grows, research on MPAs has increasingly focused on designing absorbers with a wide absorption bandwidth across a frequency range [14]. To realize broadband absorption, several MPA design approaches have been proposed. [PERSON] et al. suggested external resistors to absorb energy, which reduced the \(Q\)-factor of the metamaterial unit cell and broadened the absorption bandwidth [15]. [PERSON] et al. proposed a multilayered metamaterial structure to extend the absorption bandwidth, based on the principle of superposition [16]. [PERSON] et al. utilized a special material, a resistive film, to obtain the absorption over a frequency range from 4 to 24 GHz [17]. Several other studies by [PERSON] et al. and [PERSON] et al. utilized materials such as Ti\({}_{2}\)C\({}_{3}\)Tx MXene or doped silicon to develop broadband MPA structures operating in the THz frequency range [18; 19]. One of the methods used to design the broadband MPAs is to use the spoof surface plasmon polariton (SSPP) effect in the GHz and THz frequency range. Materials with plasmonic effects have also been studied and found to have numerous applications in the optical wavelength region [20]. The SSPP effect in metamaterial structures was first proposed by [PERSON] et al. in 2004 [21]. Metamaterial structures that responded to EM waves in the GHz and THz frequency range exhibited behavior similar to SSPPs in the optical range [22; 23]. The properties of SSPP, such as local field enhancement, strong dispersion, and propagation characteristics in the deep subwavelength regime for application in waveguiding, plasmonic circuits, conformal circuits, and diffractive neural networks, have been extensively studied in recent years [24; 25; 26; 27; 28]. These properties were highly effective in designing broadband metamaterial absorbers. Many broadband metamaterial absorber structures have been proposed, based on the SSPP effects in metamaterials [29; 30]. For example, periodic arrangements of metallic strips with overlapping lengths were designed to achieve the superposition of neighboring resonance peaks [31]. This resulted in a wide absorption band for the metamaterial structure. However, these designs typically utilized longitudinal arrays to obtain the superposition along the propagation direction of incident EM waves. While this approach was highly effective in developing metamaterial structures with wide absorption bands, it required the structures to be in a 3D form [32]. This made them bulky and challenging to fabricate.