# Design of a Polarization-Insensitive and Wide-Angle Triple-Band Metamaterial Absorber
## 1 Introduction  
In recent years, due to the wide application prospects of metamaterials, more and more scientific researchers have begun to focus on various applications based on metamaterials, of which metamaterial absorbers are a very typical application field [1]. Additionally, computational methods of artificial intelligence have been applied to the research field of metamaterials [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Specifically, in 2008, [PERSON] [12] proposed the design concept +of a metamaterial absorber. The MMA designed based on this concept has broadband and multi-band absorption characteristics of 0.75 THz-1.5 THz. Most absorbers adopt the design concept of a metal-dielectric-metal sandwich structure. Incoming waves excite surface currents in opposite directions on the two metal planes surrounding the dielectriclayer, leading to electromagnetic resonance. This resonance allows for energy dissipation at specific frequencies, creating an effective mechanism for selective frequency absorption. [PERSON] [13; 14] discovered the difference in dispersion characteristics of planar waveguides based on left-handed materials under different polarization states, which indicates that the resonance realized by the absorber at different frequencies can maximize the absorption of electromagnetic wave energy at that frequency point. In addition, if the thickness or other dimensions of the absorber change, it will also affect the absorption frequency. [PERSON] [15; 16; 17; 18; 19; 20; 21; 22; 23; 24] and colleagues have designed various tunable absorber devices using materials such as Dirac semimetals, silica, and graphene, achieving electromagnetic wave absorption through surface plasmon resonance and a bound-state continuum. By adjusting the Fermi level of graphene, the resonance frequency of the absorber device can be modified. This series of work provides a novel approach for tunable terahertz metamaterial absorbers, which is of significant importance in fields such as detection and sensing. Electromagnetic waves interact on the surface of the absorber, causing electrons to oscillate, thus exciting a current on the metal layer. The energy of the electromagnetic field is converted into electromagnetic resonance for dissipation, and the remaining electromagnetic waves are reflected on the metal backplane. [PERSON] pointed out that the electromagnetic waves on the upper and lower metal surfaces generate anti-parallel currents. At a specific frequency, most of the energy of the electromagnetic wave is converted into electromagnetic resonance. In fact, the reflection coefficient \(S_{11}\) of the electromagnetic wave will infinitely approach 0 at some frequency points; such points are called resonant frequencies [25]. The top and bottom layers of absorbers are often made of metal, with copper being a popular choice due to its affordability and high compatibility with PCB manufacturing processes [26; 27; 28]. Alternatives include silver [29], nickel [30], MXene [31], and gold [32]. For the dielectric layer material selection in metamaterial absorbers (MMA) [33; 34], FR4 is the most commonly used due to its excellent compatibility with PCB processes and its strong bonding with copper [35; 36]. Other dielectric materials include ABS [37], PVC [38], PET [39], rubber [40], and polylactic acid [41]. Non-dielectric options for the middle layer can also be used, such as carbon [42] and water [43]. We propose a triple-band metamaterial absorber (MMA) based on the metal-dielectric-metal design concept. The simulation data show that the MMA achieves an absorption performance of at least 97% at three frequency points of 4.23 GHz, 7.403 GHz, and 14.813 GHz. We use the equivalent medium theory to study the impedance and equivalent electromagnetic parameters of the MMA at the absorption frequency point. We find that at the corresponding frequency point, the MMA achieves impedance matching with the vacuum. At the same time, the real parts of the equivalent permittivity and permeability also show consistency.