Objective Long-wave infrared (LWIR, 6-18 μm), as the core band of the atmospheric window, holds significant potential for applications in fields such as infrared imaging, target detection, solar photothermal power generation, and electromagnetic stealth. Traditional infrared absorbing materials, limited by the intrinsic electromagnetic properties of natural materials, generally face technical bottlenecks including narrow bandwidth, large thickness, and angle sensitivity. As an artificial composite material with subwavelength periodicity, metamaterials can break the limit of electromagnetic regulation of natural materials by precisely designing the equivalent permittivity and permeability of the unit structure, thus paving a new way for broadband infrared absorption technology. Researchers have gradually expanded the absorption bandwidth and performance of metamaterial absorbers through strategies such as coupling of multi-resonator units, design of composite dielectric layers, and optimization of topological structures. Nevertheless, the design of broadband metamaterial absorbers still faces three core challenges: first, most design schemes rely on the planar integration of multi-sized resonant units (such as multi-ring and multi-disk structures), and such horizontal integration tends to increase the unit cell size, which is unfavorable for device miniaturization; second, although multi-layer structures can improve absorptivity and broaden the bandwidth to a certain extent through resonant coupling, an excessive number of structural layers complicates the device architecture, hindering its engineering applications; third, the fabrication of some high-performance devices depends on noble metals (e.g., gold and silver), resulting in high manufacturing costs and poor scalability, which limits their large-scale engineering applications. Therefore, this work aims to design a metamaterial absorber with a simple structure, high absorptivity, and broad operational bandwidth, which is expected to have important practical application prospects in infrared imaging, infrared stealth, infrared sensing and other fields.

Methods To address the above issues, a broadband high-absorption embedded multilayer metamaterial absorber is proposed based on the concepts of multimode resonance coupling, composite dielectric layer regulation, and impedance matching theory. First, through the design of multilayer resonant units and embedded cylindrical structures, localized surface plasmon resonance (LSPR), propagating surface plasmon resonance (PSPR) and cavity resonance are simultaneously excited within the subwavelength unit, enabling broadband superposition of multiple resonance peaks. Second, a Ti-Si₃N₄ composite structure is adopted, which introduces a low-permittivity Si₃N₄ layer and a high-loss metal Ti layer. The Ti layer enhances the local confinement of the surface plasmon resonance (SPR) field, while the Si₃N₄ layer broadens the bandwidth through intrinsic absorption and energy coupling effects. Meanwhile, the optimized impedance matching design for the two material layers effectively reduces interface reflection loss. Finally, the centrosymmetric design of the unit structure ensures excellent absorption performance over an incident angle range of 0° to 60°, satisfying the angle adaptability requirements in complex application scenarios. The finite-difference time-domain (FDTD) method is used to analyze the absorption performance and explore the physical mechanisms underlying the broadband high-absorption characteristics, providing a theoretical basis for the design of the absorber.

Results and Discussion The resonant unit of the metamaterial absorber consists of stacked Si₃N₄, Ti, and Si₃N₄ rectangular layers, with Ti cylindrical structures integrated within the upper Si₃N₄ layer. FDTD simulations show that the absorption spectrum of the absorber exhibits absorption peaks of 99.5%, 99.9%, and 99.8% at wavelengths of 8.61 μm, 14.76 μm, and 15.27 μm, respectively. In addition, the absorber maintains an absorption of over 90% in the wavelength range of 7.92 μm to 16.45 μm, corresponding to an absorption bandwidth of 8.53 μm, and exhibits excellent impedance matching within this band. The absorber features polarization insensitivity, with an average absorption of 91.9% across the 0° to 90° polarization angle range. This polarization insensitivity is mainly due to the structural symmetry of the designed absorber, which results in similar optical responses at all polarization angles, thus reducing the influence of polarization angle. Furthermore, the absorber exhibits wide-angle absorption characteristics; under 60° oblique incidence, the average absorptivity for the TM and TE modes remains at 90% and 76%, respectively. Mechanism analysis demonstrates that the broadband and high-absorption characteristics of the absorber mainly arise from the synergistic coupling of LSPR, cavity resonance, and PSPR.

Conclusion Based on multilayer stacked structures and impedance matching theory, a broadband high-absorption metamaterial absorber is designed, whose unit consists of a three-layer Si₃N₄-Ti-Si₃N₄ stack with Ti cylinders embedded in the upper Si₃N₄ layer. The high-loss metal Ti can excite SPR to enhance broadband absorption, and Si₃N₄ exhibits favorable intrinsic absorption and SPR coupling effects in the infrared band; thus, the Ti-Si₃N₄ composite structure achieves improved absorption efficiency and a broadened absorption bandwidth. FDTD simulations show that the designed absorber exhibits excellent absorption performance in the 7 μm to 18 μm band, with an absorption rate exceeding 90% in the wavelength range of 7.92 μm to 16.45 μm, corresponding to an absorption bandwidth of 8.53 μm. The adoption of a symmetric patterned structure design endows the absorber with polarization insensitivity, and the average absorption rate remains at 91.9% at all polarization angles. In addition, the absorber also demonstrates excellent wide-angle absorption performance; under 60° oblique incidence, the average absorption rates of the TM and TE modes reach 90% and 76%, respectively. Mechanism investigation demonstrates that the broadband and high-absorption characteristics of the absorber mainly arise from LSPR excited on the surface of Ti cylinders, cavity resonance between units, and PSPR at metal-dielectric interfaces. The coupling between various resonances further enhances the absorption performance and broadens the absorption bandwidth. This absorber integrates the excellent properties of broadband absorption, high absorption rate, polarization insensitivity, and wide-angle absorption, showing potential application value in fields such as energy harvesting and optical detection. It also provides an important reference for the structural design and performance optimization of broadband high-absorption metamaterial absorbers in the infrared band.