Ultrabroad and Wide‐Angle High‐Temperature‐Stability Infrared Metamaterial Absorber Based on All‐Dielectric Silicon Nitride (Si 3 N 4 )

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Abstract
Ultrabroadband absorption across the long‐wave and ultralong‐wave infrared (LWIR/ULWIR) spectrum is essential for applications ranging from thermal imaging to spectroscopy, yet remains a persistent challenge. Herein, a refractory metamaterial absorber (MMA) composed of four vertically tapered silicon nitride (Si 3 N 4 ) rods on a tungsten substrate is proposed. Finite element method (FEM) simulations show that the MMA achieves an absorbance of over 90% from 7.15 to 31.62 μm, with a relative bandwidth of 126.23%. Notably, the absorbance peak of the MMA is up to 99.97%, 97.92%, 98.42%, and 99.96% at 7.653, 9.045, 16.762, and 26.731 μm, respectively. An equivalent circuit model corroborates these results and elucidates the underlying impedance matching. The ultra‐broadband performance arises from synergistic coupling of multiple resonant modes, including waveguide resonance (WGR) with higher‐order, Fabry–Pérot (F–P) cavity resonance (FPCR), local surface/propagating plasmon resonance (LSPR/PPR), as well as intrinsic losses of Si 3 N 4 material. The MMA also exhibits wide‐angle stability, maintaining high efficiency for incidence up to 50° under both TE and TM polarizations. It's simple, tunable geometry and fully dielectric composition facilitate fabrication and ensure high‐temperature resilience. This design offers a robust platform for advanced infrared systems, including thermal energy harvesting and spectral imaging.

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