Two types of ultra–broadband long wavelength infrared (LWIR) absorbers with small period and super thin thickness are designed. The absorption with high absorptivity and large bandwidth is achieved through combined propagating and localized surfaced plasmon resonances. We first design a three-layer absorber with a Ti–Ge–Ti configuration, the period of the structure is only 1.4 µm (nearly 1/8 of the center wavelength), the thickness of its dielectric is only 0.5 µm (1/22 of the center wavelength), and the average absorption is 87.9% under normal incident from 8µm to 14µm. Furthermore, the four-layer absorber with a Ti–Ge–Si3N4–Ti configuration is designed to obtain more average absorption increasing to 94.5% from 8 µm to 14µm under normal incident, the period of the structure increases to 1.6 µm and the total thickness of dielectric increases to 0.6µm. The proposed absorber is polarization–independent and possesses a good tolerance of incident angle. We calculate that the average absorption of the four-layer absorber for both TE– and TM–modes still exceeds 90% up to an incident angle of θ = 40° (90.7% for TE–mode, 91.9% for TM–mode), and exceed 80% up to an incident angle of θ = 60° (80.2% for TE–mode, 82.1% for TM–mode).
A metamaterial absorber is proposed that functions in the medium‐ (3–5 µm) and long‐wavelength (8–12 µm) infrared (medium‐wavelength infrared, MWIR, and long‐wavelength infrared, LWIR, respectively) regions. The proposed design, which consists of periodic cells, can be tuned to achieve single‐band or dual‐band light absorption by changing the periodicity of the structure. Each cell forming the metamaterial absorber consists of a bottom metal plate (Al), a top metal disk (Ti), and an intermediate dielectric medium (Si or ZnS) in which a metal disk (Ti) is embedded. For a period of 0.85 µm, the absorber achieves broadband absorption in the LWIR region, with an average absorption of 92.1%. Further, the absorber shows acceptable tolerance to irradiation at oblique incidence. For a period of 2 µm, a peak absorption of 99.05% is achieved in the MWIR region, thereby providing dual‐band absorption. Tuning the periodicity of the structure enhances the localized surface plasmon resonance, with the absorption mechanism explained by establishing an equivalent parallel LC circuit. The absorption properties demonstrated by the proposed metamaterial absorber are promising for thermal imaging and infrared spectroscopy.
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