Small-period Ultra-broadband Long-wavelength Infrared Metamaterial Absorber

In this paper, we describe the design of a small-period ultra-wideband long-wavelength infrared metamaterial absorber. The two-layer metal (titanium) resonators and the underlying metal reflective layer are vertically stacked, and the layers are filled with a dielectric (silicon), where the two cross-shaped resonators have slightly different sizes. The absorption spectrum of the structure reaches absorbance values above 90% in the wavelength interval of 8 μm and 13.15 μm in the case of normal incidence. Through a field distribution analysis, we found that the propagation surface plasmons and localized surface plasmons excited by titanium resonators caused two broad absorption peaks (8.5 μm and 12.2 μm), which were close to each other, to obtain the final broadband absorption. Furthermore, the thickness of the structure is less than 1/13 of the centre wavelength (11 μm), and the period is only 1/5 of the centre wavelength (11 μm), which makes the structure easy to integrate on small-pixel detectors. Therefore, such designs may be useful in infrared imaging, thermal detection, and thermal emitters.