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High-performance optical absorbers have garnered great interest because of their wide applications in solar thermophotovoltaic systems, sensors, photo-thermal modulators, etc. In this work, we experimentally realize an omnidirectional ultra-broadband metamaterial absorber with a 90% absorption bandwidth from 440-2124 nm and an average absorption higher than 95%. The peak absorption reaches up to 99.57%. Apart from multiple substructures in the pattern layer, which preliminarily broadens the absorption bandwidth due to the corresponding multiple resonance modes, a silica anti-reflection layer is additionally coated on the top to manipulate the impedance of the nanostructure to match that of the air. Physical mechanism is discussed in detail in the paper. With the assistance of the anti-reflection coating, the proposed nanostructure exhibits an increase of the absorption bandwidth by more than 590 nm while maintains high average absorptance. The anti-reflection layer can be replaced with silicon nitride to broaden the absorption bandwidth further. Nearly angle-independent spectral performance for both TE and TM polarizations are observed in both case. Moreover, for the fabrication, only one electron beam lithography is required to construct the metasurface atoms, eliminating the process of multiple lithography or pattern alignment. The proposed absorber shows a promising prospect in versatile utilizations such as solar energy harvesting, thermal emitters, photodetectors, and so on.
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