Multi-octave metasurface-based refractory superabsorber enhanced by a tapered unit-cell structure

n ultra-broadband metasurface-based perfect absorber is proposed based on a periodic array of truncated cone-shaped TiO_2 surrounded by TiN/ TiO_2 conical rings. Due to the refractory materials involved in the metasurface, the given structure can keep its structural stability at high temperatures. The proposed structure can achieve a broadband spectrum of 4.3 µm at normal incidence spanning in the range of 0.2–4.5 µm with the absorption higher than 90% and the average absorption around 94.71% . The absorption can be tuned through the angle of the cone. By optimizing geometrical parameters, a super absorption is triggered in the range of 0.2–3.25 µm with the absorption higher than 97.40 % and substantially average absorption over 99 % . In this regard, the proposed structure can gather more than 99% of the full spectrum of solar radiation. Furthermore, the absorption of the designed structure is almost insensitive to the launching angle up to 50^∘ for TE polarization, while it has a weak dependence on the incident angle for TM polarization. The proposed structure can be a promising candidate for thermal energy harvesting and solar absorption applications.