Ultra-broadband infrared metasurface absorber: comment.

In a recently published report, an ultra-broadband gradient-metasurface-based absorber (GMBA) with a single layer of metasurface is proposed by Guo et al [Opt. Express24(18), 20586 (2016)] to realize the ultra-broadband perfect absorption. Moreover, the bandwidth of absorption can be broadened by increasing a layer metasurface on the basis of single-layered GMBA. This comment demonstrates that the cross-polarization reflection is neglected by the authors, when they calculate the total absorption in the proposed GMBAs. It is found that there are only two absorption peaks at 8.09 μm and 9.84 μm with real absorption rates of 58.4% and 57.1% in a single-layered GMBA.