# Cascaded Nanorod Arrays for Ultrabroadband, Omnidirectional and Polarization-Insensitive Absorption
## 4 Results and Discussions  
To further evaluate the robustness of the absorption performance of the CNA-based absorber, we first investigated the influence of the height and diameter of the top Ti nanorods on absorption spectra of the CNA-based metasurface. As indicated in Figure 5a, the absorption performance of the CNA-based metasurface is almost immune to the variation of the height of the top Ti nanorods, and ultralbroadband absorption can be maintained unless the top Ti nanorods are totally eliminated. As seen in Figure 5b, the overall absorption of the CNA-based metasurface can be improved with the increase of the diameter of the top Ti nanorods, and the absorption performance will be saturated as \(D_{1}=140\) nm. Increasing the diameter of the top Ti nanorods will deteriorate the absorption performance of the structure as \(D_{1}>140\) nm, and the ultrabroadband absorption performance will be cancelled as \(D_{1}=D_{2}=276\) nm. However, excellent absorption performance of the structure can be maintained around \(D_{1}=100\) nm even when the diameter of the top Ti nanorods has been changed dramatically. Figure 6 shows the influence of the buffer layer on absorption performance of the CNA-based metasurface. As shown in Figure 6a, the variation of \(t_{b}\) almost does not change the absorption performance of the structure in the longer wavelength region, but the absorption in the shorter wavelength region will be slightly altered as \(t_{b}\) is varied. Because the absorption enhancement of the  
Figure 4: (**a**) Impedance and reflection curves of the CNA-based metasurface. (**b**) FDTD result and theoretical results of absorption spectral of the CNA-based metasurface. Figure 5: Influence of the top Ti nanorods on absorption performance of the CNA-based metasurface. Other parameters are the same as those in Figure 2b. (**a**) Absorption as a function of the height of the top Ti nanorods. (**b**) Absorption as a function of the diameter of the top Ti nanorods. longer wavelength primarily results from the PRC mode confined between the multilayer nanorods, the absorption performance is almost independent of \(t_{b}\). However, the variation of \(t_{b}\) will affect the electromagnetic coupling between the top Ti nanorods and the multilayer nanorods, and the LSP mode confined by the top Ti nanorods will be slightly changed as well. As a result, the corresponding absorption will be slightly altered as \(t_{b}\) is varied. In Figure 6b, it can be seen that the absorption of the CNA-based metasurface is insensitive to the variation of the refractive index of the buffer layer. Here, although five arbitrary dielectric materials, such as MgF\({}_{2}\) (\(n\) = 1.38), SiO\({}_{2}\) (\(n\) = 1.47), Al\({}_{2}\)O\({}_{3}\) (\(n\) = 1.77), HfO\({}_{2}\) (\(n\) = 1.97) and TiO\({}_{2}\) (\(n\) = 2.50), are used as the buffer layer, the ultrabroadband absorption of the structure can be maintained in the whole wavelength region. As can be seen in Figure 3, because the electric field related to absorption wavelength is highly confined by the top Ti nanorods and the cavities of the adjacent multilayer nanorods, the variation of the refractive index of the buffer layer has little influence on the absorption performance of the CNA-based metasurface. This will facilitate the selection of materials in application. Figure 7 shows the influence of the multilayer nanorods on absorption performance of the CNA-based metasurface. As indicated in Figure 7a, the absorption of the structure can be enhanced with the increase of \(D_{2}\) in the longer wavelength region, and its absorption will be saturated as \(D_{2}\) = 276 nm. In particular, as \(D_{2}\) is increased to 300 nm (\(D_{2}\) = \(P\)), the narrow cavities that can support the PRC mode between the neighboring multilayer nanorods disappear. Thus, the absorption is reduced in the longer wavelength region.