# Cascaded Nanorod Arrays for Ultrabroadband, Omnidirectional and Polarization-Insensitive Absorption
## 4 Results and Discussions
In Figure 7b, it can be seen that there is no obvious absorption enhancement for the longer wavelength as \(N\) = 1; this is because the thickness of the multilayer nanorods is too small to support the PRC mode. Perfect absorption performance of the structure can be achieved with \(N\) = 5, and further increasing \(N\) will cause a decrease of the overall absorption in the longer wavelength region. Because the increase of \(N\) will increase the overall reflection of the structure, and the absorption of the structure is reduced as well, there is a trade-off between the absorption performance and the number of the multilayer nanorods. Note the absorption mechanism of the CNA-based metasurface is different from those of HMM-based absorbers [33; 34], in which the absorption is improved by increasing the number of the metal/dielectric stacks. In fact, the CNA-based metasurface required only several film stacks to achieve the ultrabroadband absorption. Thus, it is also an ultracompact absorber with a total thickness of 385 nm as \(N\) = 5. Figure 6: Influence of buffer layer 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 thickness of buffer layer. (**b**) Absorption as a function of the refractive index of buffer layer. Figure 8 shows the influence of period and material on the absorption performance of the CNA-based metasurface. As shown in Figure 8a, the absorption performance is robust to the variation of the period, and ultrabroadband features are retained even when the period is significantly altered. However, the variation of the period will change the width of the cavity between the neighboring multilayer nanorods, and the PRC mode confined in the cavity will be affected as well, thus the absorption in the longer wavelength is slightly altered as \(P\) is varied. In Figure 8b, three different metallic materials (Cr, Ni, Fe) and two different dielectric materials (MgF\({}_{2}\), HfO\({}_{2}\)) are selected to evaluate the absorption performance of the CNA-based metasurface; the dielectric constants of Cr, Ni, and Fe were obtained from Palik [52]. As shown in Figure 8b, ultrabroadband absorption performance can be maintained even if different metal/dielectric materials are chosen with the structural parameters kept the same. Therefore, the CNA-based metasurface may provide a general architecture to realize ultrabroadband absorption with high absorption efficiency based on low-cost metallic materials. Finally, we evaluate the angular robustness of the proposed CNA-based absorber. As shown in Figure 9, the CNA-based absorber exhibits a nearly omnidirectional absorption performance for both the TM and TE (electric field along \(y\)-axis) polarizations. For the TM polarization, ultrabroadband absorption enhancement can be kept almost the same even if the incident angle is significantly altered,  
Figure 8: Influence of period and material 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 period. (**b**) Absorption as a function of different metal/dielectric materials. Figure 7: Influence of multilayer 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 diameter of multilayer nanorods. (**b**) Absorption as a function of the number of film pairs of multilayer nanorods. and an average absorptivity of 87.6% can be achieved in the range of 0.38-2.65 \(\upmu\)m even if the incident angle is increased to 60\({}^{\circ}\). In particular, for the TE mode, the average absorptivity can be kept at 90.0% in the range of 0.38-2.65 \(\upmu\)m as the incident angle is increased to 60\({}^{\circ}\).