# Cascaded Nanorod Arrays for Ultrabroadband, Omnidirectional and Polarization-Insensitive Absorption
## 3 Absorption Performance Analysis  
Figure 2 shows the absorption performance of different CNA-based structures. As indicated in Figure 2a, ultrabroadband absorption enhancement can be obtained for the CNA-based absorber from the visible to the near-infrared region, and three absorption peaks occurred at the locations of 0.524 \(\upmu\)m, 1.001 \(\upmu\)m, and 1.952 \(\upmu\)m. An average absorptivity with A\({}_{av}\) = 98.2% and RAB = 149.8% can be achieved in the 0.38-2.65 \(\upmu\)m wavelength range. Note the RAB of the CNA-based absorber is larger than many types of broadband absorbers such as multi-sized MIM absorbers [16; 17; 18; 19; 20; 21; 22; 23; 24; 25], multi-thickness absorbers [26; 27], and graded HMM absorbers [35; 36; 37; 38; 39; 40; 41; 45; 46; 47]. The RAB of the CNA-based absorber is also comparable  
Figure 1: (a) Sketch map of the CNA-based metasurface for ultrabroadband light absorption. (b) A unit cell of the CNA-based metasurface. with that of multi-sized graded HMM absorbers [48; 49]. Specifically, compared with the similar structure of \(3\times 3\) cascading metal-dielectric pairs, in which the ultrabroadband absorption results from the overlapping of the magnetic resonances of three subunit cells [57], the absorption performance (i.e., RAB, average absorptivity, polarization and angle independent features) of the CNA-based absorber is better because it combines the advantages of LSP and PRC within a comparatively simple and low-cost configuration. However, for the CNA-based structure with \(D_{1}=D_{2}\), the broadband absorption performance of the structure is out of function, and the absorption band with absorptivity above 90% disappears completely in the whole spectral wavelength region. In particular, for the CNA-based structure without top Ti nanorods, the absorption of the structure is significantly reduced in the shorter wavelength region. In Figure 2b, we calculate the contributions of the top Ti nanorods and the rest of the CNA-based metasurface to total light trapping. As shown in Figure 2b, the absorption of the CNA-based absorber mainly results from the multilayer nanorods and substrate, yet a non-ignorable amount of absorption originates from the top Ti nanorods in the region of 0.38-1.5 \(\upmu\)m. Therefore, the top Ti nanorods are essential for the absorption enhancement of CNA-based absorbers in the short wavelength region. To better understand the physical mechanism of the ultrabroadband absorption enhancement of the CNA-based metasurface, the distributions of electric field and energy flows are investigated at the locations of three absorption peaks. As indicated in Figure 3a, for the short wavelength of 0.524 \(\upmu\)m, the electric field is enhanced mainly around the corner of the top Ti nanorods, exhibiting the features of localized surface plasmon (LSP) mode [58; 59]. In addition, the distribution of the Poynting vector shows that energy of the incident light primarily flows and dissipates around the surface of the top Ti nanorods, resulting in the absorption enhancement of the short wavelength. As can be seen in Figure 3c, for the long wavelength of 1.952 \(\upmu\)m, the electric field is mainly enhanced and localized in the narrow cavities formed by the adjacent multilayer nanorods, showing the features of plasmonic resonant cavity (PRC) mode [60; 61; 62]. The energy flow of the incident light propagates along the \(-z\) direction, and the energy of light is mainly confined and consumed by the narrow cavities and the metallic substrate. As shown in Figure 3b, for the middle wavelength of 1.001 \(\upmu\)m, the electric field is enhanced by the top Ti nanorods and the narrow cavities simultaneously, and the energy flows are trapped by both of them, indicating the hybrid LSP and PRC modes. Therefore, the ultrabroadband absorption of the CNA-based metasurface originates from the synergistic effect of the LSP and PRC modes.