# Cascaded Nanorod Arrays for Ultrabroadband, Omnidirectional and Polarization-Insensitive Absorption
## 1 Introduction  
Plasmonic absorbers can function as crucial components for various applications such as sensing [1, 2], photodectors [3], thermal emitters [4, 5], photovoltaics [6, 7], etc. In the past decade, the metal-insulator-metal (MIM) nanopatterned structures are the main schemes to achieve perfect light absorption enhancement [8]. In this configuration, strong electromagnetic coupling could be excited between the metallic pattern and the metallic substrate in a small wavelength range due to magnetic or electric resonance. By choosing different shapes of metal patterns such as strip [9, 10], square [11, 12], hole [13], disk [14] and # [15], perfect absorption can be obtained as impedance matching is satisfied at a specific wavelength. However, the resonant characteristics of the conventional MIM absorbers result in narrow bandwidths, which may limit their applications in the fields of IR cloaking, solar cells, and imaging. To meet the demand of broadband absorption, some broadband absorbers have been proposed by using the concept of a superlattice. By integrating multi-sized resonators such as strips [16, 17], squares [18, 19, 20, 21], disks [22, 23], and crosses [24, 25] into the unit cell, or using multi-thickness metasurfaces [26, 27], multi-band or broadband absorption can be obtained due to theoverlapping of multiple resonance frequencies. Unfortunately, the bandwidths of these absorbers are limited by the possible number of resonators that can be integrated into the subwavelength superlattice. In recent years, hyperbolic metamaterials (HMMs) have become an active research area due to exotic hyperbolic dispersion, which originates from one of the principal components of their electric or magnetic effective tensor having the opposite sign to the other two principal components [28]. It has been shown that HMMs can be used to improve light absorption due to their high-wavevector propagating modes and enhanced photonic density of states [29; 30]. In general, a thin-film multilayer consisting of alternating metal/dielectric layers will give rise to hyperbolic dispersion because the layer thicknesses can be ignorable compared with the operating wavelength. By using hyperbolic metal/dielectric multilayers [31; 32], nanostrips [33], or nanopillars [34], light absorption can be efficiently enhanced in a small wavelength region. To broaden the absorption bandwidth, more complicated HMM nanostructures with graded metal/dielectric patterns, such as sawtooths [35; 36; 37; 38; 39; 40], pyramids [41; 42; 43], and nano-cones [44; 45; 46; 47] are required. Similarly, by integrating multi-sized, graded HMM patterns into the unit cell, the absorption bandwidth of the graded HMM absorbers can be further expanded [48; 49]. Generally, the physical basis for the intriguing ultrabroadband absorption of these HMM-based absorbers is associated with the slow-light effect. As the incident light is coupled into the taped HMM-based waveguide, the group velocity of the waveguide mode at different wavelengths will be reduced dramatically at their corresponding critical widths, which enables the collective excitation of slow-light waveguide modes over a broad wavelength region, also known as the trapped rainbow phenomenon [50; 51]. Although the achieved absorption results of the HMM-based absorbers are remarkable, their fabrication process is quite challenging due to the graded metal/dielectric patterns with a large number of film stacks. Furthermore, despite the elaborate study of nanostructured plasmonic absorbers, absorption performance that combines the advantages of high absorption efficiency, ultrabroad bandwidth, and omnidirectional characteristics is still highly desired. Inspired by these earlier works, here we demonstrate a novel ultrabroadband, omnidirectional, and polarization-insensitive absorber based on cascaded nanorod arrays (CNAs).