# Numerical Study of an Ultra-Broadband All-Silicon Terahertz Absorber
## 1 Introduction  
Terahertz (THz) waves refer to the electromagnetic waves whose frequency ranges are between 0.1 and 10 THz [1]. Owing to the unique properties of THz radiation, including its capacity for increased penetration compared to visible light in many dielectrics [1], low photon energy [2], and high spectral-resolving ability, it is extensively used in various fields, such as in THz imaging, explosives detection, communication, food quality control, and others [3; 4; 5; 6]. Metamaterials are a new kind of artificial material, which are cell structures periodically arranged by subwavelength dimension [7; 8]. Metamaterial-based absorbers can absorb incident light efficiently and convert it into ohmic heat or other forms of energy. Because only a few substances in nature respond to THz frequencies [9], it is possible to achieve efficient reactions with artificial metamaterials by designing the structural units. Silicon is a common semiconductor and has many advantages, including its high electron mobility, easy integration, and low cost. In 2008, [PERSON] et al. designed a \"perfect\" metamaterial absorber that achieved nearly 100% absorption in the GHz frequency band. Later in the same year, [PERSON] et al. proposed an absorber with a metal ring resonator and a split wire separated by a dielectric, whichwas supported by gallium arsenide (GaAs). This proposed absorber, which operated in the THz band on the basis of [PERSON]'s findings, achieved a nearly perfect absorption in this band for the first time, and reached a structure at the micron scale [10]. In 2009, [PERSON] et al. designed an absorber with a modified electrically coupled ring resonator with a cross structure that realized \(\sim\)95% absorption at 1.13 THz [11]. The proposed absorber created new research opportunities in the research of metamaterial absorbers [12; 13; 14]. These characteristics led to the research boom of the THz absorber [10]. In subsequent studies, various schemes were proposed for perfect absorption based on metamaterials with operations which ranged from the microwave to THz [11; 15; 16], infrared [17; 18; 19; 20], and in visible wavelength bands [21; 22]. In order to achieve perfect absorption in the visible and infrared wave bands, the biggest challenge was to make the size of the unit structure smaller than the wavelength of the incident electromagnetic waves. Nanometer lithography technology can realize the fabrication of devices, but it is difficult to guarantee the machining accuracy. In 2017, [PERSON] et al. proposed 2D all-metal gradient nanostructures that could realize \(>\)90% absorption over a wide wavelength band of 378-626 nm [23]. This all-medium absorber greatly simplified the manufacturing process. In the same year, [PERSON] et al. designed a metal stripe placed on a thin SiO\({}_{2}\) layer, which was supported by the metal-dielectric layer. This absorber, with metal resonator, could retain absorption above 95% at the wavelength range of 400 to 1500 nm [24]. In 2018, [PERSON] et al. proposed an ultra-broadband absorber based on a metamaterial nanostructure composed of a periodic array of titanium-silica (Ti-SiO\({}_{2}\)) cubes and aluminum (Al) bottom films that showed an average absorption of 97% from 354 to 1066 nm [25]. Traditional absorbers based on metal-dielectric-metal sandwiched structures mainly achieve single-frequency and multiple-band absorption. Because of the complexity of this multilayered structure, the manufacturing process is complicated, and the cost of fabrication is expensive. Recently, all-dielectric broadband THz metamaterial absorbers have become a research hotspot [26]. In 2014, [PERSON] et al. designed an all-silicon circular groove absorber that achieved a bandwidth of 360 GHz above 90% absorbance [27]. In 2018, [PERSON] et al. proposed an all-silicon H-shaped microstructure with two rectangular cavities on each sidebar based on a substrate that achieved the peak absorption absorbance \(\sim\)99.99%, with a bandwidth of \(\sim\)900 GHz for 90% absorbance [26].