## 1 Introduction  
In recent years, terahertz (THz) waves gained popularity due to their applications in wireless communication, sensing, and imaging. [1; 2; 3; 4]. To meet the challenges posed by the emerging THz applications, several metamaterial-based devices were suggested, including filters [5; 6; 7], absorbers [8; 9; 10], polarity converters [11], and others. Metamaterial perfect absorbers are one of these valuable gadgets that play a central role in many THz applications such as emitters of heat [12; 13], solar-powered cells [14; 15], and stealth technology [16; 17]. Therefore, realizing ultra-wideband efficient absorbers has always been of profound interest to the research community and industry. In this regard, studies in the literature suggested multi-band, dual-band, and narrow-band absorbers [18; 19; 20; 21]. Aside from a specific absorption bandwidth, these metamaterial perfect absorbers (MPAs) are inflexible in their electromagnetic responses. It is demonstrated that different sized resonant structures in a single unit cell can be used to produce broadband absorption [22; 23; 24]. Another alternative is a multilayer construction with different thickness dielectric layers [25; 26; 27]. However, MPAs made with the earlier methods are difficult to realize and control [28; 29; 30; 31]. Several progress reports integrate metamaterials with semiconductors, graphene, and liquids crystals, while others use phase transition materials [28; 29; 30] to achieve active control.  
The insulating material can be transformed into metallic by using electrical, thermal, or optical stimuli, such as UV light. The rapid reaction time and wide modulation depth of VO\({}_{2}\) make it more suitable for integrating into tunable absorbing structures. Various VO\({}_{2}\)-based absorbers with broadband and adjustable absorption capabilities were recently discovered. Several studies demonstrate that electrical [32; 33; 34], thermal [35; 36], or optical excitation [37; 38] can induce the phase shift. Using metamaterials with VO\({}_{2}\) seems to be a potential approach to achieving THz re-configurability [39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50]. The electrical conductivity of vanadium dioxide (VO\({}_{2}\)), a phase transition material, can alter by several magnitudes as the material goes through its phase transition [51; 52]. In this regard, a tunable broadband terahertz absorber constructed of four identical vanadium dioxide (VO2) square loops with a 2.45 THz absorption bandwidth is presented in [53] under normal incidence. A quatrefoil and circle-loaded complementary square split ring resonator (CSSRR) are presented in [54], which show two peaks at 0.88 and 1.42 THz. The in-plane and out-of-plane components of its surface susceptibility tensor can be reliably determined with the optical detection of the susceptibility tensor in two-dimensional crystals, as presented in [55]. Table 1 has summarized MPAs with broadband and adjustable VO\({}_{2}\) absorption properties (see [39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 52; 53; 54] in Table 1).  
This paper proposes a tunable ultra-wideband absorber composed of a three-layer (VO\({}_{2}\)/quartz/Au) configuration. The absorber achieves excellent absorption of approximately 90% in the range of 2.7 THz-5.7 THz. The absorptivity of this absorber varies between 4% and 100% with the changing conductivity of VO\({}_{2}\). According to the comparative Table 1, the absorber performs much better than the other VO\({}_{2}\)-based absorbers in terms of operating band and efficiency. Power loss density and impedance-matching theory are applied to elaborate on the physical origin of ultra-wide absorption.