# Terahertz Broadband Adjustable Absorber Based on VO2 Multiple Ring Structure
## 1 Introduction  
In recent years, metamaterials composed of artificial subwavelength cells have attracted extensive attention from many researchers. A great variety of functional devices stemming from metamaterials have promoted the development of electromagnetic wave control technology, because of their supernormal electromagnetic properties [1; 2; 3; 4; 5] that differ from natural materials [6; 7; 8; 9; 10]. The metamaterial absorber is an important field of metamaterial research, which can realize perfect absorption through electromagnetic resonance under the action of incident waves, causing significant energy loss. Since [PERSON] et al. [11] first designed and validated the perfect metamaterial absorber in the microwave frequency band in 2008, the perfect metamaterial absorbers have played a significant role in the field of optics and even THz. At present, the broadband metamaterial absorber mainly realizes the superposition of multiple resonance absorption modes through the multilayer resonant structures so as to achieve broadband absorption [12; 13; 14; 15; 16].  
The metamaterial perfect absorbers are widely used in THz applications [17; 18; 19; 20; 21], especially a variety of tunable metamaterial perfect absorbers based on liquid crystals, graphene, semiconductors, and phase-change materials have been shown to obtain excellent reconfigurable properties [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38]. Among these materials, the phase-change material of VO2 has the advantage of rapid modulation characteristics, which can achieve great transformation of conductivity at 340 K [39]. Using this characteristic of VO2, researchers have developed broadband or even dual-band adjustable absorbers. In 2020, [PERSON] et al. reported a tunable dual broadband terahertz absorber based on the VO2 structure with two groups of symmetric elliptical hollow structures, and its two bandwidths of 90% absorption were 2.32 THzand 2.03 THz, respectively. In 2020, [PERSON] et al. proposed a metal-dielectric-phase material tunable broadband absorber, whose top structure was made up of four identical VO2 rectangle rings, and the absorption bandwidth reached 2.45 THz. In 2022, [PERSON] et al. presented a broadband terahertz absorber based on a classical three-layer structure, with the absorption bandwidth up to 3.78 THz. However, although some metamaterial perfect absorbers based on VO2 have shown broadband absorption characteristics in recent years, their bandwidth and adjustable range still need to be further expanded.  
Aiming at addressing the above problems, this paper proposes a THz ultra-wideband tunable absorber based on a VO2 multiple square ring structure, which leaves a gap with a certain thickness in the middle of the ring to enhance the resonant absorption effect. The absorption bandwidth is up to 4.26 THz, which is broader than that of other absorbers, as shown in Table 1. The absorber is not only simple and convenient for experimental preparation, but can also control its absorption intensity through temperature regulation. It has a broadband absorption bandwidth in the metal phase of VO2, and has polarization and incidence-insensitive absorption properties.