## 1 Introduction  
Terahertz (THz) waves generally refer to electromagnetic waves with a frequency ranging from 0.1 to 10.0 THz. They have a wide range of applications in the fields of wireless communication, sensors and imaging [1; 2; 3]. Because of their bright future, they have aroused great interest in the past few decades. With the rapid development of THz technology, the performance requirements of related functional devices are gradually increasing. Traditional natural materials have inherent shortcomings such as performance limitations, bulky and difficulty of integration, which can no longer meet the current needs for optic applications. Metamaterials (MM) composed of periodic subwavelength antenna arrays can solve the above problems. Because MM have the advantages of high efficiency, tunability and easy integration, various functional devices based on MM have been proposed, such as optical filters, absorbers and polarization converters [4; 5; 6; 7; 8; 9]. Among various devices, the metamaterial perfect absorber (MPA) has received widespread attention due to its significant role in imaging, stealth technology and sensing applications [10; 11; 12]. The first MPA working in the microwave band was proposed by [PERSON] et al. in 2008 [13]. A metal-insulator-metal (MIM) configuration is used to demonstrate MPAs. The basic unit structure of MPAs usually consists of three functional layers: a patterned metal layer on the top layer, a metal layer on the bottom layer and a dielectric layer in the middle. Under the guidance of this design, many types of MPAs have been reported, and the research on them is not limited to the microwave frequency range but gradually extends to all spectra related to technology, including THz, infrared, visible light and ultraviolet ranges [14; 15; 16; 17; 18; 19; 20]. So far, various MM have realized absorbers that work at THz frequencies. However, once the structure of the traditional MM absorber is determined, the electromagnetic response cannot be adjusted, and it has a fixed peak frequency and absorptance, which limits their practical application. MPAs based on semiconductors, graphene and liquid crystals have been reported to realize the dynamic tunable characteristics [21; 22; 23]. Vanadium dioxide (VO2), as a phase change material, varies greatly during the phase transition from insulator to metal by changing the temperature [24]. This is caused by the transformation of the structure from the insulating monoclinic phase to the metallic tetragonal phase around 68 \({}^{\circ}\)C. Therefore, combining MM with VO2 is a promising method to achieve dynamic tunable characteristics in the THz range. In 2018, [PERSON] et al. presented an active absorption device based on VO2 metamaterials, 90% absorptance bandwidth reaches 0.33 THz with a central frequency of 0.55 THz, and absorptance at peak frequencies can be continuously tuned from 30% to 100% [25]. In order to improve operation bandwidth, in 2019, [PERSON] et al. designed a broadband THz absorber that the bandwidth of absorption rate above 90% is 1.25 THz, and the absorption rate can be continuously tuned from 15% to 96% [26]. To achieve multi-band absorption, in 2020, [PERSON] et al. presented a dual broadband absorber, the bandwidths with the absorption above 80% were 0.88 THz and 0.77 THz in the frequency ranges of 0.56-1.44 THz and 2.88-3.65 THz, and the absorptance can be continuously adjusted from 20% to 90% [27]. Although the performance of absorbers based on vanadium dioxide metamaterials has been improved, the characteristics of the broad bandwidth, multi-band and active control still require more research to meet the expectations of practical applications. For the classic three-layer structure absorber, the material and the structure of the top layer design are the most critical parts. In this work, we propose a tunable dual broadband THz absorber, which consists of a VO2 structure with two perpendicularly intersecting ellipses and a circular hole in the center, and a metal ground plane separated by a dielectric spacer.