THz waves, spanning from 0.1 to 10 THz, occupy the electromagnetic spectrum between microwaves and visible light. Renowned for their high resolution, ample information capacity, and facile beam focus, THz waves find applications in communication, radar, detection, and beyond [1], [2], [3], [4]. Unfortunately, the innate limitations of natural materials in interacting directly with THz waves have hindered the progress of THz technology. The introduction of metamaterials, artificial materials with distinctive electromagnetic properties, has revolutionized this landscape. Comprising periodic unit cells with unique physical characteristics absent in conventional materials, metamaterials have paved the way for a plethora of THz devices with diverse functionalities, including filters, absorbers, and polarization converters [5], [6], [7], [8], [9], [10], [11]. This metamaterial paradigm has injected vitality into THz technology, enabling the development of high-performance, multifunctional devices. Presently, researchers are concentrating on the integration of multiple functions into a single structure, aligning with the trend of multifunctional and switchable THz metamaterial devices.

An effective strategy in designing such metamaterial devices involves incorporating tunable materials like graphene, liquid crystal, and VO2 [12], [13], [14]. The functions and performance of these devices can be tailored through external excitation. In 2020, Song et al. introduced a switchable metasurface based on VO2, enabling seamless transitions between broadband absorption and broadband polarization functions [15]. In 2022, Yang et al. proposed a THz reconfigurable metasurface based on VO2, facilitating dynamic switching of reflection beam splitting, reflection focusing, vortex beam control, and narrowband absorption functions [16]. Subsequently, in 2023, Xiao et al. presented a multifunctional tunable VO2-based coding metasurface, achieving dynamic tunability of reflected beams [17]. As for coding metasurface, Cui et al. first proposed the concepts of digitally coding metasurface and programmable metasurface in 2014 [18]. Based on this coding metasurface concept, the modulation of the metasurface reflection states is achieved by a coding sequence of discrete phase units, enabling functionalities such as focusing [19] and beam steering [20]. As a unique phase-change material, VO2 undergoes a transition from the insulating state to the metallic state with changes in external conditions such as temperature, voltage, and light. This transition occurs at approximately 68 °C, and the reverse process occurs when cooled to room temperature [21], [22]. Leveraging these characteristics, VO2 presents a promising avenue for designing multifunctional integrated metasurface devices.

While many studies have explored devices achieving either broadband absorption or beam steering, few have investigated establishing a relationship between these two functions within the same structure. This study introduces a bifunctional VO2-based THz metasurface. The device transitions from broadband absorption to dual-beam, four-beam, and anomalous reflection as VO2 shifts from the insulating to the metallic state. Moreover, adhering to the principles of phase coding, different beam forms can be realized by altering the coding form and sequence of the unit cell. This approach enhances the flexibility of THz wave modulation, offering a novel perspective for creating versatile THz metasurface devices.