A multifunctional terahertz device based on vanadium dioxide metamaterials that switches between ultra-broadband absorption and ultra-high-<i>Q</i> narrowband absorption

Terahertz (THz) absorbers with ultra-broadband and ultra-narrowband absorption capabilities are crucial for integrated and efficient terahertz modulation. This study proposes a dual-mode tunable terahertz absorber based on the phase transition characteristics of vanadium dioxide (VO₂), enabling dynamic switching between narrowband and broadband absorption through its insulating-to-metallic transition. In the insulating state, the excitation of quasi-bound states in the continuum (Q-BIC) resonance <i>via</i> geometric parameter modulation of silicon pillars is investigated, with its physical mechanism elucidated <i>via</i> impedance matching theory and multipole analysis. This mode demonstrates exceptional sensing performance at 8.017 THz: a refractive index sensitivity of 3.735 THz RIU^-1, a quality factor (<i>Q</i>) of 4800.89, and a figure of merit (FOM) of 3822.93 RIU^-1. When VO₂ is transformed into the metallic state, the device achieves more than 90% ultra-broadband absorption in the range of 3.93 THz to 9.25 THz, and its broadband absorption properties originate from the electric dipole resonance. In addition, the performance of the device remains stable at different structural parameters. Compared to existing technologies, this design integrates dual functionalities in a single-layer hybrid structure, significantly reducing fabrication complexity.