Design and Refractive Index Sensing Investigation of a Multi-Mode Switchable VO₂-Graphene Terahertz Absorption Device

Terahertz waves combine the dual characteristics of electronics and photonics and boast tremendous application potential in fields including nondestructive testing, telecommunications and biochemical sensing. Natural materials generally interact weakly with terahertz waves. In contrast, the reversible metal-insulator transition of VO₂ and the tunable Fermi energy of graphene provide an effective route for developing dynamically reconfigurable terahertz devices. Existing VO₂-graphene hybrid absorbers generally suffer from limitations such as single absorption mode, unsatisfactory absorption performance and complex multi-layer configurations. In this work, a reconfigurable five-layer terahertz metamaterial absorber supporting three operating modes is proposed. It consists of a patterned graphene top layer, an embedded patterned VO₂ layer, and a bottom metallic reflector that suppresses terahertz transmission. The study is primarily conducted using the electromagnetic simulation software CST, and the physical mechanism behind the absorption peaks' formation is elucidated via impedance matching theory and electric field energy distribution profiles. Simulation results demonstrate that the device supports three operating modes-dual narrowband, single narrowband and broadband-with peak absorptivity above 99% in each mode. The absorption spectral response can be dynamically modulated by regulating the Fermi energy level of graphene and modifying the material properties of VO₂ via thermal regulation. Meanwhile, the proposed absorbing device sustains robust high-absorption performance across polarization orientation angles of 0-80° and incident angles of 0-50°. The dual-narrowband and single-narrowband modes are applicable to refractive index sensing, delivering a maximum sensitivity of 1750 GHz/RIU and a figure of merit (FOM) as high as 5.13. Finally, compared to existing similar devices, the proposed five-layer structure integrates multi-mode switching capability and high sensitivity, thus presenting promising application prospects. The fabrication of the patterned graphene and VO₂ layers at different planes, however, still requires careful multilayer alignment and should be considered in practical implementation.