Terahertz (THz) technology holds immense potential in wireless communication, sensing, and biomedical applications, yet its development is hindered by the limitations of conventional metamaterials, such as fixed resonance frequencies, polarization dependence, and environmental inflexibility. To address these challenges, we proposed a novel polarization-independent dual-mode THz metamaterial integrating graphene and vanadium dioxide (VO₂). The primary objective is to achieve switchable functionalities between triple plasmon-induced transparency (PIT) and quadruple narrowband perfect absorption in the THz regime, while ensuring polarization insensitivity and dynamic reconfigurability. This design overcame the shortcomings of previous works, such as polarization-dependent responses, structural complexity, and limited operational bandwidth, thereby advancing the development of multifunctional photonic platforms for applications in optical switching, sensing, and slow-light systems.

In this paper, the proposed metamaterial comprises a vertically stacked structure with patterned graphene arrays (square and circular rings), a VO₂ phase-change layer, and dielectric spacers (SiO₂ and ion-gel). The graphene layer was patterned into concentric nested square and circular rings (G1 and G2), with optimized geometric parameters (period P=6 μm, square side length l=5.09 μm, width w=1 μm, and ring radii R₁=1.10 μm and R₂=0.81 μm). The VO₂ layer, acting as a dynamically tunable medium, enabled dual-mode operation through its insulator-to-metal phase transition. In the insulating state (σ=10 S/m), the structure supported triple PIT via bright-bright mode coupling between graphene resonators. In the metallic state (σ=200000 S/m), the device transitions to a quadruple narrowband perfect absorber. The finite-difference time-domain (FDTD) method with periodic boundary conditions and perfectly matched layers was used to investigate electromagnetic responses within the structure. Coupled mode theory and impedance-matching analysis were employed to explain the underlying physical mechanisms. Key parameters, including graphene’s Fermi level (E_F) and VO₂’s conductivity (σ), are independently and dynamically modulated via gate voltage and Joule heating, respectively.Results and Discussions We revealed the multifunctional electromagnetic response characteristics of the proposed structure under different operational states. When VO₂ is in the insulating state (σ=10 S/m), the device exhibits triple PIT with transmission dips at 3.90, 3.36, 5.01, and 6.83 THz (Fig. 2). The coupling of three bright modes from square-ring (G1) and one from circular-ring (G2) creates distinct transparency speaks with strong dispersion, resulting in a significant slow-light effect. A maximum group delay of 0.73 ps is achieved (Fig. 6). The PIT spectrum is dynamically modulated by adjusting graphene’s Fermi level between 0.9 and 1.1 eV, resulting in a blue shift of the transmission spectrum (Fig. 3 and Fig. 5). Transitioning VO₂ to its metallic state (σ=200000 S/m), the structure exhibits four narrowband absorption peaks at 2.31, 3.90, 6.02, and 7.91 THz, each exceeding 98% absorption (Fig. 8). Impedance-matching analysis reveals near-unity real impedance and near-zero imaginary impedance at these frequencies (Fig. 8), ensuring perfect absorption. Field distributions indicate local plasmon resonances in graphene and Fabry?Pérot-like magnetic resonances between graphene and VO₂, which together enhance absorption (Fig. 9). The Fermi level of graphene enables continuous blue-shifting of both PIT and absorption spectra, which is linearly correlated with the applied gate voltages. The conductivity of VO₂ modulates the transition between PIT and absorption modes without changing the structural parameters. In addition, the key design advantage lies in the centrally symmetric architecture, which ensures stable performance at different polarization angles (0°?90°). This polarization independence was verified by consistent transmission and absorption spectra under different polarization conditions (Fig. 11).

In this study, a polarization-independent and dual-mode THz metamaterial integrating graphene and VO₂ is theoretically proposed. By leveraging VO₂’s phase transition and graphene’s tunable plasmonic, the device dynamically switches between triple PIT (with slow-light capabilities) and quadruple narrowband perfect absorption, validated by theoretical models (coupled-mode theory and impedance matching) and numerical simulations. Key achievements include a high group delay of 0.73 ps, absorption peaks of more than 98%, and robust polarization insensitivity. These results advance the development of integrated THz devices for applications in optical modulation, sensing, and slow-light systems, providing a versatile platform for next-generation photonic technologies. Future work will focus on experimental validation and further optimization for industrial scalability.