Terahertz radiation yields electromagnetic responses when interacting with metamaterials, thus resulting in the reflection and transmission at specific frequencies. Currently, studies regarding terahertz demultiplexers based on metamaterials are few; most studies focus on photonic crystals and insulated silicon demultiplexers. However, photonic-crystal fabrication is challenging and expensive, which hinders its large-scale implementation. Silicon has a large thermo-optic coefficient, which changes the refractive index significantly even under slight environmental temperature variations, thus causing shifts in the center frequency. Moreover, the existing metamaterial demultiplexers can only achieve two-channel demultiplexing. Therefore, this study proposes a tunable three-channel terahertz demultiplexer based on metamaterials that utilizes the phase-transition characteristics of VO₂. By employing the Drude model, we obtain simulated S parameters and the equivalent parameters, as well as analyze the group delay curves for two states. Subsequently, we analyze the operating mechanism of the demultiplexer. The proposed demultiplexer exhibits wave separation at 0.30, 0.46, and 0.50 THz within the communication window. Additionally, it exhibits isolation exceeding 22 dB, insertion losses of less than 0.40 dB, low group delays, and minimal sensitivity of performance indicators to parameter variations, thus satisfying the design requirements for operational performance. Compared with conventional photonic crystals and insulated silicon demultiplexers, the proposed demultiplexer exhibits higher isolation and lower insertion loss. Meanwhile, compared with existing metamaterial demultiplexers, it offers higher channel capacities and enables channel tunability. Its application prospects include multichannel transmission in terahertz wireless-communication wavelength-division multiplexing systems.

The metamaterial unit cell comprises an upper metal square ring and a metal line, an intermediate dielectric layer of polyimide, and a bottom VO₂ rectangular line structure [Figs. 2(a) and (b)]. It is arranged along the x- and y-directions to form a 3×3 array [Fig. 2(d)]. The metal material is gold, with a conductivity of 4.56×10⁷ S/m; the relative dielectric constant of the intermediate dielectric polyimide is 3.4; and the loss tangent is 0.0027. The dielectric constant of VO₂ is described using the Drude model. The angle between the incident-wave direction and the z-axis is 10°, and the array structure is simulated and analyzed using the CST 2022 software. First, a frequency-domain solver is used for analysis. The S-parameter curves of VO₂ in the insulating and metallic states are obtained (Fig. 3), and the isolation and insertion losses are analyzed to satisfy the performance design requirements. Next, the electromagnetic parameters and group delay curve of the demultiplexer are derived via a reverse deduction of the S-parameters (Fig. 4) to investigate the resonant characteristics of the metamaterial. We use the field monitors in CST 2022 software to obtain the magnetic-field intensity distributions and surface current distributions at the corresponding frequencies to portray the mechanism of the demultiplexer (Fig. 6). Finally, we analyze the effects of structural parameters on the performance of the demultiplexer.

Based on the phase-transition characteristics of VO₂ and the structural design of metamaterials, when VO₂ is in the insulating state, the transmission and reflection frequencies are 0.302 THz and 0.460 THz, respectively (Fig. 3). This implies that at 0.302 THz, the terahertz wave and metamaterial structure are impedance matched, whereas magnetic resonance occurs at 0.460 THz [Figs. 4 (a) and (c)]. When VO₂ is in the metallic state, the transmission and reflection frequencies shift to 0.298 THz and 0.500 THz, respectively (Fig. 3), thus indicating impedance matching at 0.298 THz and magnetic resonance at 0.500 THz [Figs. 4(d) and 4(f)]. Further analysis of the magnetic-field intensity and surface current distributions at these frequencies confirms the occurrence of magnetic resonance at specific frequencies. As the temperature increases from room temperature to 68 ℃, the conductivity of VO₂ increases from 200 S/m to 2×10⁵ S/m. Consequently, the coupled resonance frequency between the wave and metamaterial shifts, thus causing the magnetic resonance frequency points to shift to the right. Simultaneously, the isolation, insertion loss, and group delay satisfy the communication requirements. Compared with existing photonic crystals and insulated silicon multichannel demultiplexers, this design offers higher isolation and lower insertion loss, thus addressing the performance limitations of photonic crystals and insulated silicon. It overcomes the current limitations of metamaterial-based demultiplexers, which can only achieve two-channel demultiplexing, thereby demonstrating potential for use in wavelength-decomposition multiplexing in future terahertz wireless-communication applications.

In this study, the different electrical conductivities of VO₂ at different temperatures are employed to design a tunable tri-channel demultiplexer based on metamaterials. The multiplexer can separate waves at 0.30, 0.46, and 0.50 THz within the communication window. Additionally, it exhibits isolation levels of 28.35 dB, 37.93 dB, and 22.74 dB, with insertion losses of 0.11 dB, 0.10 dB, and 0.40 dB, respectively, thus reaching the design goals. The equivalent parameters of the metamaterial demultiplexer are obtained via S-parameter inversion. Next, the group delay of the demultiplexer is calculated, which shows minimal variation at both ports, thus ensuring undistorted signal transmission. Subsequently, the magnetic-field intensity and surface current distributions at the corresponding frequencies are analyzed to understand the mechanism of the demultiplexer. Finally, the effects of structural parameters on the demultiplexer performance are discussed. Variations in the structural parameters result in only slight frequency shifts at the two ports, with no significant effect on the insertion loss and isolation. The performance indicators of the proposed demultiplexer are superior to those of photonic crystals and insulating silicon demultiplexers; furthermore, the proposed demultiplexer can support more channels than the existing metamaterial demultiplexers.