基于超材料的可调谐三信道太赫兹分波器设计

Design of Tunable Three⁃Channel Terahertz Demultiplexer Based on Metamaterials

太赫兹波在超材料表面结构中会发生电磁响应,使得特定频率的电磁波发生反射与透射。利用VO2的相变特性,设计了一种基于超材料的可调谐三信道太赫兹分波器。采用Drude模型,分析了该分波器两种状态下的仿真S参数曲线、等效参数曲线和群延迟曲线,并研究了分波器的工作机理。处于通信窗口的0.30、0.46、0.50 THz太赫兹波实现了分离,分波器的隔离度均大于22 dB,插入损耗均小于0.40 dB,群延迟低且性能指标受参数变化的影响较小,满足工作性能的设计要求。与传统的光子晶体和绝缘硅分波器相比,所设计的分波器具有隔离度高和插入损耗低等特点。与超材料二信道分波器相比,所设计的分波器提高了信道容量,实现了信道可调谐功能。所设计的分波器在太赫兹无线通信波分复用系统中,展现出实现多信道传输的潜力,具有较好的应用前景。

Objective 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 threechannel 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.Methods 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 magneticfield 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.Results and Discussions 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.Conclusions 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.