环状烯烃聚合物基底材料的太赫兹复介电常数和损耗特性研究

Analysis of Complex Dielectric Constant and Loss Properties of Cyclic Olefin Polymer Substrate by Terahertz Time-Domain Spectroscopy

太赫兹时域光谱(THz-TDS)已被用于研究包括液体,半导体,爆炸物和气体等多种材料。然而自由空间太赫兹光谱系统存在着一些检测局限性,如微量物质难以被检测、系统尺寸难以实现微型化、空气中水的强烈吸收引起的信号衰减较大等问题。为了解决这些问题,研究人员设计了基于金属波导传输线结构的太赫兹芯片集成器件,通过飞秒红外激光激励传输线上的光电导材料实现太赫兹波的产生和检测。然而,在这些芯片上传输的太赫兹信号的频谱宽度很难达到自由空间太赫兹时域光谱系统的频带宽度,一个重要原因是由于传输信号受到随频率增加的传输线损耗所导致的衰减。通常这些损耗主要由三个部分组成:导体损耗,介电损耗和辐射损耗。研究表明:使用低介电常数材料作为共面传输线的衬底,将减少这种介电常数的失配,从而避免冲击波辐射损失;使用具有低损耗角正切的基底材料可以减少太赫兹传输线的损耗。环烯烃聚合物(COP)是一种具有环状烯烃结构的非晶性透明共聚物的材料,在太赫兹波段具有很高的透射率,为了探究这种材料是否能用于共面传输线的衬底,需要通过太赫兹时域光谱技术和介电函数理论分析它在太赫兹频段的光谱和介电特性,以及对这种材料作为基底时用在太赫兹传输线的传输特性进行仿真计算分析。使用透射式太赫兹时域光谱系统,对三种COP、熔融石英和PMMA进行了光谱测试,提取了它们的透射时域信号,采用Dorney和Duvillaret等提出的物理模型计算复介电函数。实验表明:与其他两种材料相比,COP材料在1THz处的透过率更高,可以达到94.5%,同时介电损耗和介电常数更低,其中介电损耗在1THz处达到4.31×10^(-4),因此将COP作为传输线基底材料时能有效减少基底的介电损耗。同时COP材料的介电常数在0.2~2.8THz范围内维持在约2.3的水平,也有效减弱了辐射损耗。对实验材料基底组成的共面波导传输线进行了HFSS模拟,获得了它们的正向传输衰减系数(S21 parameter),并对由基底引起的介电损耗和辐射损耗进行了计算分析。模拟和计算结果也表明在同一传输线结构下,与其他材料相比COP作为基底时的损耗更小。通过太赫兹时域光谱法与介电响应分析,表明了在太赫兹波段具有较低介电常数的COP材料更适合作为太赫兹传输线结构的基底材料,它可以有效的降低因基底引起的介电损耗和辐射损耗。这为太赫兹传输线的设计过程中衬底材料的选择与应用提供了实验和理论依据。

Terahertz time-domain spectroscopy(THz-TDS)has been used to study various materials including liquids,semiconductors,explosives,and gases etc.However,the conventional free-space terahertz spectroscopy technology has fundamental limitations in the field of trace analysis,and the signal attenuation caused by the absorption of water in the air is also great.To solve these problems,the researchers designed the integrated terahertz chip based on a metal waveguide transmission line structure.What’s more,the terahertz waves are generated and detectedby the photoconductive materials laid on the transmission lines.However,thespectral width of the terahertz signal transmitted on chip is difficult to reach the bandwidth of the free-space THz-TDS signal,because the signal is attenuated due to the loss of the transmission line with increasing frequency.Theselosses are mainly composed of three parts:conductor loss,dielectric loss and radiation loss.It has shown that low permittivity of the materials as the substrates for coplanar transmission lines will reduce this mismatch of dielectricconstants and avoid shock wave radiation losses,and the usage of substrate materials with low loss tangents can reduce the dielectric loss.Cyclic olefin polymers(COPs)are considered as alternative substrate materials that can reduceterahertz transmission line losses due to their high transmission in the terahertz band.To clarify whether this material can be used for the substrate,we need to analyze its spectral and dielectric properties by both THz-TDS and dielectric theory,and the transmission characteristics of the substrate materials onthe terahertz transmission lines were simulated and analyzed.Three typesof COPs,fused silica and PMMA were tested by transmission THz-TDS.In addition,the complex dielectric function were calculated from the time-domain data according to the physical model proposed by Dorney and Duvillaret et al..Compared with the other two materials,the COP material has a higher transmittance of 94.5%at 1THz,while the dielectric loss with 4.31×10-4 at 1 THz and the dielectric constant are lower.Therefore,when the COP is used as the substrate of thetransmission line,the dielectric loss of the substrate can be effectivelyreduced.The dielectric constant was about 2.3 in the range of 0.2~2.8 THz,which also effectively reduced the radiation loss.What’s more,the coplanar waveguide transmission lines made from different substrate materials were simulated by HFSSand their forward transmission attenuation coefficients(S 21 parameter)were obtained.Meanwhile,the dielectric loss and radiation loss caused by the substrates were also compared.Both the simulation results and dielectric analysis show that the COP has lower loss compared to other materials as the substrateof transmission line structure.Based on terahertz dielectric response analysis,COP material with lower dielectric constant in the terahertz band is more suitable as the substrate material for the terahertz transmission line,which can effectively reduce the dielectric loss and radiation loss caused by the substrate.This provides experimental and theoretical basis for the choice and applicationof suitable substrate materials in the design of terahertz transmission lines.