Abstract：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 detected by the photoconductive materials laid on the transmission lines. However, the spectral 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. These losses 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 dielectric constants 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 reduce terahertz 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 on the terahertz transmission lines were simulated and analyzed. Three types of 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 1 THz, 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 the transmission line, the dielectric loss of the substrate can be effectively reduced. 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 HFSS and their forward transmission attenuation coefficients (S21 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 substrate of 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 application of suitable substrate materials in the design of terahertz transmission lines.
Key words：THz-TDS; Cyclic olefin polymers (COPs); Terahertz transmission line
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