A low-loss terahertz (THz) hollow-core pipe waveguide constructed of plastic material is demonstrated. The structure is designed to be especially effective in transmitting THz waves of 110 GHz, which has important applications in communications, imaging, and sensing. Guiding of the electromagnetic wave is based on the principle of antiresonant reflection. Through careful theoretical analyses and systematic modeling and simulation, followed by a thorough experimental investigation, we show that the proposed structure can successfully transmit THz waves with low attenuation. Furthermore, when the structures of the pipe waveguides are varied for optimization, we find that cladding thickness and the refractive index under antiresonant conditions as well as the core diameter are important physical parameters in designing the low-loss THz waveguide. Considering not just attenuation loss but such factors as volume, weight, and flexibility of the tube waveguide, along with other practical issues such as cost, we arrive at an optimal design of the pipe waveguide, which has an inner diameter of 35 mm and cladding thickness of 5 mm. Teflon is chosen as the material for a guiding 110-GHz THz wave. The attenuation constant is determined by the simulation to be as low as 0.0228 m−1. However, due to nonuniformity of the waveguide wall thickness and random small bending of the waveguide, as well as moisture content in the air filling the pipe core, all of which strongly impede propagation in the waveguide, the experimentally measured attenuation loss (3.65 m−1) of the waveguide is much more significant than the theoretical prediction, with the latter serving as a design benchmark under perfect conditions.
Carbon-fiber-reinforced plastic (CFRP) composites are widely used in aerospace and concrete structure reinforcement due to their high strength and light weight. Terahertz (THz) time-domain spectroscopy is an attractive tool for defect inspection in CFRP composites. In order to improve THz nondestructive testing of CFRP composites, we have carried out systematic investigations of THz reflection and transmission properties of CFRP. Unidirectional CFRP composites with different thicknesses are measured with polarization directions 0 deg to 90 deg with respect to the fiber direction, in both reflection and transmission modes. As shown in the experiments, CFRP composites are electrically conducting and therefore exhibit a high THz reflectivity. In addition, CFRP composites have polarization-dependent reflectivity and transmissivity for THz radiation. The reflected THz power in the case of parallel polarization is nearly 1.8 times higher than for perpendicular polarization. At the same time, in the transmission of THz wave, a CFRP acts as a Fabry–Pérot cavity resulting from multiple internal reflections from the CFRP–air interfaces. Moreover, from the measured data, we extract the refractive index and absorption coefficient of CFRP composites in the THz frequency range.