A Novel Polarization-Maintaining Antiresonant Waveguide for Low-Terahertz Transmission

In this article, a high birefringence and low loss polarization-maintaining terahertz waveguide based on the antiresonant mechanism is proposed, of which the cladding comprises six circular tubes and four curved thin walls. The thickness difference between the horizontal outer tubes and the vertical circular tubes brings a high birefringence, which can be further improved by the structural difference attributable to the nested tubes. Moreover, the nested tubes and four curved thin walls also reduce waveguide losses. The simulated results show that the birefringence of the designed waveguide at 0.14 THz is 0.00165, and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i>-polarized and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i>-polarized loss is 0.096 dB/cm and 0.104 dB/cm, respectively. In addition, fabricated waveguides by three-dimensional printing technology are characterized by a single-frequency system working in 0.14 THz and a THz time-domain spectroscopy (THz-TDS) system. The experimental results by the THz-TDS system show that the waveguide has two low-loss antiresonant windows (0.13–0.16 THz and 0.26–0.33 THz), which agree with the simulated results. The birefringence of the waveguide at 0.14 THz is 0.0038, and the transmission loss of the two polarizations is 0.17 and 0.165 dB/cm, respectively. Moreover, the waveguide achieves minimum experimental <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i>-polarized losses of 0.077 dB/cm @0.29 THz and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i>-polarized losses of 0.073 dB/cm @0.29 THz. The polarization ratio of the waveguide measured by the single-frequency system also verified the polarization-maintaining ability, which changes from 96.81% to 96.46% after the 0.14 THz wave passes through the waveguide. This proves that our proposed waveguide can effectively transmit low-terahertz waves while maintaining the direction of incident linearly polarized waves.