###### Abstract  
Terahertz (THz) photonic crystal (PC) waveguides show promise as an efficient and versatile waveguiding platform for communication, sensing, and imaging. However, low-frequency THz PC waveguides with a low-cost and easy fabrication remain challenging. To address this issue, a THz PC waveguide with a illac-shaped defect has been designed and fabricated by 3D printing based on stereolithography (SLA). The reflection and transmission characteristics of the proposed waveguide have been analyzed using the finite difference frequency domain (FDFD) method. The waveguide spectral response is further optimized by changing the distance of the illac-shaped resonant cavities. Consistent with the results of numerical modeling, the measured results show that the waveguide performs a resonant reflection in the region of 0.2 to 0.3 THz and low-pass transmission in the 6G mobile communication window. Furthermore, in order to characterize the performance of the proposed waveguide, parameters have been analyzed, including the Q factor, resonant frequency, and bandwidth. This work supplies a novel pathway for the design and fabrication of a low-frequency THz PC waveguide with potential applications in communication, sensing, and imaging. t 2022 202  
of the existing surface plasmon polaritons (SPPs) [12], metallic PCs enable confinement of THz waves in the sub-millimeter scale. However, metallic PC waveguides are still inevitably accompanied by Ohmic losses, which limit the quality factors of resonance and compromise the efficiency of metallic-based devices [8]. In the past few years, a growing number of reports have shown that this problem could be solved by employing dielectric waveguides, which mostly rely on high-index and low-loss particles (such as silicon, SiO\({}_{2}\), and TiO\({}_{2}\)) [7; 9; 10]. Consequently, interest in dielectric waveguides has increased with the application of dielectric antennas, resonators, polarizers, etc. [4; 8; 9; 10; 14]. Meanwhile, a number of researches have been focused on dielectric waveguides because of the wide variety of available materials and the greater flexibility of the waveguide design. Many dielectric waveguide structures, including planar [15], rectangular [16], circular [1], strip [17], and photonic crystal waveguides [10; 18; 19] have been investigated. Among them, photonic crystals can realize strong light confinement due to the nature of photonic band gaps, and show great potential in high-performance dielectric waveguides [10]. Currently, THz waveguides are mostly fabricated by photolithography, which requires multiple steps, including spin-coating, prebaking, the preparation of masks, exposure, etc. [20; 21; 22], leading to a long preparation cycle and a high cost [16; 23]. In addition, most of these devices have a substrate, which significantly lowers the transmittance and causes internal interference. Therefore, it is of great significance that THz PC waveguides are fabricated in a simple, low-cost, and efficient way [16]. Furthermore, most of the related works are conducted at a microwave and millimeter wave; it remains challenging to obtain low-frequency THz PC waveguides with a low-cost and easy fabrication. Direct writing technology has been applied to create complex THz waveguides, such as microfluidic three-dimensional photonic crystals [24]. High-accuracy 3D printing based on stereolithography (SLA) [13; 25; 26; 27; 13] as an emerging technology shows great potential in high-performance dielectric waveguides, but it has not been applied in THz PC waveguides. In this work, a THz PC waveguide with a iliac-shaped defect is demonstrated. This proposed waveguide is fabricated with a high-density photosensitive resin by 3D printing based on SLA. The reflection and transmission properties of the waveguide in the 6G mobile communication window have been investigated. The waveguide spectral response is optimized by changing the distance of the iliac-shaped resonant cavities.