**4. Conclusions**

In conclusion, a THz PC waveguide with a lilac-shaped defect is demonstrated in this work. The reflection and transmission characteristics of the proposed waveguide have been analyzed both in simulation and experiment. The waveguide spectral response is further optimized by changing the distance of the lilac-shaped resonant cavities. The designed waveguide performs a resonant reflection in the region of 0.2 to 0.3 THz and a low-pass transmission in the 6G mobile communication window. These results have demonstrated that the waveguide performance can be controlled by the gap distance. For some special occasions, the gap distance can be treated in order to evaluate the pros and cons of this device. In addition, the proposed waveguide has many advantages, including a designable structure, low-cost and easy fabrication, and operation in a low-THz frequency. Although its reflection and transmission loss are larger than other types of waveguides, the performance of the proposed waveguide will be optimized with the development of SLA technology. It holds great promise for extensive applications in communication, sensing, and imaging.

**Author Contributions:** Conceptualization, J.S. and Y.D.; methodology, J.S., Y.D. and X.L. (Xiuyan Li); software, H.B., X.L. (Xianguo Li), and W.F. (Wei Fan); validation, L.T., W.F. (Weiling Fu), and X.Y.; writing—original draft preparation, J.S. and Y.D.; writing—review and editing, J.S., X.Y. and P.N.; visualization, W.F. (Weiling Fu); supervision, P.N. and J.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is sponsored by the National Natural Science Foundation of China (61905177, 61901297, 61903273, and 81802118); the Natural Science Foundation of Tianjin City (19JCQNJC01400); the Open Research Fund of National Mobile Communications Research Laboratory, Southeast University (2022D12); and the Open Fund of IPOC, BUPT (IPOC2021B02).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
