**7. Conclusions**

In summary, a bi-layered metasurface-based THz dual-wide-band LP-to-CP converter based on transmission characteristics is proposed. For incident linear X or Y polarized waves, it transmits dual polarized dual-band circularly polarized waves. It works over 1.16 THz to 1.634 THz (34% fractional bandwidth) and 3.935 THz to 5.29 THz (29% fractional bandwidth) within two bands for an axial ratio of 3 dB. Considerable energy transfer is observed in dual bands except for the band 5 THz to 5.29 THz. Close agreemen<sup>t</sup> between HFSS (FEM) and CST (FDTD) results ensures the validity and reliability of the proposed design. Physical mechanisms, electrical circuit representation and quantitative explanation of the working principle behind the dual-wide-band LP-to-CP conversion justify the designed structure. Such a simple and dual-wide-band, dual polarized LP-to-CP converter can be used in potential applications in THz wireless communication, including satellite communication, ground segment, imaging, spectrometry, antenna technology, and many other THz manipulating devices.

**Author Contributions:** Writing-Original Draft Preparation, A.K.F.; Writing-Review and editing, C.R. and K.C.; methodology, A.K.F.; software, A.K.F.; validation, K.C.; and C.R.; supervision, C.R., project administration, C.R.

**Funding:** This research was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 61831001, the High-Level Talent Introduction Project of Beihang University, gran<sup>t</sup> number ZG216S1878, and the Youth-Top-Talent Support Project of Beihang University, gran<sup>t</sup> number ZG226S1821.

**Acknowledgments:** Authors convey their thanks to Prof. Jinjun Feng in the Vacuum Electronics National Laboratory, Beijing Vacuum Electronics Research Institute, China for his sincere supports for the CST software.

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