In this paper, we have proposed bandwidth enhancement of transmissive type linear to circular polarization converter based on direct coupled metasurfaces in terahertz region. It is concluded that as compared to aperture coupling, TEM wave coupling through via holes between the two surfaces increases the bandwidth from (1.1 THz-1.48 THz) 29% to 100% (1.1THz to 3.46 THz). Incident vertical polarization can be converted into transmitted left circular polarization while maintaining the performance within 3dB of axial ratio. Physical mechanisms involved in polarization conversion were thoroughly considered. Through holes can be fabricated by soft-lithography with reactive plasma etching techniques. Compared to previous designs, the proposed polarization converter is simpler, transmissive and wideband which may be widely used in the THz wireless communications.
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