# A Wideband Terahertz Transmissive Polarization Manipulator Based on Metasurfaces
## 1 Introduction
This concept of chiral metasurfaces has been used in manipulating polarization states in transmission modes to achieve linear to circular polarization conversion in both microwave and optical frequencies [48; 49; 50]. However, most chiral metasurfaces suffer from the disadvantage of narrow bandwidth due to their resonant characteristics. Anisotropic metasurfaces have been used for wideband polarization control in reflection mode [39; 42; 43]. Broadband transmission-based polarization manipulation is still a challenging problem. [PERSON] et al. [51] quoted transmissive linear to circular polarization conversion in the microwave frequency range using a single layer of metasurfaces but the bandwidth achieved was just 12.5%. Similarly, different structures for LCPC operation have been proposed [52; 53; 54] using single/double-layered structures, but the maximum bandwidth achieved was 15%. Moreover, for the structures proposed in [53; 54], the diagonal portion used to obtain right-handed circular polarization (RHCP) wave was the opposite to that for left-handed circular polarization (LHCP). Such LCPCs [40; 51; 52; 53; 54] limit the choice of selection when a wide range of operation and both LHPC and RHCP are required. The wideband operation for LCPCs has been achieved using multilayered metasurfaces. For example, [PERSON], et al. [55] proposed wideband transmissive LCPC using subwavelength capacitive and inductive patches and achieved 40% bandwidth; however, the structure was based on six layers of metallic patches which is too complicated to fabricate. [PERSON] et al. [56] achieved 40% linear to circular polarization conversion but they used three layers of metasurfaces. [PERSON] et al. [57] proposed a design technique for broadband LCPC in reflection and transmission mode but the structure working in transmission mode works over 17% bandwidth and is based on three layers stacked together [PERSON] et al. [58] proposed a wideband LCPC in the microwave band that maintains an axial ratio of 3 dB within 33% bandwidth; however, the structure consists of three layers stacked between two dielectric spacers. Substantial attention is being devoted to developing terahertz satellite links for geostationary satellites [59]. Filters play a vital role in any communication system, including THz communication. Scientists have proposed different frequency-selective surface (FSS)-based filtering structures, such as free-standing resonant mesh filters, inverse metal resonant grids, and split ring resonators [60; 61; 62; 63; 64], [PERSON] et al. [63] proposed a THz bandpass filter consisting of a metal-dielectric-metal (MDM) sandwich with a periodic slot structure. [PERSON] et al. [64] reported an ultrawide THz bandpass filter based on a metamaterial. Transmit reject filters (also called receive filters) in THz wireless communication are essential for receiver systems that require high isolation at transmitting frequency. Since receive systems do not deal with high power, the typical requirements for transmit reject filters include low insertion loss in the passband, small size, and high rejection in the transmit band. These filters had been implemented in waveguide topology in microwave and millimeter-wave bands [65]. In the THz band, where Y-band waveguides can have fabrication issues, a solution based on low-profile metasurfaces is an attractive choice when high power is not a concern. In this paper, a metasurface-based structure is proposed which serves two purposes: transmission-based linear to circular polarization conversion covering 43.9% bandwidth, and a wideband transmit reject filter with 67% fractional bandwidth. The proposed structure is, to the best of our knowledge, the widest among any transmissive type LCPC [40; 48; 49; 60] that works with both horizontal (x) and vertical (y) polarization and converts the transmitted wave into LHCP and RHCP, respectively, within 3 dB of the axial ratio. Moreover, with slant (xy) polarizations, the proposed structure behaves as a transmit reject filter.