# A Wideband Terahertz Transmissive Polarization Manipulator Based on Metasurfaces
## 1 Introduction  
Terahertz (THz) electromagnetic waves ranging from of 0.1 THz to 10 THz located between the microwave and infrared bands is a rapidly growing technology. With the advancement in THz sources and detectors [1; 2; 3; 4], THz technology has been investigated in wide applications such as spectroscopy [5] imaging [6], remote sensing [6; 7] and telecommunications [8]. Polarization of an electromagnetic field is the direction in which the electric field oscillates while traveling through the medium. In wireless communications, the choice of polarization of the antenna depends upon the application as well as the medium. Manipulation of this polarization state has been used in a wide range of applications, including wavefront control and optical communication [9; 10]. Circular polarized THz beams have been used in biomedical imagining because of the different behavior of chiral structures of biomolecules towards a circular polarized THz beam. For example, Bessel beams can be used to control the motion of biomolecules for applications such as biosensing and drug delivery [11; 12]. For THz wireless communication, including satellite communication, circularly polarized waves are preferred because of their lower sensitivity towards multipath fading as compared to linearly polarized waves. There are two ways to achieve circular polarization: using circularly polarized antenna; or converting linearly polarized waves generated by an antenna into circularly polarized waves using LCPC. The latter is sometimes advantageous when we have readily available linearly polarized antenna. Polarization manipulation can be categorized into two types, reflection-based and transmission-based. The former uses a ground conducting plate to reflect electromagnetic waves, whereas the latter consists of bilayered or multilayered, differently patterned conducting patches. Stealth technology is a major application for reflection-based manipulation [13; 14; 15]. Transmission-based manipulation is used in transmission systems where transmitted waves and incident waves need to be aligned. Terahertz wave engineering to obtain polarized waves has been explored using metasurfaces numerically [16; 17; 18; 19] or experimentally [20] for a wide range of applications, including cloaking, camouflage, amplifiers, isolators, duplexer antenna systems, and mixers. Birefringent structures have been used to convert one type/state of polarization into another polarization state/type. Examples include waveplates [21; 22; 23; 24; 25; 26; 27], liquid crystals [28; 29; 30], and wood and paper [31; 32]. Such structures are not only bulky as they have thickness much larger than the operating wavelength, but also have complex fabrications. Metamaterials, chiral metamaterials, and quasiperiodic planar arrays of subwavelength elements have attracted research attention because of their distinguished properties such as asymmetric transmission and polarization conversion with tenability and flexibility [33; 34; 35]. Metasurfaces classified as 2D metamaterials have become a hot research topic because of their low profile. Phase manipulation of THz waves using metasurfaces has been investigated in recent years [36; 37; 38], including linear to circular polarization conversion capability due to their smaller profiles [36; 39; 40; 41; 42; 43; 44]. Graphene-based wideband LCPCs have also been investigated to dynamically control the manipulation [39; 40; 41]. However, in all these polarization converters [39; 40; 41; 42; 43; 44; 45; 46], most of the researchers have worked on reflection-based polarization conversion. Recently, [PERSON] et al. numerically investigated an excellent and broadband LCPC using reflective layers [47]. Such converters cannot be used where incident linear waves and outgoing required circular waves need to be aligned with each other. Metasurfaces can be classified into chiral and anisotropic metasurfaces. Chiral metasurfaces use chirality inclusions to break the mirror symmetry of the structure.