## 1 Introduction  
Terahertz (THz) electromagnetic waves ranging from 0.1 THz to 10 THz have been investigated in numerous applications including imaging, spectroscopy, environmental surveillance, remote sensing, high-resolution radars and high-speed communications [1; 2; 3; 4]. With the advancement in THz sources and detectors, these investigations have gained great interest from researchers. Polarization manipulators, in their applications to rotate or convert polarization states of electromagnetic waves, have been comprehensively explored for stealth, cloaking and in diode-like applications [5; 6; 7; 8]. Circular polarization is a preferred choice for THz wireless communication due to its lower sensitivity towards multipath fading and polarization mismatch with receiver mobility. Dual-band circular polarized waves are required in any satellite communication for the uplink (U/L) and downlink (D/L) operational bands. The handedness for U/L and D/L operations needs to be opposite to offer adequate polarization diversity. A generic solution for this is to use a linearly polarized antenna in cascade with orthomode transducer (OMT) as a feeder to a transmit-array or reflector. This solution is quite complicated, bulky and expensive. Another solution is to use phased array dual-band patch antennas. Both of these solutions are not feasible at THz frequencies. There is another possibility to obtain dual-band dual polarizations, i.e., using a linearly polarized wave generated by a linearly polarized antenna in cascade with dual-band linear polarization to circular polarization (LP-to-CP) converters. This solution is advantageous in terms of size and complexity. In addition, dual-band operation for LP-to-CP can be used to merge multiple systems for cost and volume reduction. In the past, birefringent structures have been used to convert one state or type of polarization into another, including waveplates [9; 10; 11], liquid crystals [12; 13; 14], and wood and paper [15; 16]. However, these solutions are bulky and complicated to integrate with existing THz systems. Metamaterials, chiral metamaterials and quasi-periodic planar arrays of sub-wavelength elements have attracted many researchers because of their distinguished properties, such as asymmetric transmission and polarization conversion with tenability and flexibility [17; 18; 19]. Metasurfaces, as the 2D equivalent of metamaterials, have been explored for the possibility of polarization manipulation, including linear to circular polarization conversion [20; 21; 22]. Particularly, they have been explored for single band linear to circular polarization conversion capability [20; 23; 24; 25; 26; 27]. Wideband linear to circular polarization conversion operation has been explored using reflection and transmission modes [23; 24; 25; 26; 27]. Controlling electromagnetic fields, termed 'wave engineering', has been explored using metasurfaces [28; 29; 30; 31; 32]. [PERSON] et al. [28] proposed transverse temporal gradient-based metasurfaces for efficient transmission that is otherwise difficult using thin layered metasurfaces. [PERSON] et al. [29] proposed an advanced wave engineering technique based on unidirectional frequency generation and spatial decomposition in space-time-modulated slabs. [PERSON] et al. [30] realized an extraordinary beam splitter with one-way beam splitting-amplification. The proposed technique offers high isolation, transmission gain and zero beam tilting. [PERSON] et al. [31] proposed a nonreciprocal metasurface that can achieve optical circulation and isolation. [PERSON] et al. [32] proposed a technique for nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit-cell whose reflection coefficient oscillates at low frequency. Such devices have been demonstrated theoretically [28; 29; 30; 31] or experimentally [32] in an excellent manner for a wide range of applications, including cloaking, camouflage, amplifiers, isolators, duplexer antenna systems and mixers.