# A Wideband Terahertz Transmissive Polarization Manipulator Based on Metasurfaces
## 2 Materials and Methods  
It is well known that the basic polarization manipulation properties of metasurfaces originate from cross-coupling between electric and magnetic field resonances in the presence of an incident wave with a specific polarization state. In order to obtain LCPC, the unit-cell structure of the metasurface needs to be tailored to the cross-coupling effects. This helps to obtain the orthogonal components of transmitted waves with different or the same polarizations. In the past, many diagonal symmetric/semisymmetric structures have been proposed for this purpose [36; 52; 53; 54; 57; 66]. However, they were either based on single or double resonating elements or were single-layered with different structures for LHCP and LHCP [53; 54]. Such structures cannot achieve a wide band of operation. Here, a bilayered diagonal symmetric structure consisting of multiresonating structures is proposed. The proposed structure is shown in Figure 1. It consists of two sheets of conductors with a substrate layer sandwiched in between. Flexible polyimide quartz with a dielectric constant \(\varepsilon\)r of 4.0 and dielectric loss tangent of 0.004 was used as a substrate [67]. Gold was used as a conductor and was characterized by a conductivity \(\sigma_{\mathrm{dc}}=e_{0}\omega_{\mathrm{p}}{}^{2}\tau\). \(\omega_{\mathrm{p}}\) is the plasma frequency and \(\tau\) is the relaxation time for scattering [63]. The shaded region in Figure 1b shows the conductor layer which consists of two parts: the outer part is a square ring with a conductor width of \((\mathrm{a}-1)/2\) and the inner part consists of a large square of dimensions \(\mathrm{b}\times\mathrm{b}\) with three identical squares of dimensions \(\mathrm{c}\times\mathrm{c}\) eliminated from it along its diagonal. The bottom layer is identical to the top layer. The optimized parameters for the unit cell were determined as follows: \(\mathrm{a}=50\)\(\mathrm{\SIUnitSymbolMicro m}\), \(\mathrm{b}=\)30\(\mathrm{\SIUnitSymbolMicro m}\), \(\mathrm{c}=12\)\(\mathrm{\SIUnitSymbolMicro m}\), \(\mathrm{d}=5\)\(\mathrm{\SIUnitSymbolMicro m}\), \(\mathrm{l}=47.5\)\(\mathrm{\SIUnitSymbolMicro m}\). Full-wave electromagnetic (FWEM) simulation was carried out in the standard electromagnetic software--High Frequency Structure Simulator (HFSS). The design was simulated using master-slave boundary conditions. Floquet ports at the input and output of the unit cell were applied so that an infinite periodic array structure may be realized. Inorder to validate the performance of the proposed structure, the optimized unit cell was resimulated in the Computer Simulation Technology (CST) time-domain solver. The electric equivalent circuit for the proposed structure is as shown in Figure 2 at the microscopic level where two adjacent cells will be represented by coupled capacitors [68]. We have considered thin metasurfaces as an impedance sheet with negligible resistance to simplify the equivalent circuit. When the incident wave is x-polarized, the equivalent microscopic circuit is as shown in Figure 2a. The outer square ring's upper and lower arm behave as inductors, represented as 'L1'. The eliminated square on the top right and bottom left corners behave as capacitors (C1) aligned with inductors (L2), whereas the diagonal eliminated patch acts as a capacitor (C2). The conductor patch at the top left and bottom right is modeled as inductor and is represented as L3. Inductances of L3 will be lower than inductances of L1 and L2. Coupling between two adjacent unit cells in the horizontal direction is represented by C3. Figure 1: Schematic of the design: (**a**) Two-dimensional (2D) periodic array structure (**b**) Top view (X-Y) (**c**) 3D perspective view. Figure 2: Equivalent circuit for the proposed converter under (**a**) X-polarization, (**b**) Y-polarization. With an incident \(y\) polarized wave, we considered two unit cells in the \(y\)-direction with the equivalent circuit shown in Figure 2b.