# Transmissive Polarizer Metasurfaces: From Microwave to Optical Regimes
## 6 Review of Recent Progress of MSs Based Polarization Conversion
### Single Band Transmissive MS-Based Converters  
MS was firstly used by [PERSON] et al. in 2013 [PERSON] et al. (2013) as a single-band polarizer. They proposed a singly layered structure whose each unit cell comprises a rectangular loop and a diagonal microstrip, as shown in Figure 4a. With the help of a diagonal strip, the pattern on the MS realized a diagonal symmetric structure. They could achieve around 9% bandwidth for a 3 dB operation of the axial ratio. However, the performance of the polarizer for oblique incidences was not good enough as the polarizer remained stable for only 10\({}^{\circ}\) oblique incidences. One year later, [PERSON] et al. (2016) proposed four-layered split rings bisected by a metal strip based structure, as shown in Figure 4b, to realize LP-to-CP conversion. They used ring resonators instead of the square resonators as used by [PERSON] et al. because of the good reflection properties at the resonant frequency. They used a slant incident wave (tilted to 45\({}^{\circ}\)) to the x-axis so that if the incident wave could be decomposed into two orthogonal components, they could face different structures along the \(x\) and \(y\) axes. Since the structure was based on four layers, the fabrication of the design was complex, where they used acrylic glass frames as spacers to ensure the required distance. Metal dowel pins were used to align the elements of the layers. Due to the multiple layered structures, the bandwidth was improved to about 31%. Since the structure was based on a circular ring, the polarizer was relatively stable over 25\({}^{\circ}\) oblique incidences. In later years, significant advancements in operational bandwidth were reported using simpler structures [PERSON] et al. (2013); [PERSON] et al. (2013). For example, [PERSON] et al. proposed an LP-to-CP converter based on self-complementary zigzag MSs [PERSON] et al. (2013). The structure consists of a bi-layered surface. The unit cell of the structure consisted of a zigzag metallic pattern, which acts as equivalent capacitances for incident horizontal polarization, whereas, for incident vertical polarization, it acts as equivalent inductive units. Such a structure proved to achieve very wide operational bandwidths for numerical simulations (70%); however, experimentally, they could achieve around a 40% axial ratio bandwidth. Figure 3: Scheme for the article. Several research groups reported wideband polarization converters using square rings enclosed in different patterned structures [73; 74; 75; 76]. This is due to the interesting property of the square ring as a wideband filtering structure without polarization conversion [77]. Most of the work reported used square ring based unit cells to achieve wideband polarization conversion performance. Transmissive MS-based polarizers (called quarter waveplates) have been reported in the optical regime as well. They used Babinet-inverted or silicon MSs based on Fano-resonances. Babinet-inverted MSs have been an attractive research field because of their ultra-thin properties and maximum transmission efficiency at resonance [78]. [PERSON] et al. reported polarizers based on cross-shaped metallic cross apertures with unequal lengths to support two orthogonal resonant modes in the THz range [71]. Figure 4: Different multi-layered single-band MSs (**a**) for linear to RHCP converter [67], (**b**) four-layered linear to circular polarization converter [68], (**c**) bi-layered polarization converter [69], (**d**) bi-layered zigzag structure, Reprinted, with permission [70], (**e**) bi-layered Babinet-inverted MSs [71], (**f**) Fano-resonant Si-MSs [72]. By selecting proper suitable lengths of the apertures, equal transmission amplitudes, and 90\({}^{\circ}\) phase differences were observed. Table 1 enlists the key findings for a few MS-based LP-to-CP converters in transmission mode. In order to describe polarization conversion using equivalent circuit theory, ref. [81] introduced a new unit cell, as shown in Figure 5a,b.