# Transmissive Polarizer Metasurfaces: From Microwave to Optical Regimes
## 6 Review of Recent Progress of MSs Based Polarization Conversion
### Multiband Transmissive MS-Based Polarization Converters  
Multiband operation of communication systems to merge multiple systems into a single one has resulted in a reduction in size, cost, and complexities. A multiband circular polarization operation is a challenging task. A way to meet the challenge is through multiband circularly polarized (CP) antennas, such as multi-feed, helical, spiral, and shared aperture antennas [82; 83; 84]. Multiband CP waves can alternatively be obtained using multiband LP-to-CP converters. This solution is particularly attractive at high frequencies because of the complex feeding structures associated with CP antennas. There has been an increasing trend towards multi-functional polarization conversion devices [85; 86] in the microwave regime. In this way, multiple systems can be merged to miniaturize the systems. However, multiband LP-to-CP converters generally operate over a narrow range of frequencies and have low stabilities over oblique incidences. The design principle for multiband and multi-functional polarization converters is essentially the same as those of the single-band polarization converters, i.e., a given LP incident wave, when transmitted through an MS with a 90\({}^{\circ}\) phase difference between two orthogonal components, converts the incident linear wave into a CP wave, however, this time, in multiple bands [65]. Either a systematic equivalent circuit design approach has been proposed to design such multiband converters [65; 87] or a step-by-step design guideline in an EM simulator can be followed [88; 89; 90; 91; 92]. For the multiband operation of such converters, one possible way is to use multiple resonant structures with the same periodicity on each side of the MS so that two multiple resonances can exist. Another approach is to use the same geometry and shape different sizes according to the required multiple frequencies both present on a unit cell. This concept was used in multiband converters. Only asymmetric elements based unit cells along the x- and y-axes can result in the transmission of orthogonal components. Therefore, an anisotropic MS can be the right choice for such converters. The split rings have been proven to be a very good candidate for such multiband converters [65; 87; 89; 91; 92; 93]. These splits react differently to two orthogonal LP fields and their mutual couplings between their ends play an important role in achieving opposite handedness for the transmitted CP waves. Either a design operation using the equivalent element extraction can be followed [65; 87; 93], or step-by-step design guidelines using an EM simulator can be used to design such converters [88; 89; 94]. This design technique for multi-resonant structures was used by [PERSON] et al. in [86] to obtain dual-band polarization conversion operation. The structure is a four-metallic layered structure and consists of a cascaded structure of split-ring resonators and metallic patches divided by a metallic strip. The amount of 31% and 13% bandwidths were obtained for the two bands of operation under the provision of 25\({}^{\circ}\) oblique incidences. A similar concept was used by [PERSON] et al., however, the structure was a tri-layered structure and the bandwidth of operation was just 2.5% and 1.7% [88; 93]. The concept of multi-resonant structures within the same periodicity was used by [PERSON] et al. using Jerusalem-cross MSs to obtain a dual-band LP-to-CP converter in Ku- and Ka-bands [85]. They could achieve 13% and 10% bandwidths for both bands of operation. Figure 6 shows the different multiband polarization converters with experimental setups. It is important to mention here, however, that MS-based LC-to-PC converters in the microwave and THz regimes have different fabrication processes, characterization, and applications. Nevertheless, the design principle remains the same, i.e., an anisotropic structure resulting in co- and cross-polarized components having the same transmission value but a phase difference of 90\({}^{\circ}\).