# Transmissive Polarizer Metasurfaces: From Microwave to Optical Regimes
###### Abstract  
Metasurfaces, a special class of metamaterials, have recently become a rapidly growing field, particularly for thin polarization converters. They can be fabricated using a simple fabrication process due to their smaller planar profile, both in the microwave and optical regimes. In this paper, the recent progress in MSs for linear polarization (LP) to circular polarization (CP) conversion in transmission mode is reviewed. Starting from history, modeling and the theory of MSs, uncontrollable single and multiple bands and LP-to-CP conversions, are discussed and analyzed. Moreover, detailed reconfigurable MS-based LP-to-CP converters are presented. Further, key findings on the state-of-the-arts are discussed and tabulated to give readers a quick overview. Finally, a conclusion is drawn by providing opinions on future developments in this growing research field. MSs polarization converters; polarizers; multiband transmissions; configurable converters 1222202202202202202202202202202202202202202202202202202202202220220222022022202220220222022022202202202217 May22022222202222022220222022220222022202  
of bulk MM is very complex, particularly when they lie in the optical region, as it requires bulk MM to be 3D nanostructures. Moreover, wave propagation in metamaterials covers a substantial distance, therefore, they encounter high ohmic losses. Recently, it was realized that 2D metamaterials can be a more profound type of metamaterial, having the ability to shape/control electromagnetic waves. They have been termed MSs (MS). They can have subwavelength inclusions to tailor electric and magnetic fields, to control transmitted and reflected fields. They are ideal candidates for many novel microwave and optical devices. The basic properties of MSs are defined by their elementary subwavelength patterns, which result in unusual resonant behavior. Importantly, MSs are patterned with subwavelength structures, so homogeneous or nearly homogenous MSs transmit and reflect plane waves. Since MSs are thin structured and 2D in nature, they are less lossy and easier to fabricate. Another major advantage of using MSs is their easier integration with existing microwave and nanophotonics systems. Several factors influencing the applications and properties of MSs are the type of structure/pattern, their mutual/cross-couplings between adjacent cells, and the substrate used. The most attractive feature of the MS is its ability to control reflected and transmitted fields, which makes it highly effective for certain applications and replaces conventional bulky equipment. There have been excellent review articles on MSs covering theory, fundamentals and applications [10; 11; 12; 13; 14] and their complex fabrication [12]. For example, [PERSON] et al. reviewed optical fiber-integrated MSs for applications, such as telecommunication, sensing, imaging, and biomedicine [10]. [PERSON] et al. focused on the development in recent years of such MSs from microwave to optical regimes [11]. [PERSON] et al. reviewed the fabrication and applications of MSs [12]. [PERSON] et al. reviewed different types of MSs for a broad range of the operational wavelengths for wavefront shaping, lenses, and polarization transformation [13]. In this review paper, we focus on a special application of the MS, i.e., polarization conversion. This conversion using MS can be categorized into three main types: transmission-based, reflection-based and dual functional (both transmission and reflection). Figure 1 depicts the functionality of MS for polarization conversion. Figure 1a shows transmission-based polarization conversion, where incident linear polarization is converted into transmitted circular polarization. In this scenario, we termed MS as MS-T. Figure 1b shows the polarization conversion phenomenon using reflection-based MS. We termed this as MS-R. In Figure 1c, it is shown that MS performs polarization conversion for both transmitted and reflected waves. We termed such an MS as MS-T/R.