# Transmissive Polarizer Metasurfaces: From Microwave to Optical Regimes
## 6 Review of Recent Progress of MSs Based Polarization Conversion
### Reconfigurable Transmissive MS Based Converters
#### 6.3.2 Controllability through State Change  
Another mechanism to control MSs for their LP-to-CP conversion is based on different materials, such as graphene. This is a 2D layered material based on carbon atoms arranged in a honeycomb lattice and has excellent electrical properties, which can help in controlling the interaction between light and matter. Graphene layers placed on the top of the insulator have proved to exhibit strong plasmonic behavior [107]. [PERSON] et al. proposed a graphene-based LP-to-CP converter in transmission mode [108]. The phase difference between transmitted orthogonal waves can be controlled by varying the Fermi level of the graphene. Starting from conceptual diagram for controlling the polarization state of the outgoing wave using the Fermi energy level, the unit cell is designed and the phase difference between transmitted co- and cross-polarization components is controlled by varying Fermi energy levels. Similarly, [PERSON] et al. used graphene to dynamically regulate the operating band in the THz regime by controlling the Fermi energy of the graphene sheets [109]. The polarizer performs over 2.64-3.29 THz (21.92%) in the case of Ef = 0.1 eV. Figure 8a shows the periodic array configuration and corresponding controllability in the band of operation in Figure 8b.  
Figure 7: (**a**) DC circuit diagram for polarization control and unit cell of the polarizer (Reprinted, with permission [104]); (**b**) unit cell of the active MS with side view of the top layer and dielectric substrate (**c**) state 1 (**d**) state 2 (**e**) state 3. (**f**) Equivalent scheme for state 1; (**g**) equivalent scheme for state 2 (**h**) Equivalent scheme for state 3 [105].  
A similar scheme is applied to the far-infrared region, i.e., the phase retardation between two orthogonally polarized wave components is constructed through light propagation. [PERSON] et al. implemented an LP-to-CP converter based on ultra-thin MS to operate in the mid-infrared region (Figure 9a) [110]. Simulated results for transmission are shown in Figure 9b. It is clear that in the wavelength 2729 \(\sim\) 3094 nm, the phase difference of 90\({}^{\circ}\) between transmitted orthogonal waves is achieved. Figure 9c shows the SEM image for the fabricated sample.  
Figure 8: Schematic of an ultra-thin quarter-wave plate MS made of an array graphene and gold (**a**) periodic array configuration [109]. **(b)** The **AR** curves when Ef runs from 0.1 eV to 1.0 eV [109].  
Figure 9: (**a**) Schematic of the design of MS as polarization converter in mid-infrared region. (**b**) Full-wave simulations of the transmittance (solid curve) and phase (dashed curve) for the MS; (**c**) SEM image of a nanoimprinted MS [110]; (**d**) experimental schematic for frequency switchable of the THz LP-to-CP converter; (**e**) schematic backside view of the resistive heater with a square aperture (6 \(\times\) 6 mm\({}^{2}\)) milled at the center to allow THz to pass through [111].  
In another exemplary work, [PERSON] et al. proposed a switchable ultra-thin terahertz LP-to-CP converter by hybridizing a phase change material, vanadium dioxide (VO2), with an MS, as shown in Figure 9d. Meta transition for VO2 changes the operational frequency from 0.468 THz to 0.502 THz. MS performs transmissive linear to circular polarization conversion at two stated operational frequencies. They fabricated MS using pulsed laser deposition (PLD) on a c-cut sapphire substrate using a metallic vanadium target in an oxidizing background. Further, iron milling was used to pattern cross-shaped resonators. Figure 9e shows a resistive heater with a square aperture, which was milled at the center to vary the temperature controlled by an external voltage source.