Figure 10.

Continuous or multi-state active polarization engineering. (a) Graphene integrated with an anisotropic metasurface for linear to elliptical polarization conversion upon reflection. (b) Reflection spectra of x- and y-polarized beams with different gate biasing voltages. The design induces a strong Fano resonance only when the polarization is along the bar direction, i.e., the y direction. The Fano resonance dip in y polarization shifts with the voltage, and the x polarization is unaffected. (c) The tilt angle is adjustable with constant ellipticity by changing the biasing voltage as −200 V, 0 V, and 250 V at $7.72\mathsf{\mu }\mathrm{m}$. Reproduced with permission from [100], Copyright The authors, 2017. (d) Graphene-loaded slot-shaped metasurface for polarization conversion due to strong resonance along the short edge and weak interaction along the long edge. (e) Reflection phase difference between x and y polarization states under different voltages. The phase differences of ${90}^{°}$, ${180}^{°}$, and ${270}^{°}$ are obtained at 7.1 $\mathsf{\mu }$m with proper voltage applied. (f) The ${61}^{°}$ linear polarized light is changed to LCP, cross-LP, and RCP when the voltage is 34 V, 89 V, and 170 V relative to the charge neutral point (CNP) for encoding and multiplexing applications. Reproduced with permission from [101], Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2015. (g) Unit cell of a tunable polarization rotator composed of ${45}^{°}$-rotated bi-layer metallic gratings and orthogonal bi-layer graphene gratings. The polarization rotation is enhanced by the Fabry–Perot interference between the top and bottom gratings and dynamically changed via modification of the graphene conductivity (h) The polarization rotation angle varies continuously from ${20}^{°}$ to ${45}^{°}$ and ${45}^{°}$ to ${70}^{°}$ by sequentially tuning the Fermi level of the orthogonal graphene gratings with a high transmission efficiency of above $75\%$ in a wide band between 0.83 and 1.2 THz. Reproduced with permission from [102], Copyright Elsevier Ltd., 2018.