Electro-tunable Metasurface for Tri-State Dynamic Polarization Switching at Near-Infrared Wavelengths.

Control of polarization states of light is crucial for any photonic system. However, conventional polarization-controlling elements are typically static and bulky. Metasurfaces open a new paradigm to realize flat optical components by engineering meta-atoms at sub-wavelength scale. Tunable metasurfaces can provide enormous degrees-of-freedom to tailor electromagnetic properties of light and thus have the potential to realize dynamic polarization control in nanoscale. In this study, we propose a novel electro-tunable metasurface to enable dynamic control of polarization states of reflected light. The proposed metasurface comprises a two-dimensional array of elliptical Ag-nanopillars deposited on ITO (indium-tin-oxide)-AlO-Ag stack. In unbiased condition, excitation of gap plasmon resonance- in the metasurface leads to rotation of x-polarized incident light to orthogonally polarized reflected light (i.e. ypolarized) at 1.55 μm. On the other hand, by applying bias-voltage, we can alter the amplitude and phase of the electric field components of the reflected light. With 2V applied bias, we achieved a linearly polarized reflected light with a polarization angle of -45. Furthermore, we can tune the epsilon-near-zero (ENZ) wavelength of ITO at the vicinity of 1.55 μm wavelength by increasing the bias to 5 V, which reduces y-component of the electric field to a negligible amplitude, thus, resuting in an x-polarized reflected light. Thus, with an x-polarized incident wave, we can dynamically switch among the three linear polarization states of the reflected wave, allowing a tri-state polarization switching (viz. y-polarization at 0 V, - 45linear polarization at 2 V, and x-polarization at 5 V). The Stokes parameters are also calculated to show a real-time control over light polarization. Thus, the proposed device paves the way towards realization of dynamic polarization switching in nanophotonic applications. .