Phase-change metasurface for switchable waveplates

In this paper, a transmissive tunable metasurface based on phase-change alloy Ge₂Sb₂Te₅ (GST) has been proposed, exhibiting the function of switching between a quarter wave plate (QWP) and a half wave plate (HWP) in mid-IR region, i.e., providing circular-to-linear and circular-to-circular polarization conversions at amorphous state and crystalline state, respectively. For the amorphous GST, linear polarized (LP) transmitted beams with angles of linear polarization (AoLPs) close to ±45° can be obtained with the degrees of linear polarization (DoLPs) close to 1 under normal circular polarized (CP) excitations at the wavelength varying from 4.8 to 5.2 μm. As for the crystalline GST, the normally incident CP lights can be converted into the corresponding orthogonal CP beams with the degrees of circular polarization (DoCPs) close to ±1 within the wavelength range from 4.95 to 5.05 μm. The polarization state diagrams of the transmitted beams with good profiles further prove the good performance of the designed GST metasurface. Our finding paves the way for the development of flat polarization optical components with switchable functionalities.