Phase-change metasurface for switchable vector vortex beam generation

Metasurfaces have attracted increasing attention due to their unprecedented capabilities of molding optical fields with exceedingly low losses. Despite significant achievements, most existing metasurfaces are passive and lack dynamic modulation post-fabrication. Here, we propose a tunable phase-change metasurface for switchable vector vortex beam generation in the mid-infrared range. The phase-change meta-molecule, consisting of two coupled Ge2Sb2Te5 (GST) bricks, is firstly designed to perform the switching between a quarter-wave plate and a usual transmissive plate over a wavelength range from 4.95 to 5.05 μm under the amorphous and crystalline phases of GST, respectively. Furthermore, a general method is derived to achieve a switchable vector vortex beam generator at the target wavelength of 5 μm by spatially orienting GST meta-molecules to locally tailor phase and polarization distribution. Under the amorphous phase, radially polarized and azimuthally polarized beams, composed of the co-polarized circularly polarized (CP) component carrying orbital angular momentum (OAM) with a topological charge of l = 0 and cross-polarized CP component carrying OAM with a topological charge of l = }2 are obtained for the left circularly polarized (LCP) and right circularly polarized (RCP) incident waves, respectively. The mode purity values of the cross-polarized component and the co-polarized component are calculated to be ∼ 0.949 and ∼ 0.955. When GST transits to its crystalline phase, the vector vortex beam disappears, and the incident CP beams pass through directly. Our finding paves the way for advanced applications targeting photonics integration with switchable functionalities.