Design of Dynamically Focus-switchable Fresnel Zone Plates Based on Plasmonic Phase-change VO2 Metafilm Absorbers

Novel thermo-optically focus-switchable Fresnel zone plates based on phase-change metafilms are designed and analyzed at a visible wavelength (660 nm). By virtue of the large thermo-optic response of vanadium dioxide (VO2) thin film, a phase-change material, four different plasmonic phase-change ab-sorbers are numerically designed as actively tunable Gires-Tournois Al-VO2 metafilms in two and three dimensions. The designed phase-change metafilm unit cells are used as the building blocks of actively focus-switchable Fresnel zone plates with strong focus switching contrast (40%, 83%) and high numeri-cal apertures (1.52, 1.70). The Fresnel zone plates designed in two and three dimensions work as cylin-drical and spherical lenses in reflection type, respectively. The coupling between the thermo-optic effect of VO2 and localized plasmonic resonances in the Al nanostructures offer a large degree of freedom in design and high-contrast focus-switching performance based on largely tunable absorption resonances. The proposed method may have great potential in photothermal and electrothermal active optical devic-es for nonlinear optics, microscopy, 3D scanning, optical trapping, and holographic displays over a wide spectral range including the visible and infrared regimes.