Fig. 2

Rewritable flat polaritonic lenses. a Plano-convex lens schematics for 3D and 2D semi-spherical and semi-circular lenses. In the 2D case the material refractive index is replaced by the effective index of the polaritons on amorphous or crystalline GST. b–f Optical images of the written lens. The written patterns are clearly visible in the pictures because the refractive index of a-GST and c-GST also differs at visible wavelengths. First a plano-convex semi-circular lens (radius R = 5 µm) is written and measured, then it is erased, re-written (with same dimensions), erased again and finally the same area is reconfigured into a plano-concave lens (R = 5 µm). g, h Diagram of wavefronts for 2D plano-convex and plano-concave lenses, respectively. i s-SNOM image of the plano-convex lens after the first writing. A focal spot is clearly visible. j Dependence of the focal length on the wavenumber. Error bars represent the uncertainty of the fitted waist position, approximately one fringe). k s-SNOM scan after first erasing. l s-SNOM scan of the re-written plano-convex lens. m s-SNOM scan of the plano-concave lens (third writing). s-SNOM images in i, l and m have been processed to remove the fringes outside the main beam (see Supplementary Note 4, Supplementary Table 3 and Supplementary Fig. 5). Scale bars are 5 µm