Fig. 4

Reconfigurable metalenses. a Adaptation of metalenses to the 2D case. Each discrete element changes the local phase of light, so the wavefronts converge to a focal spot. b, c Local phase (in degrees) of each element as a function of the element parameter and the frequency. b refers to unit cells with period 1.8 µm, 1 µm width and variable lengths. c refers to unit cells with period 1.2 µm, 9 µm length and variable widths. d Designed phase profile of the two metalenses (see Supplementary Note 2 for additional information). The design frequency was 1452 and 1448 cm⁻¹ for metalens 1 and metalens 2, respectively. e Optical images of the written metalenses. Metalens 1 was written, characterised, erased and subsequently the same area was reconfigured into metalens 2. f s-SNOM image of metalens 1 showing focusing of polaritons at 1452 cm⁻¹. g s-SNOM image of metalens 2 at 1445 cm⁻¹. h Intensity profile (square of s-SNOM signal) at the cross-section of the diffraction-limited focused polariton beam for both lenses (dashed vertical lines in f, g). Scale bars are 5 µm