Figure 4. Complex lenses.

(a) A dual-layer STED element composed of one layer designed to focus a green laser (λ=550 nm) at f=1 mm and a second spiral-shaped layer designed to focus a doughnut-shaped red laser beam (λ=650 nm) at the same focal distance. The topological charge of the spiral beam was set to l=1. (b) Obtained image at the focal plane for illumination with super-continuum source. (c,d) Corresponding element bright-field image and focal plane image as in a,b, respectively, for the case of topological charge l=4. The interference between the generated vortex beam and a background beam transmitted by the plate gives rise to the spiral-shaped vortex in b and the four lobe vortex in d. Scale bar, 35 μm (a,c) and 5 μm (b,d). See the recorded propagation of the beams in space showing their red vortex and green focusing characteristics in Supplementary Movies 1 and 2. (e) Demonstration of the functionality of a lens that was designed to show anomalous chromatic aberration of its RGB foci, which is shorter wavelengths focus before longer wavelengths (contrast was enhanced for clarity).