Sub-Abbe resolution: From STED microscopy to STED lithography

We review the emergence of three-dimensional sub-Abbe optical nanoscopy, which in its original version deployed stimulated-emission-induced depletion (STED) of the excited state of the fluorophores to break the diffraction limit. We focus first on different methods to realize a donut-shaped point spread function (PSF), which are key to distribute the intensity of the depleting beam around the originally excited focal volume, and second on methods to suppress fluorescence, which are alternative to stimulated emission. In the second part, we elaborate on the transition of STED microscopy to STED lithography. We present data which show the state of the art of minimal axial feature sizes (around 55 nm). Again, stimulated emission is not the only technique to decrease the feature size and the resolution in optical lithography. Alternative techniques are up-conversion within the triplet system or excitation of polymerization stoppers inside the donut-shaped depletion PSF. We conclude with a comparison of the record resolutions in STED and STED-related lithography techniques, which are in the range of some tens of nanometers, far below the wavelength of visible light.