Hell, S. W., Schmidt, R. and Egner, A.
Diffraction-unlimited three-dimensional optical nanoscopy with opposing lenses. Nature Photonics 3: 381-387 (2009).

The resolution of far-field optical microscopy stagnated for a century, but a quest began in the 1990s leading to nanoscale imaging of transparent fluorescent objects in three dimensions. Important elements in this pursuit were the synthesis of the aperture of two opposing lenses and the modulation or switching of the fluorescence of adjacent markers. The first element provided nearly isotropic three-dimensional resolution by improving the axial resolution by three- to sevenfold, and the second enabled the diffraction barrier to be overcome. Here, we review recent progress in the synergistic combination of these two elements which non-invasively provide an isotropic diffraction-unlimited three-dimensional resolution in transparent objects.