Hell, S. W.
Fluorescence Nanoscopy: Breaking the Diffraction Barrier by the RESOLFT Concept. NanoBiotechnology 1 :296-297 (2005).

A general concept is described for breaking the diffraction barrier in far-field microscopy through reversible saturable optical transitions. This concept is based on the huge optical nonlinearities required for the saturation that are realized at relatively low light intensities. Thus, subdiffraction resolution in the nanometer scale is feasible under realistic physical conditions. The subdiffraction resolution achievable is expressed in a simple equation as a function of the saturation level of the involved transitions, and experimental examples of fluorescence microscopy techniques with resolution of a few nanometers are given.