Hell, S. W.
Strategy for far-field optical imaging and writing without diffraction limit. Physics Letters A 326: 140-145 (2004).

A general concept is described for subdiffraction imaging and writing with focused light and conventional optics. It is unique in the fact that the giant optical nonlinearities required for breaking the diffraction barrier are realized at low light intensities. The result is that far-field resolution on the nanoscale becomes feasible at realistic physical conditions. The predicted subdiffraction resolution or feature size is formulated in a simple equation.