Editing Diffraction-limited system (section)

===Far-field techniques===

Far-field imaging techniques are most desirable for imaging objects that are large compared to the illumination wavelength but that contain fine structure. This includes nearly all biological applications in which cells span multiple wavelengths but contain structure down to molecular scales. In recent years several techniques have shown that sub-diffraction limited imaging is possible over macroscopic distances. These techniques usually exploit optical [[Nonlinear optics|nonlinearity]] in a material's reflected light to generate resolution beyond the diffraction limit.

Among these techniques, the [[STED microscope]] has been one of the most successful. In STED, multiple laser beams are used to first excite, and then quench [[fluorescent]] dyes. The nonlinear response to illumination caused by the quenching process in which adding more light causes the image to become less bright generates sub-diffraction limited information about the location of dye molecules, allowing resolution far beyond the diffraction limit provided high illumination intensities are used.