“The diffraction barrier responsible for a finite focal spot
size and limited resolution in far-field fluorescence microscopy
has been fundamentally broken.” With that blunt assessment,
published in the Proceedings of the National Academy of Science
in 2000, Stefan Hell introduced the world to a “super-
resolution” microscopy technique called stimulated emission
depletion, or STED. Or rather, he reintroduced the technique.
Hell had first described STED—which cracks the diffraction limit
that has for so long bounded optical microscopy—in a brief two-
and-a-half page mathematical treatise six years earlier. “I had
a very hard time to convince people it would work,” Hell
recalls. The idea of a “diffraction barrier,” a physical
constraint on light microscopy resolution, was simply too well
ingrained. Following his 2000 article, though, researchers
started paying attention; the publication has racked up some 310
citations, according to the ISI Web of Knowledge. It also
attracted the notice of microscopy manufacturer