Hell, Stefan W.
Light microscopy has continually played a key role in science, but diffraction has limited the imaging of details that are smaller than about half the wavelength of light. For the important contrast mode of fluorescence, which is crucial to modern cell and molecular biology, the diffraction barrier has now been broken. In spite of relying on focused visible light, stimulated emission depletion (STED) microscopy is not limited by diffraction, but only by the perfection of its implementation. To date, current schemes of STED-microscopy have delivered 50 nm (1/12 of the wavelength) resolution on cell membranes and even 16 nm in optimized experiments. In STED microscopy, the key physical element for overcoming the diffraction barrier is a reversible saturable optical transition between two fluorophore states. The generalization of this concept, termed RESOLFT, has the fascinating potential to provide molecular scale resolution with visible light and regular lenses, even at ultralow intensities of light.