Following conventional wisdom, the resolution of light microscopy is limited by diffraction to about half the wavelength of light, which is why conventional light microscopes fail to distinguish object details that are closer together than ~200 nanometers. Stefan Hell and co-workers have broken this century-old barrier by developing, since the early 1990's, novel fluorescence microscopes featuring diffraction-unlimited spatial resolution. Thus, they also laid the foundation of a new scientific field: super-resolution fluorescence microscopy, also known as nanoscopy .
Stefan Hell's department is developing light microscopes with a spatial resolution down to a few nanometers, particularly, but not exclusively, for imaging living cells and tissues. Prominent methods include STED and RESOLFT microscopy, as well as concepts based on stochastic single-molecule switching such as GSDIM microscopy. To surpass the diffraction barrier, all these methods utilize a reversible transition or switch of fluorescent labels between a bright and a dark state. In combination with 4Pi microscopy, which is another concept developed by this group that uses two opposing lenses, the resolution can be increased in all spatial dimensions down to the nanometer scale. Since these super-resolution concepts fundamentally rely on transitions between molecular states, novel labels are required that can be optically prepared in at least two different states. Consequently, the group also pioneers the chemical synthesis and application of new labeling methods and techniques to improve the performance of the labels’ switching behavior to separate close-by molecules.
Current efforts of this interdisciplinary group of physicists, chemists and biologists aim to improve resolution, contrast, speed and versatility of optical nanoscopy. The outstanding biomedical research environment in Heidelberg and beyond allows for close collaborations (e.g. ) with investigators from various fields of molecular physiology and pathology.
 Hell S.W. "Far-Field Optical Nanoscopy." Science 316, 1153-1158 (2007).
 Chojnacki J., Staudt T., Glass B., Bingen P., Engelhardt J., Anders M., Schneider J., Müller B., Hell S.W., Kräusslich H.-G. "Maturation-Dependent HIV-1 Surface Protein Redistribution Revealed by Fluorescence Nanoscopy." Science 338, 524-528 (2012).