July 06, 2011
How deeply can we penetrate into the details of the visible world with optical microscopes? Previously, the law formulated by Ernst Abbe in 1873 was regarded as the absolute lower limit. Objects lying closer to each other than 200 millionths of a millimetre, i.e. about one two hundredth of a hair's breadth, can no longer be distinguished from one another. The reason for this is the wave nature of light, the half wavelength of which roughly corresponds to those 200 nanometres.
The STED (Stimulated Emission Depletion) microscopy, which the Göttingen based physicist Stefan Hell invented and developed to application readiness, allows scientists to gain insights into the nano world far beyond this limit. Biologists and physiologists in particular value this breakthrough, because living cells or tissue can only be observed using optical microscopes. In 2008, for instance, neurophysiologists using the new resolution of only a few dozen nanometres succeeded in visualising the movements of tiny synaptic components for the first time. In addition, the concept underlying STED microscopy opened up new prospects for the further development of optical Stefan Hell overcame Abbe's barrier in the imaging of fluorescent objects.
In this process, which is used widely in biology and medical research, the specimens to be examined are marked with fluorescent molecules and illuminated – e.g. with a focussed laser beam. The beam excites the molecules so that they emit fluorescent light, thereby making the marked cell components visible. Here too, the fluorescent light emitted by the closely adjoining dots also becomes an indistinct blur, but Hell found a simple trick to break through Abbe's barrier. This ensures that the cell components illuminated by the excitation beam do not emit fluorescence simultaneously, but sequentially. To achieve this, Hell applies a second beam (STED beam) which temporarily prevents the fluorescent markers from emitting light, i.e. it switches them off.