Optical microscopes have been around for more than 400 years, steadily improving, but for the past century it was thought they had reached the limit of their ability. The 2014 Nobel Prize for chemistry was awarded to three men who showed that such limits could be overcome, if not actually broken.
In the 1590s the first microscopes were more like telescopes with additional lenses to examine items close up. With a magnification of x9, they were more a novelty than anything else. Within a century, however, they were an invaluable tool for scientists – Robert Hooke’s illustrated Micrographia of 1665 includes detailed drawings of anything from fleas to snowflakes and the first observation of what we now know as plant cells.
In the 1850s German engineer Carl Zeiss, with the aid of optics specialist Otto Schott, began systematically improving lenses and construction of microscopes, but Zeiss also hired Ernst Abbe to help the project. Abbe used theoretical principles of optics to improve the design, but declared in 1873 that there was a finite limit to a microscope’s power – because optical microscopes rely on wavelengths of light, and therefore nothing smaller than half a wavelength or 200 nanometres (millionths of a millimetre) could be observed. Although electron microscopes have since then been developed, capable of registering items just 1 nm across, they have their drawbacks and cannot be used on living cells and tissue. Hell says he felt “instinctively” that optical microscopes based on lenses could go beyond Abbe’s limit, but that it “hadn’t been thought through”.
Stefan Walter Hell was born in Arad, Romania, in December 1962, originating actually from nearby Santana (German: Sankt Anna), a larger rural community founded by German immigrants in the 18th century. There he grew up and attended primary school. He spent one year of secondary education at Nikolaus Lenau High School in Timisoara before persuading his parents to move to West Germany in 1978. The family (his father was an engineer and his mother a teacher) settled in Ludwigshafen in the south-west. He enroled at Heidelberg University in 1981 and left with his PhD in physics (studying confocal microscopy) in 1990.
There followed a brief period as an independent inventor, during which Hell developed his ideas for improving depth resolution in confocal laser microscopy. He then joined the European Molecular Biology Laboratory in Heidelberg (from 1991-93) where he further developed his ideas into what is now known (slightly erroneously and not his choice) as 4Pi microscopy, which allows up to seven times sharper in depth resolution than previous models.
Feeling that a more radical step would be feasible, and not finding a receptive place in Germany, Hell took his plans to the University of Turku in Finland where, apart from a six month placement as visiting scientist at Oxford University, UK, he worked from 1993-96 as a senior scientist. A few weeks after his arrival, Hell realized the principle of stimulated emission depletion (STED) microscopy which he then worked out and published with Jan Wichmann. This was to become the first microscope to radically cross Abbe’s threshold and by and large his Nobel-winning design. Yet his idea was met with disbelief, not only in Germany, but worldwide.
However, in 1996, senior scientists at the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany realized the potential of Stefan Hell’s vision. In December that year he returned to Germany to join the Max Planck Institute for Biophysical Chemistry in Göttingen, where he has worked ever since, establishing a group and rising to the rank of director in 2002.
It was there, in 1999, that he and Thomas Klar first demonstrated a working model of a STED microscope. The key idea behind this microscope was to make some of the molecules that are illuminated by a beam of light non-fluorescent. Typically, a focused beam of laser light designed to excite fluorescence molecules to the fluorescent state is overlaid with a ring-shaped beam designed to keep them non-fluorescent (by instantly stimulating their back-transition to the ground state). Fluorescence photons are released only from molecules residing at the very centre of the ring-shaped beam. Since this region can be made much smaller than a diffracted beam, scanning these ‘on-and off-turning’ beams jointly across the specimen causes features residing within subdiffraction proximity to emit consecutively. In this way, features just 20 nm across can be imaged, and high-speed recordings made of active biological processes. In principle, the region in which the molecules are allowed fluoresce can be tuned further down to the size covered by a single molecule, providing molecular scale resolution.
The STED microscope has proved particularly useful for investigating diseases and cells, and since 2003 Hell has also been the leader of the department Optical Nanoscopy at the German Cancer Research Centre in Heidelberg. He is also an adjunct professor of physics at the universities of Heidelberg and Göttingen.
For his work, Hell has received many major awards since 2000, culminating in the Nobel Prize, which he shares with the Americans Eric Betzig and William Moerner.
By Volker Steger
The diffraction barrier is a black line dividing Stefan Hell’s sketch: His Nobel discovery broke through it, revealing a new world of detail in light microscopy.
To do this on paper took a lot of wax crayons - I had to sharpen 5 after Stefan Hell’s visit to the studio.This man sketches with a forceful stroke…