Stefan Hell and microscope revolution

Stefan W. Hell is a Romanian-born German physicist who won a Nobel Prize in Chemistry in 2014 for his contribution to the development of microscope with super-resolution. His work, with two other scientists, Eric Betzig and William Moerner, helped scientists around the world to see tiny structure with much more detail than before. For more than a century the ability of a light microscope to discriminate two or more objects was limited to several hundreds of nanometer. Meaning that no mater how hard you try you will never be able to see a very small object with size of less than several hundreds of nanometer using your regular light microscope. The limitation was known for more than a century as Abbe’s diffraction limit. Ernst Abbe the German physicist formulated this law in 1873. This law was absolutely a limit to our understanding of many processes happened in very tiny object like biological cell.

Abbe’s diffraction law is sculptured on Ernst Abbe memorial at Friedrich Schiller Universitat Jena. (By Daniel Mietchen (Own work) [CC0], via Wikimedia Commons)

Thanks to Hell who during his time as a PhD student at the University of Heidelberg, and worked as an intern at a startup company founded by his supervisor, started to think about how to break the Abbe’s diffraction limit. At the company he was working on microscopy to evaluate semiconductor devices. To surpass the Abbe’s diffraction limit, Hell began searching for basic laws in several books of physics and struck by the idea of coupling two objectives oppositely to form almost spherical wavefront of light. This idea was then became his PhD thesis and developed more when Hell worked at the European Molecular Biology Laboratory (EMBL) in Heidelberg. The idea was later known as 4Pi microscopy.

The work on 4Pi microscopy did not make Hell satisfied since the best resolution that can be achieved by 4Pi microscopy at that time was only several times better than the Abbe’s diffraction limit. In early 1990s, when his time as a postdoctoral researcher at EMBL was nearly end, Hell decided to move to the University of Turku in Finland to work under Professor Erkki Soini who was interested in fluorescence microscopy. In Finland he was still working on the 4Pi microscopy and trying to boost its capability, but then he began to realized that by only rearranging lenses will not solve the resolution problem. He then started to think about breaking the Abbe’s law in different way using quantum optical principles. Hell began to read a quantum optics book by Rodney Loudon and inspired by the concept of ‘stimulated emission’. Hell started to think about using this concept to increase the resolution of far-field microscope using fluorescence molecules which size is much less than the Abbe’s diffraction limit. With prior assumption that the fluorescence molecules is a single emitter, he though that by isolating the single emitter from the surrounding emitters he can increase the resolution of fluorescence microscope.

Fluorescence molecule emit specific wavelength of light when excited by light with shorter wavelength, by stimulated emission this light emission can be quenched to dark by light that resonate at the same frequency that bring the molecule back to its ground state. The idea proposed by Hell was firstly only ended up in pieces of papers or theoretical due to the limitation of funding to build the real microscope. The idea was then realized in early 2000s when Hell was back to Germany to fill a position in the Max Planck Institute of for Biophysical Chemistry in Gottingen.

Display of excitation focus (left), de-excitation focus (center) and remaining fluorescence distribution in a STED microscope. (Lexic 4712 at the German language Wikipedia [CC BY-SA 3.0 (, CC BY-SA 3.0 de ( or GFDL (, via Wikimedia Commons)

In his microscope he shines two pulsed-laser with different intensity and frequency. The first laser with low intensity excites all the fluorescence molecules, in diffraction limit region, to their excited state and emit fluorescence light. The second laser with red-shifted frequency and higher intensity brings back the excited fluorescence molecules to their ground state and turned off the light. The second laser forms a doughnut-like intensity structure, a zero intensity at its focal center and growing intensity at surrounding lateral and axial position from its focal center. This configuration leaves the single emitter at the center of microscope focal to glow individually while surrounding molecules turned off. Single molecule of fluorescence can then be resolved and by scanning over biological structure that was tagged by fluorescence molecules, the biological structure can be revealed with much more details than what can be achieved by conventional far-field fluorescence microscopy.

This image shows the resolution gain possible with STED microscopy over confocal microscopy. Confocal raw data is on the left side and STED raw data on the right. The dye is Abberior STAR635P. The optical resolution of the STED image is 25nm or smaller as can be seen from diameter of the single dots (antibody clusters). (By Fabian Göttfert, Christian Wurm [CC BY-SA 3.0 (, via Wikimedia Commons)

Hell’s idea of using stimulated emission in fluorescence microscopy, which then called as stimulated emission depletion (STED) microscopy, opened up a way to deepen our understanding in many field of science like how a disease developed in cells which will certainly help to eradicate the disease. I personally quite impressed by the story of Stefan Hell in breaking the law that was seemingly impossible to be broke for over a century. Hell is a passionate and persistent scientist who is not easily give up in fulfilling his dream. Beside, he is very zeal in challenging something that other taken for granted. Hell has all the characteristics that are needed by all young scientist to be a successful scientist.

References and further readings:

[1] Stefan W. Hell – Biographical,

[2] Uta Deffke, “Outsmarting optical boundaries”,

[3] Popular science background on The Nobel Prize in Chemistry 2014: How the optical microscope became a nanoscope,

[4] Scientific Background on the Nobel Prize in Chemistry 2014: Super-resolved fluorescence microscopy,

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