Ever since Ernst Abbe set a lower limit to the size of things that can be seen under a microscope, people have been looking for ways to surpass that limit. While electron microscopes and several new types of microscopy have since broken through the limit, a method developed by physicists at the Weizmann Institute of Science suggests a new way of going below the limit with a standard optical microscope.
Abbe was a physicist and managing partner in Zeiss Optical Works – a position offered to him by the founder of the company, Carl Zeiss. Abbe set his limit in 1873, when he stated that even the most perfectly ground lenses would not be able to distinguish objects smaller than half the length of a wave of visible light. In practice this corresponds to a resolution limit of about 200 nanometers.
It would take some 60 years for the arrival of a microscope that could break the optical limit. Ernst Ruska and his doctoral adviser Max Knoll invented the first electron microscope in 1933. This microscope used an electron beam instead of light. Because the wavelength of an electron is much less than the length of a visible light wave, the beam enabled them to see things that are up to 2,000 times smaller than what can be observed with a standard optical microscope. The drawback to electron microscopes is that samples must be fixed, so living things cannot be imaged under these microscopes. It would take some 60 years for the arrival of a microscope that could break the optical limit.
Almost 70 years after this, in 1999, Stefan Hell unveiled a new type of microscope, called STED (for stimulated emission depletion), which presented a completely new way of surpassing Abbe’s limit. In STED, one laser beam excites an electron in a molecule of the material under observation. When excited, the electron emits a photon – a particle of light – in a process known as fluorescence. In Hell’s microscope, a second laser douses the fluorescence, and the lasers then quickly scan the sample, making it possible to observe objects just a few nanometers in size; they can distinguish, for example, active proteins in biological systems.
An alternative method, known as PALM (photoactivated light microscopy), followed on the heels of STED. This method relies on light-emitting molecules that are also activated by light. The developers of PALM, Eric Betzig and William Moerner shared the Nobel Prize in Chemistry with Hell in 2014, for breaking Abbe’s limit and providing new super-high-resolution microscopy for research labs around the world.
Sensitive to single photons
Profs. Yaron Silberberg and Dan Oron and their colleagues in the Physics of Complex Systems Department at the Weizmann Institute of Science thought that quantum physics might offer another way around Abbe’s limit. (Abbe, of course, did not have the benefit of quantum theory, which only appeared on the scene several decades later.) Specifically, in quantum mechanics, any molecule or nano-sized particle that is excited – for example, by shining a laser beam on it – can emit one, and only one photon. Their idea was to “photograph” the process of photon emission. Of course this process happens so quickly that even the fastest camera – one that is sensitive to single photons – still cannot capture it as it happens.
The Institute researchers swapped the standard microscope camera for a system of ultra-fast detectors that are so sensitive, they can pick up the emission of a single photon; and they installed this on a scanning optical microscope (one based on the confocal microscope invented in 1957 by Marvin Minsky). Their calculations showed that this system should break Abbe’s limit. And, indeed, an experiment with such a system succeeded in observing objects 2 ½ times smaller than those at Abbe’s limit. The scientists say their calculations suggest that further calibration and experimentation should get the limit of the combined system even lower, to around four times smaller than the size set by Abbe.