An interview with former IBM scientist Dieter Wolfgang Pohl looks at near-field optics (NFO) in context with super-resolution fluorescence microscopy that was recognised with the 2014 Nobel Prize in Chemistry
Scientific curiosity has been a driving force behind the research of retired IBM scientist Dr. Dieter W. Pohl. It drove him to put the famous Abbé diffraction limit in question. His 1982 invention, the “scanning near-field optical microscope” or SNOM (European Patent EP0112401, US Patent US 4604520) provided a groundbreaking technique for optical investigations in the nanometer regime, i.e. beyond the diffraction limit.
“Actually I dreamed of becoming an architect as a child. However, my skill of drawing was poor. So I decided it better to go into physics,” Dieter said, “and I never did regret it”.
|Dieter in the late 1980s|
Dieter’s invention thus was a forerunner to the various methods of “breaking the diffraction limit” known today. Two of them, based on complementary research on single molecule fluorescence, achieved this year’s Nobel prize in chemistry, awarded today in Stockholm to E. Betzig, W.E. Moerner, and S. Hell.
Due to the propinquity of their fields of research, Dieter came to know the three Nobel Prize winners personally. In fact, he had a collaboration with one of them, W.E. Moerner, a former IBMer himself in the 1990s. It resulted in a joint paper, “Near-Field Optical Spectroscopy of Individual Molecules in Solids”
He added, “They received the prize for the high resolution microscopy that was enabled by clever use of fluorescent molecules. This is a whole field of its own. Near Field Optics is a different approach to optics in the nanoworld. You can do high resolution fluorescence microscopy with NFO, but also a lot of other things. And not all microscopy is fluorescence microscopy.”
Dieter earned his doctoral degree at the Technical University Munich in 1968. He pursued his field of research, lasers, at IBM’s first international laboratory in Switzerland. It had been officially opened only five years before in Rueschlikon and had selected laser optics as one of its core themes. Some 10 years later, it was to become the birthplace of two of the highest resolution microscopes, scanning tunneling microscope (STM), and shortly later, SNOM in optics.
A Brief Stop in New York
After a couple of years in Switzerland, he had the chance to go to the IBM T.J Watson Research Center in Yorktown Heights, NY for an assignment — a time which would play an important role throughout his career.
“It was a very interesting and challenging situation. Coming from Switzerland, you were not so familiar with the rest of the IBM universe. I gained a lot of general knowledge and good friends, it was a very fruitful time.”
Back in Switzerland in 1980, Dieter met a young, promising physicist, future Nobel Laureate Gerd Binnig. Gerd had just invented the scanning tunneling microscope with his colleagues Heinrich Rohrer and Christoph Gerber.
|A new experimental technique called “forced thermal scattering
demonstrated by Dieter.
“It was a great thing, but it challenged me. I said ‘why can’t we do such a resolution with an optical microscope?’ And that was the motivation to look at the limitations of the existing technology.”
At that time optical microscopy was characterized by a presumed limitation: that it would never obtain a better resolution than half the wavelength of light. Helped by a small hole, however, Dieter and his team cleverly circumvented this limitation.
“I said ‘what happens if you press a tiny hole in a metal screen – in optics called an ‘aperture’ – and shine light on it?’ At 20 nanometers (a sheet of paper is 100,000 nanometers thick) will the light get completely blocked or is it transmitted? If so, could it be used to scan a surface and in this way get an optical image of it? We didn’t know, so we tried it and, lo-and-behold, it worked – we had surpassed Abbé’s diffraction limit in optics for the first time.”
This breakthrough discovery took place in 1982. Essential was that they had placed the hole at the apex of a transparent, metal-coated tip. As any good IBM inventor, Dieter filed the patent mentioned before, followed by an Applied Physics Letter.
Delighted to present his findings at an optical instruments conference in 1985, the response turned out to be nothing what he expected. “I was so proud to present these results. There were many high ranking old men in the audience. They all said ‘no, this can’t be possible.’ And I watched as they turned away, mumbling to their neighbors in utter disbelief,” said Dieter.
“After the talk, an elderly man approached me and said ‘I think there is some truth to what you have said. Don’t give up!’ It turned out that this man was Ludwig Leitz II, co-inventor of the famous Leica Camera, grandson of the founder and the former co-owner of Leitz company.”
The Ah-Ha Moment
In the early 1990s near-field optics’ interest had risen to a sizeable number of scientists in various countries; it was time for a meeting which Dieter and Daniel Courjon (from France) organized. Some 50 people participated, among them 2014 Nobel Prize winner Eric Betzig.
“I remember that he presented excellent instrumental improvements and beautiful NFO images. We all were very impressed. It’s a pity that he left the NFO field. It has become a major activity in optics in recent years.”
Because, during Eric’s absence from science, emphasis in NFO changed from mere microscopy to a broader way of looking at near field optics.
Dieter contributed to this transformation. “During a walk, I stared at a TV antenna on a roof. Suddenly it came to my mind that it’s the ideal near field arrangement! The essential part of the antenna are the two metal arms where the wires are connected. Its total length is approximately half a TV-wavelength, making it resonant, and therefore sensitive for TV waves. The gap between the arms, on the other hand, is much smaller, and an alternating voltage exists in between – this is exactly the requirement for a near-field probe.”
Dieter worked on his idea w
ith increasing excitement. “I came to rate all the different NFO probes as antennas with a different radiation efficiency. And text books taught me that the aperture in a metal screen – a slot antenna in the jargon of electrical engineers – was one of the worst for optics.”
After introducing his theoretical findings at a 1999 conference in Beijing, he took steps to demonstrate a resonant optical antenna. He and his team at University of Basel, Switzerland, prepared gold dipole antennas with dimensions of 150 to 250 nm only. The antenna concentrated the irradiated energy so much into the gap that even nonlinear effects showed up. “You could see blue light come out from the antenna region while shining in with deep red light.”
With the strongly enhanced light spot in the gap of an efficient antenna, instead of the weak one at the tiny aperture, a plethora of novel experiments and potential applications became feasible.
“The story of Near Field Optics is just beginning,” says Dieter. And he adds with a smile “Look at the NFO conferences, the number of participants has risen from some 50 people to more than 400 in the last 20 years.”
After 31 years of service Dieter retired from IBM in 1999. He was granted permission to take his experiments with him to the University of Basel. At present, he has a collaboration with the Photonics Laboratory of the ETH in Zurich.
“What always drove me was the wish to get to the bottom of something.” Dieter said. “The advice I would give to a young scientist is to be open, persevere, and always fair.”