From night shifts in a basement to the Nobel Prize

4 Dec 2023

The man who snuck up on electrons: interview with freshly minted Nobel laureate Ferenc Krausz, pioneer of attosecond physics.

Portrait of Prof. Dr. Ferenc Krausz

Professor Ferenc Krausz

is Chair of Experimental Physics / Laser Physics at LMU and Director at the Max Planck Institute of Quantum Optics. | © Stephan Höck / LMU

Professor Krausz, a few weeks have passed since you got the famous phone call from Stockholm. Has it sunk in yet?

Ferenc Krausz: Yes and no. On one hand, the sense that the award is real and not just a dream has solidified within me – not least due to the huge media attention. On the other hand, the excitement has meant I haven’t had the necessary solitude to reflect on what’s happened in all its import. That process is still ahead of me.

Presumably you’ve had precious little time for research recently?

None at all in fact, although I’m in the happy position of having several world-class researchers alongside me at the helm of our Attoworld group.They know exactly what to do even without my input. As such, this break need not have any negative effects on our progress.


Nobel Prize for LMU physicist Ferenc Krausz

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How much of Ferenc Krausz is there actually in the Nobel Prize – and how much work by postdocs, doctoral researchers, and students?

An honor like this is very much the result of teamwork over long periods of time. And so I accept the prize with all humility, bearing in mind the many people who made various contributions (scientific and otherwise) at different stages of the research. At the end of the day, all the breakthroughs and results wouldn’t have been possible without the brilliance and stamina of many fantastic colleagues, including numerous non-scientists.

And what was your specific contribution?

First of all, clearly defining the questions we wanted to answer and the concrete research objectives that we should derive from these questions – and firmly believing that we can achieve them in the short or long run despite all setbacks. Persistence and obsession are important qualities here, as are self-confidence and a positive attitude. It’s also vital to have a healthy measure of discipline, so that you do not allow yourself to be distracted by the interesting new questions that pop up on almost a daily basis but which do not bring you closer to your goal.

A tool to measure electrons

One of the duties of a Nobel laurate, ahead of the award ceremony this weekend, is to present their specialist field in a way that ordinary people can grasp. Where will you start?

It makes sense to start with the original nagging question. In my case, it was the following question that began to bug me many years ago: Will I be able one day to capture the movement of electrons? Back then, the particles were thought to be unmeasurably fast. There was no tool to observe or measure them.

What tools do you need?

In principle, a very, very fast camera. To illustrate the idea: If I wanted to photograph a Formula 1 racing car on the finish line, I’d need an extremely short exposure time. Otherwise, the image gets blurred. It’s the same with electrons.

And where does the difficulty lie in developing a camera of that speed?

The laws of physics set certain limits. Light is an electromagnetic wave. You need at least a complete wave train to make an image. In the visible light range, this wavelength is around 2.5 femtoseconds – we’re talking here about millionths of one billionth of a second. Although femtosecond laser pulses rather marvelously allow researchers to observe how chemical bonds are broken in molecules, they’re still much too slow for electrons.

How come?

Electrons are around 2,000 times lighter than the lightest atom, hydrogen. And they move correspondingly faster as a result. To capture them in an image, you need time resolutions a thousand times higher than for observing molecular processes, where whole atoms move within the molecule. And that brings us to exposure times in the range of attoseconds – billionths of one billionth of a second.

A new ultraviolet flash of light in the attosecond range

How did you get one over on the laws of physics?

Firstly, by pushing femtosecond technology to its limits. This involved developing laser pulses with a single, strong field oscillation cycle, which we could then aim at neon atoms streaming out of a nozzle in their millions. The positive half-cycle of the strong electrical field near the pulse peak knocks an electron out of a neon atom. Then the ensuing negative half-cycle forces the electron back into the atom. This releases a large amount of energy in the form of ultraviolet radiation. The process occurs simultaneously – almost perfectly synchronized – in millions of atoms, which are all exposed to the same laser pulse. From start to finish, the entire process takes place in a fraction of the wavelength of the femtosecond laser. This produces a new ultraviolet flash of light in the attosecond range, transported in a collimated, laser-like beam. Exactly what we want.

Around the turn of the millennium, you weren’t the only one pursuing this line of inquiry. Pierre Agostini, another of this year’s laureates, also generated attosecond pulses.

Yes, although all other groups worked with laser pulses consisting of many wave cycles. Consequently, the generation process described above repeated itself for every half-cycle of the laser light, resulting in a whole series of successive pulses. In contrast, we were able to limit the duration of our femtosecond laser to one oscillation period and thus generate an individual attosecond flash for the first time. The other groups worked with a camera that went clack, clack, clack. Only for a short duration, but several times consecutively. Whereas our camera managed to capture a single attosecond image – one night in September 2001, in a basement lab at TU Wien.

During the night?

Yes, night shifts were the norm for us back then. The conditions are calmest at night, especially when the laboratory – like ours – is situated above a subway line, which disrupts sensitive experiments. Although I should stress that we worked through the night many times without making a breakthrough!

Did you realize immediately what had happened?

First, we repeated the measurement a few times to be sure that we actually and unmistakably had proof of an attosecond pulse. But then we were certain: bingo! At that tender hour of the morning – about four or five o’clock – we were too jaded to celebrate properly. But our joy over the following days was all the greater for that.

Faster computers, medical applications

Did you think at that moment: Wow, I’ve discovered something big, maybe even Nobel Prize worthy?

That certainly wasn’t my first thought, or my second one either. Rather, it felt as if a door had opened on to a world that nobody had ever seen before. And that exciting times lay ahead. And the thought that immediately followed was: What are we going to do with this first?

Twenty years later, attosecond physics has gone well beyond just observing electrons. What applications does it offer?

In the longer term, we hope that it’ll be useful in the development of faster computers. To date, industry has concentrated on making integrated circuits ever smaller in order to increase computing speed. However, this will soon reach hard physical limits. One way around this is to consider how we can make further progress in the fourth dimension – time – by increasing the clock rate of the chips – in the long term, up to the frequency of visible light. In experiments, we’ve been able to demonstrate that the frequency can be increased by a factor of 100,000. Attosecond physics can play a significant role here as a measurement technique.

When the Nobel Prize was announced, there was also a lot of talk about medical applications.

Yes, in fact that’s been the main focus of our research for some years now. The idea is to irradiate blood plasma with an infrared laser flash. This excites the molecules in the blood such that they emit radiation in turn – and do so in a very characteristic way, like a sort of fingerprint. We’re able to precisely scan this signal with our attosecond measurement techniques. Insofar as a disease changes the composition of molecules in the blood and thus alters this ‘infrared’ fingerprint, doctors could use this change to detect diseases at an early stage. We’re seeing the first encouraging signs how this could work in the case of lung cancer.

Interview: Alexander Stirn

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