2023 Nobel Prize in Physics: How ultrafast lasers track electron movements

Three experimentalists have jointly won the 2023 Nobel Prize in Physics for their work in using pulses of light to probe how electrons behave inside atoms and molecules.

The winners

Anne L’Huillier is a French-Swedish professor of atomic physics at Lund University in Sweden.

In 1987, L’Huillier discovered that an infrared laser interacting with atoms in a noble gas created “overtones” of different wavelength light as the initial pulse interacts with the electrons in the atoms.

Pierre Agostini is a French experimental physics professor at Ohio State University, US.

Agostini built on this in 2001 when he successfully produced and investigated consecutive light pulses which lasted only 250 attoseconds – an attosecond is one quintillionth – or one billion billionth – of a second.

Ferenc Krausz is a Hungarian-Australian physicist, director at the Max Planck Institute of Quantum Optics and a professor of experimental physics at Germany’s Ludwig Maximilian University of Munich.

At the same time as Agostini, Krausz was working on a different experiment which produced a single light pulse of only 650 attoseconds.

Every attosecond matters

There are more attoseconds in one second than there have been seconds since the Big Bang!

It is on the scale of attoseconds that electronic changes in atoms and molecules occur.

This year’s physics Nobel Laureates helped create the tools necessary to make “films” of how electrons move inside atoms and molecules with attosecond framerates using lasers. It is like how a film camera captures a series of still images which show movement.

Hummingbirds, for example, can beat their wings 80 times a second. We can’t perceive each beat of a hummingbird’s wings with our eyes. But high-speed photography and strobe lighting can capture the flapping wings – as long as the pulses of light are separated by a time shorter than that of each beat. The faster the event, the faster the pulses have to be.

“We can now open the door to the world of electrons. Attosecond physics gives us the opportunity to understand mechanisms that are governed by electrons. The next step will be utilising them,” says Eva Olsson, Chair of the Nobel Committee for Physics.

Applications of the technology range from understanding how electrons behave in materials for electronics, and for identifying different molecules in medical diagnostics for example.

How did they do it?

Light is a wave (yes, a particle as well). Each light particle, a photon, is a vibration in the electromagnetic field. Photons have different wavelengths – this is associated with different colours in the visible spectrum (wavelengths between about 400 and 700 nanometres), or different types of radiation.

For example, red light of 700nm cycles at about 430,000,000,000,000 times a second.

The shortest possible pulse of light – the time it takes for one cycle to go from a peak to a trough and back to a peak – is on the order of femtoseconds, which is a thousand times too long for electron movements to be studied.

But the mathematics of waves tells us that any waveform can be created by adding other waveforms together. So, theoretically at least, shorter-cycle light waves could be created by combining pulses of light together.

Enter L’Huillier’s overtones. These are wavelengths of light that cycle multiple times for every one cycle of the original wavelength. The same principle applies to sound waves from a guitar string or flute which gives the same note different sounds when played on the different instruments.

Creating many different overtones of light at the same intensity meant that they could coincide to produce a series of pulses of ultraviolet light. These wavelengths have cycles in the attoseconds.

Krausz and Agostini were able to produce these attosecond pulses in 2001. For the first time, the movements of electrons became accessible to experimental physics.