Attophysics — new tools to fathom the world of electrons | Explained Premium

2023 Physics Nobel | How do the work of the three laureates demonstrate a way to create short pulses of light that can be used to measure the rapid fashion in which electrons move or change energy? What are the applications of this discovery?

The story so far: On October 3, the 2023 Nobel Prize for physics was awarded to Anne L’Huillier, Pierre Agostini, and Ferenc Krausz “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”.

An attosecond is one quintillionth of a second, or 10^-18 seconds. This is the timescale at which the properties of an electron change. So, to truly understand electrons, it should be possible to study them at these timescales. This is what the work of the Nobel laureates made possible.

Attosecond science, including attosecond physics, or attophysics, deals with the production of extremely short light pulses and using them to study superfast processes. A hummingbird’s wings beat 80 times a second, so a single beat would last 1/80th of a second. At its best, the human eye can see up to 60 frames per second, which is not good enough to see a single wingbeat as it happens. Instead, the wings’ motion would appear as a blur.

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One solution is to use a digital camera that creates photographs by capturing light coming from a source using a sensor. To capture a single wingbeat, the camera needs to capture only just as much light — which it can do if its aperture is open for exactly 1/80th of a second. An alternative is to keep the aperture open at all times and release a light pulse whose duration is 1/80th of a second towards the wing and capture the reflection. The former is much easier to do with a digital camera, but when you’re studying electrons, the latter is a better option. In attosecond science, the light pulse’s duration is a few hundred attoseconds because the electrons’ ‘wingbeats’ happen that rapidly.

The concepts underlying the production of attosecond pulses come from wave mechanics. In 1988, Anne L’Huillier and her colleagues in Paris passed a beam of infrared light through a noble gas. They found that the gas emitted light whose frequency was a high multiple of the beam’s frequency — for example, if the beam frequency was 10 arbitrary units, the emitted light had frequencies of 50 units, 60 units, 70 units, etc. This phenomenon is called high-harmonic generation, and the emitted waves are said to be overtones of the original. The researchers also found that as they increased the frequency of the original beam, the intensity of the emitted light dropped sharply, then stayed constant for a range, and then dropped again.

By 1994, researchers had worked out why passing the infrared beam through the noble gas was having these effects. A beam of light consists of oscillating electric and magnetic fields. ‘Oscillating’ means that at a given point, the field’s strength alternately increases and decreases. So an electron at this point would be imparted some energy and then have it taken away; when energy is imparted, the electron would come loose from an atom, and when it is taken away, the electron and the atom would recombine, releasing some excess energy. This energy is the light re-emitted by the gas.