The Nobel Prize in Physics: Opening up the world of electrons
D. Indumathi, The Institute of Mathematical Sciences, Chennai

As usual, October is the time of year when the Nobel Prizes are awarded. The Nobel Prizes are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." However, often the prizes are awarded to major discoveries or inventions that opened new frontiers in various fields. In this article we talk about the recently awarded Nobel Prize in Physics.

BOX Alfred Nobel
Alfred Nobel was a Swedish chemist, engineer, and industrialist most famously known for the invention of dynamite. He died in 1896. In his will, he bequeathed all of his "remaining realisable assets" to be used to establish five prizes which became known as "Nobel Prizes". Nobel Prizes were first awarded in 1901. Nobel Prizes are awarded in the fields of Physics, Chemistry, Physiology or Medicine, Literature, and Peace.
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The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics 2023 to Pierre Agostini, The Ohio State University, Columbus, USA, Ferenc Krausz, Max Planck Institute of Quantum Optics, Garching and Ludwig-Maximilians-Universität München, Germany, Anne L’Huillier, Lund University, Sweden,
“for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter.”

What is the award about?
In the words of the Nobel Prize website, the three Nobel Laureates in Physics 2023 are being recognised for their experiments, which have given humanity new tools for exploring the world of electrons inside atoms and molecules. Pierre Agostini, Ferenc Krausz and Anne L’Huillier have demonstrated a way to create extremely short pulses of light that can be used to measure the rapid processes in which electrons move or change energy. Let us try and understand exactly what they have achieved.

Why do we need such short light pulses?
A tiny hummingbird can beat its wings 80 times per second. We are only able to perceive this as a whirring sound and blurred movement. See the blurry wings in the photo taken with lower-speed camera. We need to use technological tricks to
capture or depict these very brief instants.
High-speed photography and strobe lighting make it possible to capture detailed images of fleeting phenomena. A highly focused photograph of a hummingbird in flight requires an exposure time that is much shorter than a single wingbeat. In other words, the faster the event, the faster the picture needs to be taken if it is to capture the instant.
In a molecule, atoms can move and turn in millionths of a billionth of a second, femtoseconds. These movements can be studied with the very shortest pulses that can be produced with a laser. What determines this timing? Atoms are made of a central, heavy nucleus, surrounded by light electrons which are nearly 2000 times lighter than a single proton or neutron which are inside the nucleus. So the speed of atoms is mostly determined by its slowest component, which is the nucleus. See the elaborate equipment of David Nadlinger who photographed a strontium atom and won the National Science Photogrphy prize in England in 2018. The atom is seen as a pale dot in the zoomed-in part of the central portion of the image between the two needle-like pieces which are 2 mm apart. It is held in place by electric fields.
If you want to study the motion of electrons inside an atom, we realise that they are much quicker because they are so light. So if you study the motion of electrons with these same lasers, you will again see a blur, just as we see the humming bird's wings. In the world of electrons, positions and energies change at speeds of between one and a few hundred attoseconds, where an attosecond is one billionth of a billionth of a second.

BOX What is an attosecond
One way to understand an attosecond is: the number of attoseconds in one second is the same as the number of seconds that have elapsed since the universe came into existence, 13.8 billion years ago. On a more relatable scale, we can imagine a flash of light being sent from one end of a room to the opposite wall – this takes ten billion attoseconds.
For a long time, people thought that it is not possible to have pulses shorter than a femtosecond. This year’s Nobel laureates conducted experiments that opened up the new research field of attosecond physics.
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How they achieved attosecond pulses
Light consists of waves – vibrations in electrical and magnetic fields – that move through a vacuum faster than anything else. These have different wavelengths, equivalent to different colours. For example, red light has a wavelength of about 700 nanometres, one hundredth the width of a hair, and it cycles at about four hundred and thirty thousand billion times per second. We can think of the shortest possible pulse of light as the length of a single period in the light wave, the cycle where it swings up to a peak, down to a trough, and back to its starting point (see figure). The trick to attosecond pulses is that it is possible to make shorter pulses by combining more and shorter wavelengths.
The key is a phenomenon that arises when laser light passes through a gas. The light interacts with its atoms and causes overtones. What is an overtone? It can best be understood with a musical instrument like a guitar or veena whose strings are held rigid at their end points. By plucking the string harder, you set up waves or vibrations in the string that complete more than one cycle for each cycle in the original wave, as can be seen in the figure. These are called overtones and their sound has a higher pitch or frequency. Since more wave cycles fit into the same length, the corresponding wavelength becomes shorter and so also their time period (the time taken to complete a cycle) becomes shorter. This is because the ratio of the wavelength to the time period is fixed and equals the speed.
In 1987, it was Anne L’Huillier who discovered that many different overtones of light arose when she transmitted infrared laser light through a noble gas. They are caused by the laser light interacting with atoms in the gas; it gives some electrons extra energy that is then emitted as light. Since the speed of light is constant, higher overtones have smaller time periods and so pulses shorter than femtoseconds can be achieved.
Once these overtones exist, they interact with each other. The light becomes more intense when the lightwaves’ peaks coincide, but becomes less intense when the peak in one cycle coincides with the trough of another. In the right circumstances, the overtones coincide so that a series of pulses of ultraviolet light occur, where each pulse is a few hundred attoseconds long. In 2001, Pierre Agostini succeeded in producing and investigating a series of consecutive light pulses, in which each pulse lasted just 250 attoseconds. At the same time, Ferenc Krausz was working with another type of experiment, one that made it possible to isolate a single light pulse that lasted 650 attoseconds.
These experiments demonstrated that attosecond pulses could be observed and measured, and that they could also be used in new experiments. Now that the attosecond world has become accessible, these short bursts of light can be used to study the movements of electrons. It is now possible to produce pulses down to just a few dozen attoseconds, and this technology is developing all the time.

Electrons’ movements have become accessible
Attosecond pulses make it possible to measure the time it takes for an electron to be tugged away from an atom, and to examine how the time this takes depends on how tightly the electron is bound to the atom’s nucleus. It is possible to reconstruct how the distribution of electrons oscillates from side to side or place to place in molecules and materials; previously their position could only be measured as an average.
Attosecond pulses can be used to test the internal processes of matter, and to identify different events. These pulses have been used to explore the detailed physics of atoms and molecules, and they have potential applications in areas from electronics to medicine. For example, attosecond pulses can be used to push molecules, which emit a measurable signal. The signal from the molecules has a special structure, a type of fingerprint that reveals what molecule it is, and the possible applications of this include medical diagnostics.
Sources: nobelprize.org, pictures courtesy Nobel Prize Foundation and internet