How physics Nobel laureates helped unlock secrets of shortest time frames

The groundbreaking research into attosecond pulses opens doors to a new era of scientific exploration with applications spanning physics, chemistry, engineering and medical diagnostics

When we film fast actions – to review a batsman being run out, for example – we use fast shutter speeds to split the action, frame by frame. To film processes inside atoms and molecules, you need superfast shutter speeds.

Separate bodies of research by physicists Pierre Agostini, Ferenc Krausz and Anne L’Huillier, who shared the 2023 Nobel Prize in Physics, helped create very short bursts of light that could spot such processes.

The frequency of a wave is the number of times it oscillates in a second (or in any given time period). Each oscillation, from peak to trough to peak again, is a cycle. Humans can see the electromagnetic spectrum only in-between infrared (long wavelength, low frequency) and ultraviolet (short wavelength, high frequency).

Normally, electromagnetic waves, which may or may not be visible, are composed of many different frequencies, but lasers modulate their output to one frequency. However, even lasers find it hard to hit the frequencies needed to create pulses brief enough to observe atomic processes.

Atomic processes occur in a few attoseconds. An attosecond is a second divided by 10 followed by 17 zeroes (1 divided by 10 to the power of 18). That is a billionth of a billionth of a second.

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The universe is about 13.8 billion years old (13.8 multiplied by 10 to the power of 9). There are roughly 31.5 million seconds (31.5 multiplied by 10 to the power of 6) in a year. Multiply those two large numbers and you have the universe’s age in seconds, and that’s roughly equal to the number of attoseconds contained in a single second.

In 1987, L’Huillier started to pump output from infrared lasers into noble gases (gases that are chemically inert). If you hold down the halfway point on a sitar string, you create two overtones, which are exact divisions of the tone you get from the open string. The mathematics of electromagnetic overtones is similar to that of musical overtones. The infrared light split into overtones – that is, into separate waves that were sub-multiples of the laser’s wavelength.

This happened because the laser imparted a kick of extra energy to electrons in the gas, and those electrons subsequently released the extra energy as flashes. (This light is not necessarily visible.) The energy in light is related to the wavelength of the light. The shorter the wavelength, the higher the energy. The overtones are some sub-multiple of the laser’s wavelength, and hence, shorter wavelength, and therefore, higher energy.

These overtone-waves interact with each other. When the peaks coincide, the waves are in “phase”, and the flash more intense. An infrared laser can stimulate the gas to emit ultraviolet flashes. So, the original wavelength is so low-frequency that humans can’t see it, and the overtones are so high-frequency that humans can’t see those either!

L’Huillier continued to work with these experimental set ups, producing overtones and analysing them.

In 2001, by tinkering with overtones, Agostini learnt how to produce consecutive pulses, each lasting 250 attoseconds. At the same time, Krausz figured out a way to isolate pulses lasting 650 attoseconds each.

The analogy used in the Nobel citation is that Agostini succeeded in coupling a train of pulses, while Krausz figured out how to isolate single “carriages” and decouple them. Eventually, they all managed to fine-tune their experiments to the point where individual pulses lasted just a few dozen attoseconds.

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Put these breakthroughs together and you have attosecond physics. You know ways to split wavelengths and put them into phase to create brief pulses of electromagnetic radiation; you know how to keep doing this in sequences; and you know how to isolate each of these pulses. 

These pulses are caused by electrons being kicked into a higher energy state, and then giving up that energy. Electrons orbit around the nucleus of atoms. When they receive energy, they move away from the nucleus, and they give up that energy when they move closer again. By studying these pulses, you can reconstruct these movements (or rather make very, very, good guesses).

It is an amazing feat with a wide variety of applications.

Molecules of different materials in gaseous and liquid states have different characteristic signatures in terms of attosecond pulses. This has a wide variety of applications across physics and chemistry. Attosecond pulse technology also has applications in various engineering disciplines and in the material sciences – such as checking the composition of solutions or of seawater, or the diffusion rates of water into clouds, or checking alloys for the exact composition and conductivity, or testing ores for purity.

In medicine, it can be used to detect molecular traces of diseases in blood samples. This is a cutting-edge technology, which is finding new uses all the time.