Pierre Agostini, Ferenc Krausz and Anne L’Huillier share the 2023 Nobel Prize in Physics for experiments that “have given humanity new tools for exploring the world of electrons inside atoms and molecules.” A more succinct description is that they have given us attosecond physics.
Attosecond physics is the science of the exceedingly, extremely, exceptionally [insert your own hyperbolic adverb here] fast. To put it into context, L’Huillier’s first call from the Nobel Prize’s Adam Smith after she received the news took 3 minutes 48 seconds, or 228,000,000,000,000,000,000 attoseconds. Her first heartbeat during that call lasted a second, or a billion billion attoseconds. Almost defying a description, an attosecond is an unfathomably tiny amount of time. But it happens to be the natural timescale of the near-instantaneous dance of electrons.
Being able to gain a glimpse into the incredibly tiny scale of electrons in the incredibly fast attosecond regime opens the door to directly measuring, and perhaps even controlling, quantum processes. And this, in turn, offers huge potential to advance research, not only in quantum physics but also in biology, chemistry, medicine, electronics and many more areas important to science and society.
From Milliseconds to Attoseconds
After the invention of the laser, made possible by 1964 Nobel Prize in Physics recipients Charles Townes, Nicolay Basov and Aleksandr Prokhorov, reaching the attosecond scale involved numerous breakthroughs in photonics over a period of decades – specifically, breakthroughs in the development of short wavelength, high-power pulsed laser light that can be used to ‘freeze’ and study rapid processes in action.
Some of the biggest leaps forward came in the 1980s. During this decade, Ahmed Zewail developed a technique to make lasers strobe, generating pulses lasting a few femtoseconds – thousands of attoseconds – for which he received the 1999 Nobel Prize in Chemistry. In the mid 1980s, the advent of chirped pulse amplification (CPA) also stands out as being pivotal. Introduced by Donna Strickland, Gérard Mourou (who received the 2018 Nobel Prize in Physics for their work) and others, CPA stretches, amplifies and then recompresses ultrashort laser pulses to produce extreme optical intensities. This provides the power needed, but not the short pulse generation to push into the attosecond regime.
A little after Zewail, Strickland and Mourou made their breakthroughs, in the late 1980s L’Huillier and her colleagues at Paris-Saclay, France, began making inroads into understanding a phenomenon called high-order harmonic generation (HHG) in noble gases. HHG is a process where an intense infrared laser pulse, produced by CPA, focused on a target causes strong nonlinear interactions that lead to a tiny fraction of the laser power being converted to very high harmonics, or overtones, of the optical frequency of the pulse. Under the right conditions, these harmonics can be in phase, so that a series of extremely short pulses are created, potentially even attosecond pulses.
Physicists understood the theory behind this in the 1990s, but an attosecond ‘pulse train’ was finally achieved experimentally only in 2001 by Agostini and his research group at Paris-Saclay. They produced a pulse train where each pulse lasted just 250 attoseconds. Around the same time, Krausz and colleagues at the Technical University of Vienna, Austria, had devised a different method known as streaking to divert a single pulse from a pulse train. In the same year as Agostini’s seminal result, Krausz succeeded in isolating a 650 attosecond pulse. Two years later, L’Huillier (who had moved to Lund University, Sweden) upped the ante, producing a laser pulse lasting just 170 attoseconds.
Fundamental Insights Into the Very Fast
Today, the record for the shortest light pulse ever generated – in fact, the shortest controlled event that has ever been created by humanity – stands at 43 attoseconds, set in 2017 by Hans Jakob Wörner’s team at ETH Zurich, Switzerland. But attosecond physics is much more than a race for the Guiness World Record. It is being wielded to generate fundamental insights into the physical world.
For example, both Krausz and L’Huillier have used attosecond pulses to time electrons ejected from atoms in order to test the photoelectric effect – a phenomenon first explained by Albert Einstein in 1905 (and for which he would later win the 1921 Nobel Prize in Physics).
Krausz has also started to explore biological applications. Based in part on precision attosecond technologies, his group at the Max Planck Institute for Quantum Optics in Garching, Germany, has developed a molecular fingerprinting technique that precisely monitors changes in the molecular composition of biofluids. This could be developed as a new diagnostic to detect traces of diseases in blood samples. In fact, it has already been shown to be capable of detecting whether a person has an early-stage cancer, and what kind.
Other researchers have probed the ultra-fast interactions of electrons in, for example, organic photovoltaic materials, which hold promise as new solar cell materials or catalysts but are currently either unstable or inefficient. Tracking the first moments after light strikes solar cells provides insights into how they produce electricity, and clues for how to improve them.
Another promising application being actively explored is electronics. In modern electronic circuits, microwave voltages drive electrons to switch current on and off in less than a nanosecond. But this is an age compared to the time it takes an electron to travel between neighbouring atoms, a process occurring at the attosecond scale. Attosecond physics could help realise petahertz electronics, where smaller structures take advantage of electron motion between atoms to switch current several trillion times per second – about 100,000 times faster than possible today.
So far, researchers have barely scratched the surface of the possibilities attosecond physics opens in a wide and ever-growing range of applications. Yet technology taking advantage of attosecond physics is already starting to transform our understanding of the physics of matter, and ultra-fast chemical and biological processes, just by ‘freezing’ events in action. In the not-too-distant future, once humanity take the next step and gains the ability to manipulate and design ultra-fast processes, the attosecond revolution – sparked by the work of Agostini, Krausz and L’Huillier – will truly begin.