Extreme Light for Science and Society

With only five female researchers having received the Nobel Prize in Physics (out of 224 recipients in total), hearing from one of the three living female Physics Laureates is an honour. Hearing from two is as rare as hen’s teeth. So attendees of #LINO24 enjoyed a unique treat when Donna Strickland and Anne L’Huillier took to the Inselhalle stage to share their knowledge in pushing laser technology to the extreme limits, beyond what was thought possible.

The first Laureate Lecture at this year’s meeting was L’Huillier’s, entitled Attosecond Pulses of Light for the Study of Electron Dynamics. L’Huillier shared the 2023 Nobel Prize in Physics with Pierre Agostini and Ferenc Krausz for experiments that “have given humanity new tools for exploring the world of electrons inside atoms and molecules”, and this talk told the story of how she and colleagues developed these tools.

Generously acknowledging the pioneering work of others throughout, L’Huillier began by outlining how, ever since the invention of the laser – made possible by 1964 Nobel Prize in Physics recipients Charles Townes, Nicolay Basov and Aleksandr Prokhorov – researchers have been striving to achieve shorter, more intense laser pulses. Why? Because many natural processes, such as molecular vibrations and chemical reactions, are so fast that they can only be measured (and potentially controlled in the future) with extremely short pulses of light.

By the time L’Huillier got involved in the 1980s, Ahmed Zewail had already developed a technique to make lasers strobe, generating ultrashort pulses lasting a few femtoseconds (one femtosecond is one millionth of one billionth of a second) for which he received the 1999 Nobel Prize in Chemistry. This breakthrough made it possible to see the movement of atoms in molecules for the first time, a staggering achievement. But even faster attosecond processes (of the order of a billionth of a billionth of a second) such as the movement of electrons remained frustratingly out of reach.

A Serendipitous Discovery

L’Huillier’s first breakthrough came in the late 1980s during her PhD at Paris-Saclay in France when she was experimenting with generating fluorescent effects using an infrared laser beam transmitted through a noble gas. Though fluorescence was not observed, the experiment did generate overtones – waves that complete a number of entire cycles for each cycle in the original wave. Not only did she observe many high order overtones, but surprisingly the intensity of these waves remained relatively high for all but the very highest order overtones, suggesting they could be useful.

While L’Huillier was developing the theory to explain her experimental results at the beginning of the 1990s, several physicists were already thinking about how to use these intense overtones to generate attosecond pulses. Soon they realised that under the right circumstances, constructive and destructive interference of overtones could generate a train of very short light pulses. Using this technique, the more overtones you used, the shorter the pulses you could generate.

L’Huillier soon had an opportunity to generate the required extremely high order overtones, when she moved to Lund University where physicists had built the first terawatt titanium sapphire laser. But it was still not clear whether or not she and others using this technique had actually generated attosecond pulses, because there was no method of measuring such short pulse durations.

L’Huillier’s fellow Laureates Agostini and Krausz solved this problem using a cross-correlation technique where the generated pulse (train) can be studied using the infrared laser electric field used for its generation. In 2001, Agostini and his research group at Paris-Saclay 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, devised a different method known as streaking to divert a single pulse from a pulse train, isolating a 650 attosecond pulse. Two years later, L’Huillier upped the ante, producing a laser pulse lasting just 170 attoseconds.

Since then, 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). “The challenge will be to apply the techniques to more complex systems like condensed matter or molecules,” L’Huillier concluded.

Fibre Laser Potential

Two days later, L’Huillier was on stage again, this time as moderator for the Agora Talk ‘Extreme Light Laser and Its Societal Applications’ by 2018 Nobel Prize in Physics co-recipients Strickland and Gérard Mourou. Strickland and Mourou were awarded for their work on chirped pulse amplification (CPA), which stretches, amplifies and then recompresses ultrashort laser pulses to produce extreme optical intensities – an advance that was critical in developing lasers capable of delivering the attosecond pulses L’Huillier pioneered. But from the get-go, the discussion focused strictly on more modern methods of generating extreme laser light: “Just a week before I won the Nobel Prize for CPA lasers, I got rid of all of my CPA lasers,” recalled Strickland “And this was because fibre lasers were on the rise.”

Though high intensity CPA lasers can generate huge amounts of energy in a short amount of time, Strickland explained that they are limited in terms of average power: “The average power of fibre lasers can be so much higher, and that intrigued me.”

Both Strickland and Mourou are keen advocates for fibre lasers and their potential applications. Thinking of practical small-scale applications, Strickland is developing a fibre laser system with pioneering Japanese theoretical plasma physicist Toshiki Tajima, known for inventing the laser wakefield acceleration technique. Together, Tajima and Strickland are attempting to use fibre lasers to generate kilo-electron-volt electrons capable of removing the last layer of cancerous tumours that are often hard to access during surgery. 

Mourou meanwhile is working towards the longer-term goal of developing fibre lasers that outperform today’s laser technology by orders of magnitude. Such gains would bring extraordinary applications within reach. “For instance, the accelerator at the Large Hadron Collider is 27 kilometres long because we are using microwaves in order to accelerate particles,” Mourou explained. “Now, if you’re using [visible] light waves, that can be much, much shorter; 100 meters. But if you by some means produce a very high intensity in the X-ray regime, then this becomes extremely short.”

The X-ray and gamma-ray regimes open a number of possibilities, not least the study of cosmological processes in the lab for the first time, and various applications requiring the acceleration of protons and neutrons. “We will be able to produce giga-electron-volt protons, and you can use them for proton therapy, nuclear therapy, nuclear diagnostics,” said Mourou. “And we will produce neutrons that can be used to treat nuclear waste.”

What advances need to be made to start realising these applications? “We’re trying to get very high intensity, high energy, short pulses in the X-ray regime,” summarised Mourou. For context, achieving attosecond pulses required intensities of around 10¹⁵ W/cm². Many orders of magnitude higher intensities may be needed to achieve everything Mourou and Strickland envision. “It’s time for a new Nobel Prize winning discovery to kick us up, because we want to be up in that 10²⁹ regime before I die.”