The BBVA Foundation Frontiers of Knowledge Award in Basic Sciences goes in this fifteenth edition to Anne L’Huillier (Lund University, Sweden), Paul Corkum (University of Ottawa, Canada) and Ferenc Krausz (Max Planck Institute of Quantum Optics, Germany), the three pioneers of “attosecond physics” or “attophysics” whose work has made it possible to observe subatomic processes unfolding over the shortest time scale captured by science.
The awardees, says the committee, “have shown how to observe and control the motion of electrons in atoms, molecules, and solids with ultrashort light pulses on time scales of about one hundred attoseconds. One attosecond is approximately the time for light to travel across an atom and is the natural scale for electronic motion in matter. This time scale was previously inaccessible to experimental studies due to the lack of light pulses with short enough duration.”
Thanks to attophysics, scientists can now directly observe natural processes that were once off-limits to the human eye. “It is a huge step to know that what we can imagine theoretically can now be tested experimentally. This interplay between experiment and theory is inspiring a lot of ideas,” remarked committee chairman Theodor W. Hänsch, Director of the Laser Spectroscopy Division at the Max Planck Institute of Quantum Optics (Germany) and winner of the Nobel Prize in Physics.
Attophysics, says laureate Paul Corkum, “is about making the fastest measurements that we as humans can make. And that, I think, is what places it at the forefront of knowledge.” An attosecond, he explains, “is incredibly short. So an attosecond is to a second as a second is to the age of the universe. Can you imagine something as short as that?” In figures, an attosecond is one billionth of a billionth of a second, that is, 0.000000000000000001 seconds.
“That’s the time scale for the movement of electrons across all the atoms matter is composed of, including our own bodies,” adds Fernando Martín, Professor of Physical Chemistry at the Universidad Autónoma de Madrid, Scientific Director of IMDEA Nanociencia and one of the nominators of the three awardees. “So to achieve real-time imaging of electron motion in matter, we needed a technology that would give us access to that time scale. And that is precisely what these researchers have achieved.”
An ultrafast “camera” to “film” electrons in motion
The tools developed by L’Huillier, Corkum, and Krausz act like a camera with a shutter time so dizzyingly ultrafast that it can capture the movement of a hydrogen atom electron that takes 150 attoseconds to circle the nucleus.
Prof. Martín elaborates on this example: “If you want to film how a car moves, you have to take snapshots at very close intervals, so the movement registers.”
“If you take photos with an exposure time of one minute, say, by the time you press the shutter the car has gone and all you have is a blurred image at best. In other words, to visualize an object, you need to take snapshots at intervals and with a duration far shorter than the time that object takes to move significantly. This is what the three laureates have done on the time scale of electron motion, thanks to light pulses generated with ultrafast lasers that emit for just a few attoseconds.”
Attophysics techniques not only mean that we can now capture the movement of electrons, they have also conjured the possibility of manipulating these subatomic particles. “Once you have gained the ability to visualize this movement in real time,” says Martín, “you can likely use the same light sources to manipulate it, eventually modifying its behavior and properties, with applications in multiple domains from biomedicine and electronics to the search for new clean energy sources.”
The committee’s citation ends with a similar reflection: “These groundbreaking contributions have opened exciting new frontiers in different areas, including atomic physics, photochemistry, and materials science.”
The findings that sowed the seeds of attophysics
In 1987, Anne L’Huillier made a discovery that laid the groundwork of the attophysics field. When working as a postdoctoral researcher at the Saclay Nuclear Research Centre near Paris, she became intrigued by the question of what would happen if atoms were subjected to short, intense laser pulses of infrared light. She expected to see fluorescent light, but was surprised to find that the atoms appeared to be emitting light waves at very high frequencies, that is, extremely high-energy X-rays.
L’Huillier had achieved the highest frequency ever recorded through the interaction of laser light pulses with matter. “It was truly fascinating; the first step toward generating an attosecond pulse. And I have never stopped working in the field, contributing to different aspects of the body of research.”
Retracing her steps, the scientist realized that the laser was acting on the atoms the way waves act on seaweed on a rock. Each time a wave comes in, the seaweed unfurls to its full extent, only to retract when the wave recedes. The seaweed, in other words, moves up and down in time with the waves. In similar fashion, the arrival of a laser pulse pulls away the electrons surrounding the atom, which then resume their initial position when the pulse stops. It was on the way back, it turned out, that the electrons emitted those high-frequency light waves.
The seaweed metaphor was originally devised by Paul Corkum on the basis of L’Huillier’s reconstruction. As well as coming up with an intuitive visualization of the phenomenon, he decided to study it from a theoretical standpoint, developing a model that mathematically described the interaction between laser and atoms. It was L’Huillier’s discovery and Corkum’s theoretical model that sowed the seeds of today’s attophysics.
The shortest light pulses in the history of science
While visiting Vienna in the 1990s, Corkum met Ferenc Krausz, then studying there as a young postdoc. “Corkum inspired me with his concepts like no one else,” Krausz recalls today. “What I took from them was that there might be a way to move forward into a time domain that was completely inaccessible beforehand, and render extremely fast processes observable.”
Corkum and Krausz were familiar with L’Huillier’s work and quickly agreed that it could hold the key to generating the shortest light flashes ever. They were aware that, in general, short light pulses were the vehicle to access and observe the universe of the small. Indeed with pulses just a little longer than those they were aiming for, scientists had already managed to glimpse the movement of atoms within molecules.
“The idea is the same as when you capture the motion of a Formula 1 car or a bullet. You take a series of snapshots and then reconstruct how the bullet actually hits the wall,” Krausz explains. These snapshots can then be reproduced in slow motion to see the movement in all its detail.
But they wanted to take this idea of ultrafast photography even further, and apply it to tracing the movement of electrons. These minuscule particles move up to a thousand times faster than atoms, so would require much shorter light pulses than were possible at the time. What was needed was to descend to the attosecond scale.
The light waves generated by L’Huillier seemed the ideal candidate for the task, as they oscillated at such high frequencies that they emitted light pulses of a few attoseconds duration.
The challenge didn’t end there, however. The pulses were short, certainly, but they followed each other in quick succession. And for them to perform in the style of an ultrafast camera, they would have to be isolated into individual light flashes and emitted one by one.
Krausz describes it thus: “Having a whole train of pulses is still something like having a camera that has a very high shutter speed. But it doesn’t open the shutter just once, it opens and closes the shutter all the time, which is, in many cases, not very helpful. You want to be able to open the shutter once and close it very quickly, to take just one snapshot. This is also true when we try to actually capture microscopic processes.”
The solution they came up with was simple but effective. They decided to go as far as possible in shortening the initial infrared pulse (the wave hitting the rock), so the electron (the seaweed) would rise and fall just once, and by doing so obtained a single light pulse lasting around one hundred attoseconds.
This experiment, published in 2001, marked what Krausz describes as “the birthday of experimental attophysics.” As well as opening the door to detailed observation of electron motion, it was able to corroborate a series of predictions formulated decades back by theoretical physics which had previously been inaccessible to testing in the lab.
One of these predictions is the so-called tunnel effect, a phenomenon predicted by quantum physics whereby an electron is able to pass through a barrier which, in theory, it does not have the energy to surmount. Although there was some evidence that the tunnel effect existed in nature, it had never yielded to real-time observation. Now, thanks to Corkum and Krausz’s technique, building on L’Huillier’s discovery, it could be seen on “film” for the first time.
In search of applications in fields like electronics and biomedicine
Now that attosecond physics has revealed its unquestionable potential, the awardees wish to use it to delve deeper into the mysteries of the matter all nature is made of, and to develop applications in fields such as electronics or biomedicine.
“This field of research is exploding in every direction,” says L’Huillier. “I have had the privilege to be there from the beginning, so I have seen the ideas grow and been able to follow the main steps in the process.” Asked about its future, she ventures that “it will split into different subfields,” as has occurred with other lines of research close to her own.
For this scientist, the next goal is to move towards the sphere of quantum information science. She is currently studying ways to probe more closely into processes like entanglement, one of the most surprising features of quantum mechanics whereby two separate particles, perhaps even kilometers apart, display a shared behavior that cannot be accounted for by classical physics. Arriving at as close as possible an understanding of this phenomenon could do much to hasten the advancement of quantum technologies, although L’Huillier’s motivation is not of an immediately practical nature: “This is a new aspect which I am very excited about, though I have no idea where it will go.”
Another concern is the highly specialized nature of the lasers used to generate attosecond pulses, and she is currently pondering ways to achieve the same effect using more widespread, commercially available lasers. “I think they are going to be very, very useful for more standard, maybe even industrial applications.”
Corkum, meantime, has been using attosecond pulses generated not by single atoms but by sets of atoms of semiconductors such as silicon. Semiconductors are at the heart of modern electronics, and the scientist believes that combining all previous knowledge of these materials with the new possibility of having them emit attosecond pulses is “a very powerful technology.”
Krausz too believes that attophysics can drive a new revolution in computing: “Electrons play an extremely important role in nanocircuits, they are responsible for switching the electric current on and off and thus processing information at ever higher speeds. If we want to speed up signal processing to build ever more powerful computers, again, we have to understand how electrons move in these tiny dimensions. And in doing so, we have the opportunity to advance electronic signal processing to its ultimate limit.”
He has also been exploring the potential of attosecond pulses in the detection of disease. If we remove all the cells from a blood sample, he explains, the fluid we are left with is the blood plasma or serum (depending on the treatment). The molecules it contains provide valuable cues to the donor’s health status, and the scientist is studying ways to use attosecond pulses to extract this information.
“Using incredibly sensitive measurements, we can analyze these molecules with great precision,” he explains. “And in our preliminary studies, we have been able to detect eight different types of cancers with an excellent efficiency. We have also detected one type of a very severe coronary disease, pre-diabetes, diabetes, and stroke.” These measurements, he believes, could prove game-changing in future for the early diagnosis of multiple conditions.
Results are now being subjected to testing in 10,000 individuals as part of a multi-year clinical trial, and Krausz’s hope is that the system may be operative within the next ten years.
