This year’s Nobel Prize in Physics honors three scientists who developed techniques to glimpse the movement of electrons in atoms and molecules using flashes of laser light lasting just attoseconds—billionths of a billionth of a second.
Pierre Agostini of Ohio State University, Ferenc Krausz of the Max Planck Institute of Quantum Optics, and Anne L’Huillier of Lund University share the prize of roughly $1 million for experiments that “enabled the investigation of processes that are so rapid they were previously impossible to follow,” the Royal Swedish Academy of Sciences announced this morning.
“The Nobel committee made the right choice,” says Mauro Nisoli, head of the attosecond research center at the Polytechnic University of Milan. “Now, this is a big topic with many active researchers, but these really were the pioneers.”
L’Huillier produced such pulses first, following a fortuitous discovery. In 1988, she and her colleagues shined an intense infrared laser through a noble gas and found that the gas emitted ultraviolet light with frequencies that were multiples of the frequency of the original laser light. Over the next several years, L’Huillier and colleagues explained how such “high harmonics” emerge as the laser light rips an electron from a gas atom and then shoves it back in, causing it to emit light.
Those high harmonics are spread across a range of frequencies differing by factors of 30 or more—just what you need to make a very short pulse of light. Fundamentally, any short pulselike wave must contain a spread of frequencies—and the wider the spread, the shorter the pulse. It’s the same reason why the information-packed pulses of high-speed internet require “broadband” wavelengths. In the 1990s, L’Huillier and colleagues demonstrated that the high harmonics can be combined in such a way that the waves interfere with each other, producing a train of shorter pulses where the peaks coincide.
Theory predicted that those pulses would be just attoseconds long. But physicists had no way to prove that until Agostini and his colleagues tackled the problem in 2001. He and his team used an argon gas to generate a train of ultrashort pulses and shined both those pulses and part of the original laser pulse that spawned them onto another argon sample. The researchers then took advantage of a prediction, based on interference effects, that the number of electrons emerging at a particular energy would depend on how the two laser pulses overlapped in time. They could vary the overlap by, say, moving a mirror in their optical set up by a micrometer or so. Using this now-standard technique, Agostini and colleagues showed that the pulses were just 250 attoseconds long.
Krausz pushed attosecond pulses further by creating solitary pulses that can be more useful for experiments. In 2001, he and his team produced a lone pulse 650 attoseconds long. To do that, the researchers had to first produce a very wide range of harmonics and then use a filter to select and combine just a few of the highest ones.
In the past 2 decades, applications of attosecond science have begun to grow. Scientists can measure how long it takes for a pulse to tug an electron away from an atom—a sign of how tightly it is held—and see how collections of electrons move from place to place inside molecules or materials, key to understanding their properties. Researchers have also used attosecond pulses as ultrafast electronic switches, using the light to flip a material from an insulator to a conductor. Attosecond light pulses may even help diagnose disease in blood samples, by giving molecules a shove and provoking a fingerprint response. So far the techniques have produced few major discoveries, says Karl Krushelnick, a physicist at the University of Michigan. “It’s a bit of a great technique looking for a killer app.” But Krushelnick notes that it also took lasers decades to fulfill their initial promise.
Some physicists are working to extend attosecond techniques to other parts of the electromagnetic spectrum. Researchers have now coaxed attosecond pulses out of the Linac Coherent Light Source, an enormous x-ray free-electron laser at SLAC National Accelerator Laboratory, says Linda Young, an atomic physicist at Argonne National Laboratory. In principle, ultrashort x-ray pulses would have enough energy to probe atoms and molecules with much more strongly bound electrons.
L’Huillier is just the fifth woman to win the Nobel Prize in Physics. “L’Huillier was instrumental and for a long time she didn’t get the recognition for it,” says Ursula Keller, head of the ultrafast laser group at ETH Zürich.
Speaking by phone during the press conference announcing the prize, L’Huillier said there is still plenty of fertile terrain left to explore. One goal would be to use attosecond pulses not just to investigate the movement of electrons during chemical processes, but to control them, perhaps driving a reaction that nature doesn’t prefer. “The dream—if you want, the Holy Grail—of this part of our research is to be able to control the initial time for a molecular reaction,” L’Huillier said. “But this is in the future.”
