Pierre Agostini (Ohio State University, US), Ferenc Krausz (Max Planck Institute of Quantum Optics, Germany), and Anne L'Huillier (Lund University, Sweden) are to be awarded the 2023 Nobel Prize in Physics for their work toward generating light pulses on attosecond (10−18 s) time scales, the Royal Swedish Academy of Sciences announced on Tuesday. The physicists' pioneering research in ultrafast-laser science has opened up investigations of electronic motion in atoms, molecules, and materials.
"It's a great choice," says Stephen Leone, a physical chemist at the University of California, Berkeley. "To give a prize for pushing the limits of time, one of the fundamental quantities, is really special."
The earliest laser pulses in the 1960s had lengths measured in microseconds. In the quarter century that followed, pulse durations plummeted to femtoseconds, enabling researchers to track molecular dynamics and capture the evolution of chemical reactions. But to investigate the motion of electrons, scientists needed to go beyond the performance limitations of even state-of-the-art lasers and access the attosecond regime.
One of the key insights that put that regime within reach came in the late 1980s. L'Huillier, then at a research center in Paris-Saclay, and colleagues shined an IR laser at argon gas and measured the light intensity at multiples of the laser frequency. Rather than steadily dropping off with increasing frequency, the intensity remained flat for a series of higher-order odd harmonics corresponding to UV and x-ray frequencies. The finding hinted at the possibility of manipulating those higher-frequency waves to produce ultrashort pulses.
In the following years, L'Huillier and other researchers investigated the theoretical underpinning of the high-harmonic generation process. An important advance was the proposal of a phenomenon called recollision. When laser light encounters an atom, the oscillating electric field of the pulse can pull away an electron. A half cycle later, the electron accelerates back toward the atom. Upon its return, the electron radiates light that has a higher frequency than that of the driving laser (see the Physics Today article by Paul Corkum, March 2011, page 36).
In 2001 Agostini, then at another institute in Paris-Saclay, and his colleagues finally broke the subfemtosecond barrier. They produced a train of 250-attosecond pulses that were separated by 400 nanometers, half the wavelength of the driving laser, which was consistent with the recollision mechanism. Crucial to the advance was the ability not only to generate the fleeting pulses but also to detect and characterize them. Agostini and his team succeeded using a technique they developed called RABBIT—reconstruction of attosecond beating by interference of two-photon transitions—that exploits the interference between the ultrafast pulses and those of the driving laser.
Krausz and his colleagues focused on the challenge of isolating ultrashort pulses, which is a requirement for researchers probing subatomic processes. In 2001 he (then at the Technical University of Vienna) and his team shined a 7-femtosecond optical pulse on a neon target and then filtered the output to attain isolated pulses with durations of about 650 attoseconds. The Krausz team then demonstrated the promise of attosecond spectroscopy by tweaking the time delay between the femtosecond laser pulses and the resulting ultrashort pulses to probe the electron dynamics of atoms of krypton.
Just under a decade later, the field had advanced to the point that Krausz and his team were able to quantify the time scale of the photoelectric effect for electrons in neon atoms. Combining 200-attosecond pulses with IR pulses using a technique called attosecond streaking, the researchers found that ionization from the 2p subshell took about 21 attoseconds longer than did ionization from the 2s subshell. Six years ago, L'Huillier and colleagues used the RABBIT technique to explain the deviation of Krausz's result from theory (see Physics Today, January 2018, page 18).
Recently experimentalists have created laser pulses that are mere tens of attoseconds long. Attosecond science was a major motivation for building the upgraded Linac Coherent Light Source at SLAC, says Matthias Kling, director of the LCLS science, research, and development division. The facility employs the attosecond streaking technique developed by Krausz.
Agostini, Krausz, and L'Huillier will split the 11 million Swedish kronor (roughly $1 million) prize. L'Huillier is the fifth woman to be awarded the physics prize and the first to share the prize evenly with all the other laureates. Marie Curie (1903), Maria Goeppert Mayer (1963), Donna Strickland (2018), and Andrea Ghez (2020) each split half the prize with a male counterpart, with the other half going to a man.
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