The Royal Swedish Academy of Sciences awarded this year’s Nobel Prize in Physics to three European scientists who have made significant contributions to the field of attosecond generation and measurement. Their research focuses on the experimental techniques used to produce attosecond light pulses, which have proved instrumental in investigating the intricate dynamics of electrons within various forms of matter.
Researchers Pierre Agostini, Ferenc Krausz and Anne L’Huillier successfully demonstrated a methodology for generating highly compressed light pulses, which can be effectively employed to gauge the swift phenomena associated with electron motion and energy transitions. The ability to generate and measure light pulses of attosecond duration would make it possible to study and analyze a number of quantum-mechanical processes.
Attosecond defined
Suppose we wanted to use a continuous-shooting camera to capture an event that occurs in a fraction of a second, such as the beating of a hummingbird’s wings. Capturing the dynamics of this event by obtaining a sequence of sharp, blur-free images would require a high frame rate, for example, 30 to 50 frames per second (fps). The shorter the exposure time, the greater the probability of obtaining a sharp image with the exact instantaneous position of the hummingbird.
Now suppose we applied the same reasoning to the atomic scale. To study and understand the dynamics of the extremely rapid processes in which electrons take part, such as chemical reactions, it is necessary to generate and measure the duration of vanishingly short pulses.
At the molecular level, atoms move and transform in timescales on the order of a femtosecond—a millionth of a billionth of a second, or 10–15 second. Advances in laser technology now allow molecular-level phenomena to be studied by generating femtosecond pulses.
Compared with the nuclei of atoms, however, electrons are much lighter and move much faster, and using femtosecond pulses to study electron dynamics does not yield clear results. It is as if we were to photograph a hummingbird in flight with too long an exposure time: The image of the wings would be blurred, and it would be impossible to determine its exact position.
Electrons change position at speeds of up to a few hundred attoseconds. One attosecond equals a billionth of a billionth of a second (10–18 second). There are as many attoseconds in a second as the number of seconds that have elapsed since the Big Bang, which took place some 13.8 billion years ago (Figure 1).
For a long time, the femtosecond was regarded as an insurmountable limit on atomic-scale research involving electronics, in that it was not possible to generate pulses with durations shorter than this value using the available laser technology.
In the early 1980s, however, a significant discovery paved the way for the systematic generation of laser pulses with durations on the order of attoseconds. The key to generating attosecond pulses, researchers found, lay in applying a combination of multiple and shorter wavelengths to produce them.
Research on attoseconds
In separate experiments over decades, this year’s Nobel Prize winners produced light bursts brief enough to capture individual frames of the lightning-fast motion of electrons.
The first results were obtained by L’Huillier, now a professor at Lund University, Sweden. In experiments conducted in 1987, she discovered that a noble gas hit by infrared laser radiation emits radiation with a much higher frequency, due to the presence of so-called overtones. These overtones are high-order harmonics with frequencies that are multiple of the laser frequency. In the 1990s, L’Huillier and others proposed an explanation for the experimentally observed phenomenon: The radiation emitted at high frequency is due to processes involving individual electrons.
When an electron is hit by the laser pulses, it acquires enough ionizing energy to allow it to temporarily leave its atom via the tunneling effect. Because the electric field produced by the laser pulses oscillates, when it reverses direction, the free electron can return to the nucleus of the atom (Figure 2). This process emits electromagnetic radiation of a much higher frequency (and therefore a much smaller period, or duration) than the laser radiation used to trigger the process itself.
The high-order harmonics generation (HHG) process, demonstrated in experiments conducted by L’Huillier’s group, was later analyzed and explained from a quantum perspective. Utilizing quantum mechanics, the researchers accurately predicted the general shape of the HHG spectrum by calculating the different intensities of the various overtones. As soon as they realized which harmonics to expect, they devised techniques to combine them to create a new wave with attosecond-scale peaks.
In 1994, Agostini and his group studied the frequency modulation principle in a two-color photon field. This principle was subsequently developed into a metrology technique dubbed RABBIT (an approximate abbreviation of “reconstruction of attosecond beating by interference of two-photon transitions”). By focusing the extreme-ultraviolet pulse and light from the drive laser onto a rare gas target and analyzing the photoelectrons emanating from the target, the RABBIT technique made it possible to measure the pulse duration of a train of attosecond pulses (Figure 3).
Agostini and his team combined the “pulse train” with a delayed portion of the original laser pulse to determine the phase relationship between the harmonics. This procedure also provided a measurement for the duration of the train’s pulses, revealing that each pulse lasted a specific number of attoseconds.
In 2001, research teams operating at CEA Paris-Saclay and the Vienna University of Technology demonstrated attosecond pulses. The Agostini group at CEA Paris-Saclay generated a series of pulses with a duration of 250 attoseconds, as measured by the RABBIT metrology with argon as the target gas. At the Vienna University of Technology, the Krausz group produced 650 attosecond-long isolated pulses. These accomplishments paved the way for the study of electron dynamics in atoms, molecules and condensed matter.
Why are attoseconds so important?
With the Nobel laureates’ experiments in attosecond physics having laid the groundwork, it is now possible for researchers to analyze and study the dynamics and time evolution of many processes in quantum mechanics, such as chemistry and materials science.
According to L’Huillier, prerequisites for the study of attosecond phenomena are a cutting-edge, ultrafast laser system, advanced attosecond engineering and a robust application program.2
To meet the challenging requirements of attosecond science, an attosecond source must be created using both specialized ultrafast laser technology and attosecond physics. From there, the technology can be used to investigate potential uses in numerous fields, including but not limited to atomic and molecular physics, surface physics, plasmonics and ultrafast coherent imaging.