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Proc Natl Acad Sci U S A
Proc Natl Acad Sci U S A
PNAS
Proceedings of the National Academy of Sciences of the United States of America
0027-8424
1091-6490
National Academy of Sciences

38252840
202321587
10.1073/pnas.2321587121
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Profile of Pierre Agostini, Anne L’ Huillier, and Ferenc Krausz: 2023 Nobel laureates in Physics
Leone Stephen R.
22 1 2024
30 1 2024
22 7 2024
121 5 e23215871212024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
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pmcThe 2023 Nobel Prize in Physics was awarded to Pierre Agostini, Anne L’Huillier, and Ferenc Krausz for discoveries spanning many decades that led to the shortest possible pulses of electromagnetic radiation in time, the attosecond time domain (1 as = 1018 sec). Short time measurements are a vexing subject because time is not a usual observable in quantum mechanics—it is something that experimentalists must establish within their measurement setup. This is accomplished, for example, by using two short pulses of light, knowing the fundamental constant of the speed of light and carefully measuring the difference in distances traveled by each pulse of light.

The shorter the duration of the light pulses, the better the limit of short time dynamics that can be explored. This is especially needed to investigate the very short timescales attributed to the rapid motions of electrons themselves. Electrons have their own wave-like properties and consequently uncertainties in position and momentum; thus, what scientists measure is the occupancy of energy states rather than observing the actual motion of electrons. With very short pulses of light, ultimately, the limits of time, and the time–energy uncertainty relation, are brought sharply into focus, even calling into question fundamental aspects of how fast is the absorption of light, whether an observed time delay represents a real timing delay or some other kind of phase shift, and sometimes even the validity of the measured time dynamics itself (1).

With the Nobel Prize in femtochemistry (1 fs = 1015 sec) awarded to Ahmed Zewail in 1999, and before that the Nobel Prize for nonequilibrium dynamics (approximately microsecond timescales) awarded to Manfred Eigen, Ronald Norrish, and George Porter in 1967, several Nobel Committees in Chemistry acknowledged the importance of time domain measurements in chemistry. These breakthroughs took advantage of available short-pulse lasers, flash photolysis sources, and electrical impulses. The 2018 Nobel Prize in Physics was in part awarded to Donna Strickland and Gerard Mourou for their invention of the technique of chirped pulse amplification, which is needed to achieve most of the high-energy short-pulsed lasers in use today.

The award of the 2023 Nobel Prize in Physics for attosecond pulses acknowledges, in a fundamental way, the limits of time itself, as well as a number of important strong-field atomic physics processes that allowed researchers to go beyond the pulse durations achievable with available lasers. This award establishes “time” as a subject that is basic to physics and many other fields. The Nobel-Prize-winning research provided the first time-domain measurements of inner shell decay processes (2) for which we only had a solid intuition about the short lifetimes from traditional linewidth measurements. Researchers became very good at switching between frequency and time when the processes were simple—this award emphasized that there are time-domain measurements that cannot be obtained from traditional linewidth spectroscopy, such as electron photoemission delays from different orbitals in an atom or from surfaces (3–5).

Ultimately, to achieve attosecond pulses, the three Nobel Prize winners, as well as many other colleagues working for decades, first had to understand the strong-field manipulation of electrons in atoms by lasers. In strong field processes, the electric field of the laser is as strong as the binding energy of an electron to the atom. There are many seemingly disparate and difficult-to-understand phenomena that occur simultaneously when the electrons in an atom are subjected to strong laser fields. These include tunnel ionization, multiphoton ionization, emission of extreme ultraviolet light, above-threshold ionization, and laser-induced electron diffraction. It took years to sort out the underlying dynamics of these processes, by many contributors.

One of the key processes was the generation of extreme ultraviolet emission by intense lasers focused into rare gases (6). This was all the rage at Gordon Conferences in the late 80s and early 90s. There were many detailed investigations of this electromagnetic emission process, called high harmonic generation (HHG), the foremost of which were pioneered by Anne L’Huillier (Fig. 1). The work culminated in the realization that this emission is the result of a recollision process of an electron that is first driven away from an atom, gains kinetic energy from the field, and is then driven back toward the atom by the periodic reversal of the field of the laser. It results in a grand series of high-order harmonic wavelengths. Using a longer wavelength laser than previously investigated (1,064 nm compared to 248 nm) to produce the high-order harmonic emissions, Anne, while at the French Alternative Energies and Atomic Energy Commission (CEA) Saclay, made the important and serendipitous discovery of a plateau in the intensity over many harmonic orders (7). This gave the early clue that HHG was not an ordinary nonlinear optics process, which would instead have decreased exponentially in intensity with increasing harmonic order.

Fig. 1. Anne L’Huillier. Image credit: Magnus Bergström (photographer)/Knut and Alice Wallenberg Foundation.

The large bandwidth of the spectral components in the plateau region of the high harmonics led to a number of early suggestions that combining these high harmonic emissions could support very short, attosecond, pulses. It was the theoretical predictions by Anne and collaborators (8–10), fueled by the information amassed in Anne’s experimental investigations about the high harmonic emission process, that set the stage for the production of attosecond pulses.

It was validated that electron tunneling out from the atom followed by a rescattering process of the electron with its ion was responsible for the high harmonic emissions and the highest observed harmonic photon energy was formulated (11). A popular intuitive model for the electron recollision process was born (12). As noted, the electron driven by the laser gains substantial energy from the laser field, and this energy is released when the electron recombines with the ion in the form of extreme ultraviolet emission. Moreover, it became clear that the recollision process itself was taking place on very short, attosecond timescales, definitely providing a possible pathway to make attosecond pulses of extreme ultraviolet and X-ray light. This was also appealing because in the extreme ultraviolet, the cycle frequency is sufficiently rapid to support multiple electromagnetic cycles within an attosecond pulse, whereas in the visible optical regime, barely a fraction of an optical cycle occurs in a few hundred attoseconds. The electron recollision time was certainly short enough, and, if only the phases of the high harmonics would cooperate in lock step, attosecond pulses could result from the harmonic emissions.

Pierre Agostini was an early strong-field pioneer and the discoverer of one of the other processes involved in the interaction of strong laser fields with atoms, that of above- threshold ionization (13), i.e., peaks in the ionization spectrum that result from adding the energy of an additional photon from the laser, occurring at higher energies than required for the minimum ionization process itself. To understand this process and other multiphoton processes in atoms, Pierre was developing interference methods at CEA Saclay to detect two-photon ionization pathways. A method that ultimately became affectionately known to the community as RABBITT, reconstruction of attosecond beating by interference of two-photon transitions, was invented.

Using a powerful laser at the Laboratoire d’Optique Appliquiée, Agostini (Fig. 2) analyzed electron energies from atoms that were ionized by the resultant high harmonic photons in an electron spectrometer. Sidebands in the kinetic energy of the electrons are created when the harmonics and an additional photon from the fundamental laser pulse are present, by processes such as above-threshold ionization, and these could be measured as a function of time delay between the laser pulse and the harmonics. Agostini’s measurements with the higher power laser sought specifically to extract the phases of each harmonic. The results showed that a train of attosecond pulses were created by the sum of just five high-order harmonics, and the pulse durations of that train of pulses were determined to be 250 attoseconds (14). The experiments confirmed precisely that the relative phases of the harmonics were cooperating, i.e., “locked in phase.”

Fig. 2. Pierre Agostini. Image credit: Krystal Kenney (The Ohio State University, Columbus, Ohio).

Ferenc Krausz (Fig. 3) came to the field of attosecond pulses from a somewhat different point of view, not from the subject of atomic processes but from the development of lasers themselves. Ferenc was skilled at designing lasers from scratch, as well as developing new technology to produce phase-locked pulses, i.e., where the phase of the carrier wave is locked to the envelope of the pulse, in high-energy pulsed lasers themselves. The Nobel Prize in Physics in 2005 was awarded to John Hall and Theodor Hänsch for precision spectroscopy based on what is called optical frequency comb technology, where a very high-resolution fine-toothed “comb” of frequencies is produced by specialized lasers, and that award recognized the ability “to measure both time and distance more accurately than before.” This technology was extended by Ferenc to high-power, amplified pulsed lasers by the introduction of the phase-locked pulses in each stage of the laser amplification, with the application of producing very short, high-energy optical laser pulses.

Fig. 3. Ferenc Krausz. Image credit: Peter Seidel (photographer). © 2021 www.peterseidel.de.

Ferenc drove the process of HHG with these ever-shorter strong-field laser pulses and achieved higher energy X-ray photons. There were concurrent theoretical predictions, indicating that isolated attosecond pulses could exist in the region of the highest harmonic energies of the high harmonic spectrum, rather than the plateau. By filtering the harmonic output to focus on just the highest harmonic region, Ferenc showed, first, that 1.8-fs pulses of extreme ultraviolet light were achieved (15), and then, isolated attosecond pulses of 650-attosecond duration were produced (16).

Similar to the case of the pulse trains investigated by Pierre, an atomic photoionization process was required to show that isolated attosecond pulses were produced. To do this, a somewhat different method called an attosecond streak camera was developed (17). An atom is subjected to the strong field of the master, few-cycle laser pulse, and the amount of momentum kick that the electron receives from the field at the precise time that the electron is released from the atom by the isolated attosecond pulse is recorded versus time delay. A reconstruction algorithm allows to extract not only the duration of the attosecond pulse but also the precise field cycles and envelope of the few-cycle laser pulse.

These highly visual streak traces (and RABBITT traces) became the coin of the realm for practitioners of isolated attosecond pulse technology (18). Moreover, the first lifetime of an inner shell state in an atom was extracted (2), and soon, tens of attosecond photoemission delays were observed from different orbitals in atoms (3, 4) and from the surfaces of solids (5). Not only had attosecond pulses been validated in the lab, but, with much work, viable methods were achieved to measure the intrinsic time dynamics of atoms and solids via the photoemission of electrons (19).

A decade later, the photoelectron methodologies that seemed so essential for the first attosecond pulse measurements were augmented by additional, more general methods, such as attosecond transient absorption (20), expanding the means by which attosecond pulses could be used and involving new communities of scientists across more fields. This quickly went beyond the concept of simple atomic events to relevant timescales in much more complex systems and many more processes that could be studied. Rapidly, it became possible, for example, to measure the electronic state changes at conical intersections, ring opening processes, bond breaking, and intersystem crossings.

Today, record short pulses in the X-ray of 53- and 43-attosecond pulses are reported (21, 22). The process of HHG was refined to achieve reliable fluxes of photons at harder X-ray energies (23). Moreover, attosecond pulses have now been produced at free electron lasers (24). The HHG process itself, embodied in the 2023 Nobel Prize in Physics, has been extended to solids (25). What was perhaps unexpected for a Nobel Prize based on the creation of some of the shortest pulses observed to date are the many other side benefits that have emerged from this innovative field.

Ultrafast X-ray spectroscopy has become a formidable new way to investigate dynamics in atoms, molecules, and materials (26). This is because extreme ultraviolet spectra originate from the inner shells of atoms, providing an element- and location-specific means of probing dynamics. The spectroscopic transitions associated with attosecond pulses are sensitive to spin, oxidation state, chemical environment, and even chiral sites in molecules. Thus, new fields have been founded, for example, to understand the interactions of chiral light with molecules in the X-ray and extreme ultraviolet (27). Medical applications are sought through the interaction of few-cycle infrared pulses with molecules via Fourier spectroscopy (28). Practitioners are already seeking to understand the implications of entanglement and coherence that arise among the particles created by attosecond pulses (29, 30).

The 2023 Nobel Prize winners in Physics are outstanding and exemplary members of an extremely generous and highly collaborative community. Through their leadership, amazing discoveries and advances were made. Moreover, their generosity to help and learn from others has led to many vital and exciting collaborations and successes. It is a pleasure to give tribute to their achievements in this field and their human qualities, which have touched us all.

This article is part of a series of articles in PNAS highlighting the discoveries and profiling recipients of the Nobel Prize.
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