Portrait of Pierre Agostini by his close colleagues from LIDYL at CEA-Saclay

Pierre Agostini is a French physicist and pioneer of ultrafast physics on the attosecond scale (one billionth of a billionth of a second), currently Professor Emeritus at Ohio State University after a full career at the CEA Saclay. He was awarded the Nobel Prize in Physics 2023 on October 3, 2023, for his work permformed at CEA-Saclay. He shares this prize with his colleague Anne L’Huillier, who worked in the same CEA laboratory, and Ferenc Krausz of the Max Planck Institute for Quantum Optics in Munich.

Through his pioneering studies of atomic ionization in intense laser fields, Pierre Agostini forged the tools of attosecond metrology, which would enable him to measure the shortest flashes of light ever produced, paving the way for their use to probe electronic movements in matter.

The attosecond (as = 10-¹⁸ s, or a billionth of a billionth of a second) is the natural time scale for electron motion in matter. In Niels Bohr’s model of the hydrogen atom, the electron completes its rotation around the nucleus in just 150 attoseconds.

At the microscopic level, many processes occur on this time scale, from the first stage of chemical reactions to electrical currents and photovoltaic processes. These ultra-fast electronic phenomena have long remained inaccessible to experimental study, due to the lack of sufficiently brief flashes of light to capture each instant and record the sequence of events.

Anne L’Huillier’s discovery of the generation of high-order harmonics (GHOE) in the 1980s provided the source of ultra-shortwave radiation, whose spectrum extends into the extreme ultraviolet (XUV), that was to revolutionize ultrafast physics. For his part, Pierre Agostini pioneered the high-field ionization processes which, in 2001, made it possible to measure such short pulse durations, a prerequisite for their use in probing electronic motions in matter and thus opening up the field of attosecond science.

Pierre Agostini defended his thesis at the University of Aix-Marseille in 1968. That same year, he joined CEA-Saclay, where he enjoyed a brilliant scientific career until his retirement in 2002, remaining Scientific Advisor until 2004.

As early as 1968, Pierre Agostini was studying the process of multiphoton ionization of atomic gases induced by an intense infrared (IR) laser, a process just discovered thanks to the advent of pulsed lasers. In 1979, he observed for the first time ‘over-threshold’ ionization, i.e. the ability of an atom to absorb more laser photons than necessary to cross the ionization threshold. This first observation of continuum-continuum transitions opened up a whole new field of research. Numerous other pioneering studies followed, laying the foundations for the temporal characterization of attosecond pulses.

Particularly, from 1994 onwards, Pierre Agostini carried out a series of studies on ionization induced by XUV sources in the presence of an intense laser field. He did so in collaboration with Harm Muller of the FOM Institute in Amsterdam and the teams in charge of the laser platform at ENSTA’s Laboratoire d’Optique Appliquée in Palaiseau. In particular, he uses the new ultra-brief source supplied by the GHOE, superimposed in time on a fraction of the fundamental laser. The continuum-continuum transitions induced by the latter lead to the appearance of sidebands in the spectrum of emitted electrons, which encode the cross-correlation of harmonic and laser pulses on the femtosecond scale. In fact, sidebands only exist when the two beams interact simultaneously with the atom, and they disappear as soon as they are desynchronized, i.e. in this experiment, in a few tens of femtoseconds.

To reach the attosecond scale, much finer control of multi-color ionization was required. In 1996, a team of theorists from the Laboratoire de Chimie-Physique-Matière et Rayonnement (LCPMR) at Sorbonne University, led by Alfred Maquet, showed that within the cross-correlation trace, the intensity of the sidebands is in principle modulated on the optical cycle scale by the delay between the two XUV and IR beams. These modulations are due to interference between the different ionization pathways leading to the sidebands, and therefore depend on the relative phase between successive harmonics. They then proposed the method, later named RABBIT (‘Reconstruction of attosecond beating by interference of two-photon transitions’), which measures the amplitude and relative phase of the different harmonic frequencies of the GHOE, and which, by Fourier transform, gives access to the temporal profile of the attosecond pulses. This electronic interferometry required high device stability and recurrence, which would be made possible by LOA’s state-of-the-art 1 kHz laser. The RABBIT technique was demonstrated experimentally by Pierre Agostini and colleagues in 2001, with the first characterization of a 250 attosecond pulse train.

Pierre Agostini then continued his studies at Saclay, where in 2003 he measured a train of 130 attosecond pulses and observed a frequency drift of the attosecond emission, which is still the limiting factor for the record duration of these pulses, at 43 as. After several guest research positions in major European and American laboratories, in 2005 he became Professor of Physics at Ohio State University in Columbus (USA), where he continues his research and is today Professor Emeritus.

Beyond the “pulse duration record” aspect, this attosecond metrology had an immediate impact, as it enabled the optimization of attosecond emission, paving the way for all its applications. Among these, the Nobel Committee underlines the conceptual advance represented by the study of the photoelectric effect in real time, a process theorized by Albert Einstein in 1905 through the absorption of a quantum of light (the photon) and the quasi-simultaneous emission of an electron.

The RABBIT method, as well as the related ‘streaking’ technique developed by Ferenc Krausz, co-winner of the 2023 Nobel Prize in Physics, can also be used to measure tiny emission delays of a few tens of attoseconds, providing valuable information on the cohesion of matter at the microscopic scale. These studies were initiated by Ferenc Krausz and Anne L’Huillier, with subsequent contributions from Pierre Agostini.

Winner of several international awards, including the Joop Los Award from the FOM Institute in the Netherlands, and the William F. Meggers Award from the Optical Society of America in 2007, Pierre Agostini was awarded the Nobel Prize in Physics in October 2023.

In 2013, the scientific community paid tribute to his work, along with that of theorist Alfred Maquet (LCPMR), at the SYMPA Symposium opening the ATTO international conference. The 350 participants listened to a number of presentations and tributes, including those by future Nobel Prize winners Anne L’Huillier, Ferenc Krausz and Alain Aspect, followed by Claude Cohen-Tannoudji’s closing address.

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Written by Thierry Ruchon and Pascal Salières (CEA-IRAMIS/LIDYL).

References:

• The original research publication by Anne L’Huillier on GHOE:

Multiple-harmonic conversion of 1064 nm radiation in rare gases
M Ferray, A L’Huillier, X F Li, L A Lompre, G Mainfray and C Manus
Journal of Physics B: Atomic, Molecular and Optical Physics, 21(3) (1988) L31-35

• on the method for characterizing attosecond laser pulses by Pierre Agostini, co-recipient of the 2023 Nobel Prize:

Observation of a train of attosecond pulses from high harmonic generation
P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, Ph. Balcou, H. G. Muller, and P. Agostini
Science, 292, (5522) (2001) 1689-1692

• Pierre Agostini’s publication on the first observation of attosecond frequency drift:

Attosecond synchronization of high-harmonic soft X-rays
Y. Mairesse, A. de Bohan, L. J. Frasinski, H. Merdji, L. C. Dinu, P. Monchicourt, P. Breger, M. Kovacev, R. Taïeb, B. Carré, H. G. Muller, P. Agostini and P. Salières, Science 302 (2003) 1540.