Faster than a speeding bullet—the 2023 Physics Nobel Prize

Attosecond physics is a novel research field that pursues a better understanding of electron dynamics in atoms, molecules and condensed matter by means of pump-probe experiments where the motion of electrons are tracked with attosecond (1 as = 10−18 s) time resolution. The 2023 Physics Nobel Prize was awarded to three experimental pioneers of the field, who developed the key methods to generate and characterize attosecond pulses.

On 3 October of last year, Anne L'Huillier, Pierre Agostini and Ferenc Krausz were awarded the 2023 Nobel Prize in physics for their groundbreaking research into the production and characterization of attosecond pulses, the shortest light pulses ever to be produced in a laboratory.Using attosecond pulses (1 as =10 −18 s), the motion of electrons in atoms, molecules and the condensed phase can be visualized.The fundamental insights that can be obtained in this manner have many potential applications.For example, improved insight into the motion of electrons in molecules may create opportunities to steer chemical processes towards desired outcomes, while improved understanding of charge transport may lead to the further improvement of devices such as solar cells.
The 2023 Nobel Prize in physics is strongly connected to the Journal of Physics B. Many important discoveries, including the discovery of the high-harmonic generation (HHG) process underlying the production of attosecond pulses (Ferray et al 1988) itself were first reported in the journal, and nowadays the journal continues to be an important outlet for the latest results that are achieved in the attosecond science research field.It is for this reason that the Journal of Physics B congratulates the three Nobel laureates and celebrates the 2023 Nobel Prize in physics by making a selection of key research papers by the three Nobel laureates and many others available to its readership in a special collection.
The generation and characterization of attosecond laser pulses follow a long tradition of laser development that started in 1960 with the discovery of the laser by Maiman (1960).This discovery quickly led to the demonstration of second harmonic generation (SHG) by Franken et al (1961), the first experimental example of a non-linear optical effect.In SHG a non-linear optical crystal is used to fuse two photons from an incoming laser together, producing a single new photon with twice the photon energy.
These first demonstrations of the generation and application of laser light were followed by numerous further developments, including that of the 'chirped pulse amplification' (CPA) technique, for which Gerard Mourou and Donna Strickland received the 2018 Nobel Prize in physics.In CPA, a short laser pulse is first stretched in time before being amplified, thereby avoiding damage of the laser amplifier.CPA made it significantly easier to generate high intensity laser pulses, Figure 1.High-harmonic spectrum recorded by Anne L'Huillier and co-workers in experiments where a 1064 nm Nd:YAG laser with an intensity of 3 × 10 13 W cm −2 was focused on a 10 mbar Xe gas expansion.This discovery of the high-harmonic generation (HHG) process was reported in the Journal of Physics B (Ferray et al 1988).Reproduced from Ferray et al (1988).© IOP Publishing Ltd All rights reserved.
enabling more extensive studies of non-linear optical effects, and leading to some very surprising results.
A first surprise had already come in 1979, when Pierre Agostini, one of this year's Nobel laureates, discovered the process of 'above threshold ionization' (Agostini et al 1979).Until then, it had been assumed that non-linear ionization processes would be dominated by absorption of the minimum number of laser photons needed to overcome the ionization potential, resulting in photoelectrons with a kinetic energy E e = nhν − IP, where hν is the photon energy, IP the ionization potential and n the smallest number where E e > 0. The experiments by Agostini revealed the formation of photoelectrons with a significantly higher kinetic energy, demonstrating the involvement of ionization processes where E e = (n + s) hν − IP, with s > 0.
Whereas the first experiments on strong field ionization mostly looked at the formation of ions and photoelectrons, some researchers decided to explore the role of excited state resonances in the ionization process by focusing their attention on the possible emission of light.These experiments were significantly harder, since photons cannot be guided towards a detector using an electric field and since the detection efficiency of photons is typically lower than that of ions and photoelectrons.To address this challenge, Anne L'Huillier put together an experiment where the target pressure in the laser focus was significantly higher than had customarily been the case until then.And thus the first high-harmonic spectrum was recorded (see figure 1) (Ferray et al 1988).Remarkably, the spectrum of the emitted light did not consist of atomic resonances but contained a series of harmonic peaks in the extreme ultra-violet (XUV) part of the wavelength spectrum, i.e. peaks where the photon energy is an odd multiple of the photon energy of the incoming laser: E q = nh ν .Moreover, up to a high energy cut-off all the peaks were approximately equally strong.This key result, which may in retrospect be regarded as the moment that attosecond science was born, was published in the Journal of Physics B in 1988(Ferray et al 1988).
Further experiments on this new process of HHG explored how the formation of the harmonics depended on the experimental conditions (L 'Huillier et al 1992).It turned out that the highest observed photon energy depended both on the laser intensity and the gas that was used according to E max ≈ IP + 3U p , where U p is the cycle-averaged kinetic energy of an electron that performs an oscillatory motion  Corkum (1993).After ionisation by the intense driver laser (commonly described as a tunnelling process through the Coulomb + laser field potential (black line)) the photoelectron is accelerated by the oscillatory laser electric field.Some of the electrons return to their parent ion, allowing a recombination process, where the available energy is released as a high energy photon.Because this process repeats every half-cycle of the driver laser, the spectrum of the emitted light consists of odd harmonics of the driver laser frequency.
in the intense laser field.Moreover, experiments showed that the HHG intensity scaled quadratically with the gas density, suggesting that the high-harmonic radiation results from the coherent addition of the emission amplitudes from all emitters in the laser focus (and explaining why phase-matching is important in HHG).
All in all, it took about 5 years before the major ingredients of our current understanding of the HHG process were established (L 'Huillier et al 1991).In 1993, Schafer et al (1993) and Corkum (1993) published landmark papers that led to the so-called 'three-step picture' of HHG (see figure 2), where the HHG process is understood as a sequence of (i) ionisation of target atoms under the influence of the strong oscillatory electric field of the laser, (ii) acceleration of the photoelectron by this electric field (first away, and then back towards the ion left behind), and (iii) a so-called 're-collision' between the electron and the ion where recombination occurs accompanied by the emission of an XUV photon that carries of all the energy that has been invested in the ionization (step (i)) and electron acceleration (step (ii)) processes.Since this three-step process repeats every intense half-cycle of the driving laser pulse, the emitted photon energies are constrained to have photon energies that are odd multiples of the photon energy of the driver laser, hence the appearance of 'harmonics' in the spectrum.
It did not take long before the insights of Schafer et al (1993) and Corkum (1993) led to suggestions that the formation of attosecond laser pulses might be possible.After all, the ionisation by the intense laser field (step (i)) occurs most efficiently during a small fraction of the driver laser optical cycle when the laser electric field is maximal.After the acceleration (step (ii)), all electrons in the gas medium return to their parent ion at approximately the same time, and therefore the recombination (step (iii)) does not occur continuously, but is expected to occur during a small fraction of the driver laser optical cycle, which for commonly used Ti:Sapphire lasers is itself only 2.7 fs (1 fs = 10 −15 s) long.In other words, the three-step picture describes electron dynamics that automatically brings us into the attosecond domain.
However, there was no guarantee that addition of the emission of all atoms in the gas medium would preserve any of the attosecond time-scale dynamics that is inherent in the three-step picture.Moreover, it is one thing to anticipate that attosecond pulses may be formed-it is quite another to experimentally prove that with a photon energy E q+2 = (q + 2) hν accompanied by stimulated emission of an IR photon, and absorption of a lower-energy harmonic Hq with a photon energy Eq = qhν accompanied by absorption of an IR photon, both lead to the detection of a photoelectron with a kinetic energy Ee = (q + 1) hν − IP, where hν is the photon energy of the driver laser and IP is the ionization potential of the atom that is used in the experiment; (b) reconstructed attosecond pulse train (APT) as reported in the paper by Paul et al (2001).The RABBITT measurement reveals that in the time-domain the high-harmonic emission consists of a train of attosecond laser pulses with an individual duration of about 250 as.From Paul et al (2001).Reprinted with permission from AAAS.
this is the case.And that is why it took almost ten more years before the first experimental demonstrations of attosecond laser pulses were reported.Appropriate characterization techniques only emerged in 2001, nearly simultaneously, in two different places in Europe.
Short pulses are completely characterized when their electric field is known at all times.Unfortunately, there do not exist any photo-detectors that can follow changes in the electric field of the incoming light on the femtosecond, let alone attosecond timescale.Alternatively, a light pulse is completely known if it is fully characterized in the frequency domain, i.e. if we know the frequency spectrum of the pulse as well as the spectral phase at each frequency.This phase determines how all the available frequency components add up to a pulse that is ultrashort in the time-domain.
This idea underlies the first demonstration of attosecond laser pulses as reported by Paul et al (2001).While it is easy to measure the intensity of every spectral component in the High Harmonic radiation (for that, a measurement such as the one shown in figure 1 suffices), it is a lot more difficult to measure the spectral phase of all the observed frequency components.For this, Agostini and co-workers developed an interferometric method, which became known as RABBITT ('reconstruction of attosecond harmonic beating by interference of two-photon transitions (Muller 2002)').The scheme is explained in figure 3(a).
In the simplest implementation of a RABBITT experiment, the high-harmonic radiation is used to ionize an atomic gas in the presence of a replica of the IR driver laser.Besides contributions arising from single-photon ionization by one of the harmonics, the photoelectron spectrum then also contains contributions where the absorption of one of the harmonics is accompanied by absorption or stimulated emission of an IR photon.Absorption of a higher energy harmonic H q+2 accompanied by stimulated emission of an IR photon, and absorption of a lower energy harmonic H q accompanied by absorption of an IR photon, both lead to the detection of a photoelectron with a kinetic energy E e = (q + 1) hν − IP.This leads to a situation that is comparable to a double-slit experiment.Both ionization pathways interfere with each other, and this interference depends on the phase difference between the two harmonics involved.By measuring these phase differences for a series of adjacent harmonics, Agostini and co-workers were able to determine that in their experiment a train of 250 as long laser pulse was formed (Paul et al 2001).
This first measurement of an attosecond pulse train was followed briefly thereafter by the first measurement of an isolated attosecond pulse in the laboratory of Ferenc Krausz (2001).In their experiments, Hentschel et al used a driver pulse for the HHG process that was so short (∼5 fs) that the highest photon energies within the HHG spectrum could only be generated once, during a single cycle near the intensity maximum of the driver laser pulse.In this case, the XUV spectrum no longer looks like the spectrum in figure 1, but consists of a continuum of photon energies.To characterize these XUV pulses, the team took inspiration from the concept of a streak camera, where a time-dependent deflection field is used to map the temporal structure of an electron pulse into a spatial pattern that can be recorded with a camera.In order to generate a steep enough voltage ramp for the deflection field, the IR driver field was used, leading to the concept of an 'attosecond streak camera' (Itatani et al 2002).Under the influence of the IR deflection field photoelectrons that are produced at time t 0 undergo a momentum shift that is given by ∆p ∼ − ´∞ t0 eE IR (t ′ ) dt ′ = −eA IR (t 0 ) , where A IR (t) is the vector potential of the IR field and e is the elementary charge.Measuring the distribution of momentum changes that occur in the experiment (see figure 4) then allows to determine the range of times t 0 when photoelectrons are produced, thereby providing a temporal characterization of the attosecond pulse.In this manner, Krausz and co-workers found a pulse duration of 650 as (Hentschel et al 2001) and shortly thereafter 250 as (Kienberger et al 2004).
Since these first experiments demonstrating the formation of attosecond laser pulses by HHG, attosecond physics has developed into a vibrant research field.Nowadays, attosecond experiments are being conducted in dozens of laboratories around the world to gain deep insight into the dynamics of electrons within atoms, molecules and the condensed phase.Although it is still early days for the exploitation of attosecond laser pulses, attosecond experiments currently already address research questions in diverse fields ranging from biology and chemistry to various sub-fields within physics and quantum technologies.In our research at the Max-Born Institute in Berlin, for example, we look into the role of quantum entanglement in attosecond experiments and investigate this most intriguing of quantum mechanical phenomena on the shortest timescales possible (Koll et al 2022).The future of attosecond science looks highly promising, and this is why it is justified and very satisfying that the 2023 Nobel Prize in physics was awarded to three of its experimental pioneers.Perhaps we will see Nobel Prizes in future for the significant theoretical contributions that permitted the development of experimental attosecond science or the scientific applications that are now pursued using attosecond technniques.The Journal of Physics B looks forward to keeping you informed about these developments.Until then, we hope that you will find inspiration in our selection of papers in the Journal Physics B that document the pre-history and history of attosecond science up to the present time.

Figure 2 .
Figure2.Three-step picture of HHG, introduced bySchafer et al (1993) andCorkum (1993).After ionisation by the intense driver laser (commonly described as a tunnelling process through the Coulomb + laser field potential (black line)) the photoelectron is accelerated by the oscillatory laser electric field.Some of the electrons return to their parent ion, allowing a recombination process, where the available energy is released as a high energy photon.Because this process repeats every half-cycle of the driver laser, the spectrum of the emitted light consists of odd harmonics of the driver laser frequency.

Figure 3 .
Figure3.(a) Principle of a RABBITT-experiment: absorption of a higher-energy harmonic H q+2 with a photon energy E q+2 = (q + 2) hν accompanied by stimulated emission of an IR photon, and absorption of a lower-energy harmonic Hq with a photon energy Eq = qhν accompanied by absorption of an IR photon, both lead to the detection of a photoelectron with a kinetic energy Ee = (q + 1) hν − IP, where hν is the photon energy of the driver laser and IP is the ionization potential of the atom that is used in the experiment; (b) reconstructed attosecond pulse train (APT) as reported in the paper byPaul et al (2001).The RABBITT measurement reveals that in the time-domain the high-harmonic emission consists of a train of attosecond laser pulses with an individual duration of about 250 as.FromPaul et al (2001).Reprinted with permission from AAAS.

Figure 4 .
Figure 4. (a) Ionisation of Neon atoms by an isolated attosecond pulse, measured with an 'attosecond streak camera'.The delay between the attosecond pulse and a co-propagating IR pulse influences the electron kinetic energy distribution of the measured photoelectrons.From the measurement an attosecond pulse duration of 250 as can be deduced .Reproduced from Kienberger et al (2004), with permission from Springer Nature.(b) in the presence of the IR laser field the photoelectrons produced by the attosecond laser pulse undergo a momentum shift that is determined by the value of the vector potential of the IR laser A IR (t) at the time of ionization.In this way the measured photoelectron kinetic energy distributions allow to determine the range of ionization times and thereby the duration of the attosecond pulse.