The “flashes of light” associated with Lund University are attosecond pulses, bursts of extreme-ultraviolet radiation so brief they can freeze the motion of electrons inside atoms and molecules. Anne L’Huillier, a physicist at Lund, shared the 2023 Nobel Prize in Physics for developing the experimental methods that make these pulses possible. An attosecond is a billionth of a billionth of a second, and the techniques pioneered in Lund have opened a window onto processes that were previously too fast to observe.
Why Lund and the Nobel Prize
The 2023 Nobel Prize in Physics was awarded jointly to Pierre Agostini of Ohio State University, Ferenc Krausz of the Max Planck Institute of Quantum Optics and Ludwig-Maximilian University of Munich, and Anne L’Huillier of Lund University “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter.”1Europhysics News. The Nobel Prize in Physics 2023 L’Huillier’s contribution stretches back to the late 1980s and early 1990s, when she and colleagues discovered that shining an intense laser pulse into a gas could produce a cascade of higher-frequency harmonics of light. That discovery, called high-order harmonic generation, became the foundation for creating attosecond pulses. Decades of refinement at Lund turned what began as an unexpected laboratory observation into a reliable tool for probing the fastest processes in nature.
Lund’s role is not just historical. The university continues to operate one of the world’s leading attosecond laboratories, where researchers generate short attosecond pulse trains at high repetition rates using optical parametric amplification laser technology that delivers few-cycle infrared pulses at a repetition rate of 200 kHz.2Lund University Publications. Short Attosecond Pulse Trains at High Repetition Rates for Novel Pump-Probe and Coincidence Studies That high repetition rate matters because it allows researchers to gather data much faster than older systems that fire only a handful of times per second, making experiments that once took days possible in hours.
How You Make a Flash That Short
To understand why these flashes are remarkable, consider the timescale. A single attosecond is to one second roughly what one second is to the age of the universe. Light itself travels only about three-tenths of a nanometer in an attosecond, about the diameter of a single water molecule. Producing a controlled burst of light on that timescale requires a trick of physics rather than brute-force engineering.
The trick is high-order harmonic generation. You aim an extremely intense, very short laser pulse at a gas of atoms. The laser’s electric field is strong enough to pull an electron away from its parent atom, accelerate it, and then slam it back. When the electron recombines with the atom, it releases the energy it picked up as a burst of extreme-ultraviolet or soft X-ray light at many times the laser’s original frequency. Those harmonics, when properly combined, interfere with one another to produce a pulse lasting only a few hundred attoseconds or less. Early work in this area produced periodic combs of roughly 250-attosecond pulses by superposing just four harmonics, while single pulses as short as 650 attoseconds were achieved and later pushed toward an estimated limit of about 130 attoseconds.3IOP Publishing. The physics of attosecond light pulses
A key distinction in the field is between pulse trains and isolated pulses. A pulse train is a rapid sequence of attosecond flashes, one per half-cycle of the driving laser. An isolated attosecond pulse is a single, solitary flash. Both have uses, but many experiments need a single flash to capture a snapshot of an event at one precise moment. Generating clean isolated pulses is harder and has spurred a variety of gating techniques.
Engineering a Single Isolated Flash
Getting from a train of pulses to a single isolated burst requires some form of gating, essentially a way to allow harmonic emission during only one half-cycle of the driving laser field. One widely used approach is polarization gating, which manipulates the polarization of the driving laser so that it is linearly polarized for only a brief window in the middle of the pulse. Because high harmonic generation is efficient only in linearly polarized light, emission is confined to that narrow window, producing a single attosecond pulse.
Recent work has extended these techniques beyond the traditional noble-gas targets to semiconductors. Researchers have demonstrated that polarization gating applied to molybdenum diselenide can generate clean isolated attosecond pulses, with a driving wavelength around 2.5 micrometers identified as optimal for that material.4PubMed Central. Controlling isolated attosecond pulse generation in MoSe2 using polarization gating and TDDFT simulations Similarly, cadmium sulfide has been used as a target with a two-color pulse system and polarization gating technique, producing single attosecond pulses with durations around 400 attoseconds and improved intensity compared to simpler schemes.5PubMed Central. Generation of isolated attosecond pulses in CdS semiconductor using polarization gating technique and tailored two-color pulse system Moving from gases to solid-state materials matters because it could eventually make attosecond sources more compact and more tunable, opening up new wavelength ranges.
Measuring What You Have Made
Creating an attosecond pulse is only half the challenge. You also need to measure it, and you cannot measure something that short with any ordinary detector. The pulses are far too fast for electronics, which respond on the picosecond scale at best, millions of times slower. Instead, researchers use the attosecond pulses themselves in combination with a synchronized infrared laser field to read out timing information from the electrons the pulses liberate.
One of the most widely used measurement techniques is called RABBIT, which stands for reconstruction of attosecond beating by interference of two-photon transitions. In a RABBIT measurement, an attosecond pulse train ionizes atoms in the presence of a weak infrared dressing field. The photoelectrons from adjacent harmonics can reach the same final energy by absorbing or emitting one infrared photon, and the resulting interference pattern oscillates as the delay between the attosecond pulses and the infrared field is scanned. Decoding those oscillations reveals the timing structure of the attosecond pulse train, and also any time delays introduced by the ionization process itself. Detailed simulations have examined how the spatial and temporal properties of the light fields affect the accuracy of RABBIT measurements, confirming that it can deliver reliable timing information when the experimental parameters are well controlled.6Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Science. Accuracy and precision of the RABBIT technique
The same interferometric measurement techniques that characterize the pulses have become scientific tools in their own right. By measuring tiny differences in the arrival times of photoelectrons ejected from different atomic shells, researchers can extract information about how long it takes an electron to escape an atom after absorbing a photon, a quantity that was essentially unmeasurable before attosecond pulses existed.
Timing Electrons as They Leave Atoms
One of the earliest and most striking applications of attosecond pulses was measuring the time delay in photoionization. When a photon hits an atom and knocks out an electron, that electron does not emerge instantaneously. It takes a tiny but measurable amount of time to traverse the atomic potential on its way out, and electrons from different atomic shells leave at slightly different times. Attosecond interferometry made it possible to resolve those differences.
In argon atoms, researchers measured the time delay between electrons emitted from the 3s and 3p shells at excitation energies between 32 and 42 electron volts. The experiments revealed that the probing infrared field itself contributes a substantial portion of the measured delay, a subtlety that must be carefully accounted for to extract the true atomic physics.7PubMed. Probing single-photon ionization on the attosecond time scale More recent work has pushed the spectral resolution of these measurements far enough to identify the specific electron correlations that affect photoemission timing, resolving a long-standing disagreement between theory and experiment. The key turned out to be coherent couplings with so-called shakeup channels, where the departing electron exchanges energy with the remaining electrons in the atom.8PubMed. Photoionization Time Delays Probe Electron Correlations
This line of research has expanded beyond atoms to molecules, liquids, and solids. A broad overview of the field notes that attosecond techniques can now resolve the dynamics of single-photon ionization, tunnel ionization, and a range of other processes across gas, liquid, and solid phases.9PubMed Central. Photoemission and photoionization time delays and rates What started as a measurement of a quirky quantum delay in a single noble gas has become a general-purpose probe of how electrons interact with their surroundings on the shortest timescales physics can access.
Watching Charge Move Through Molecules
Atoms are only the beginning. When you ionize a molecule, the resulting “hole” left behind by the departing electron does not sit still. It can migrate from one part of the molecule to another on a timescale of hundreds of attoseconds, driven purely by the quantum-mechanical evolution of the electronic states, with the heavier atomic nuclei essentially frozen in place. This process, called charge migration, is thought to influence how chemical bonds break and form in the earliest moments after a molecule absorbs light.
Experiments on iodoacetylene demonstrated that high-harmonic spectroscopy could spatially and temporally resolve this electron-hole migration with a resolution of roughly 100 attoseconds. By measuring both even and odd harmonic orders and matching them to theoretical calculations, the researchers reconstructed the quantum amplitudes and phases of the electronic states involved and tracked how the hole moved as a function of the molecule’s spatial orientation. They also showed that the laser field itself could be used to control the migration, steering the charge redistribution in real time.10PubMed. Measurement and laser control of attosecond charge migration in ionized iodoacetylene
A complementary approach combines ultrafast X-ray and electron diffraction to create real-space movies of charge migration dynamics. By using the ground-state nuclear structure as a reference frame and exploiting a mixed electronic-nuclear interference signal, researchers showed it is possible to invert diffraction data into movies of how electronic charge density redistributes on the attosecond timescale.11PubMed. Attosecond Charge Migration in Molecules Imaged by Combined X-ray and Electron Diffraction Imagine watching, frame by frame, as a cloud of negative charge sloshes from one end of a molecule to the other before the atomic skeleton has time to move even slightly. That is what these experiments are beginning to accomplish.
The long-term promise here extends to chemistry and biology. A major review of attosecond electron dynamics in molecules notes that the field has grown from its roots in atomic physics to tackle problems of direct chemical and biological interest, with experiments and theoretical methods now being applied to molecules whose behavior matters for real-world chemistry.12PubMed. Attosecond Electron Dynamics in Molecules Understanding how charge migrates in the very first instants after a molecule absorbs light could help explain, and eventually control, processes like photosynthesis, radiation damage to DNA, and the initial steps of vision.
Attosecond Pulses Inside Solids
Molecules in a gas are relatively simple compared to a crystal lattice, where trillions of atoms sit in a periodic arrangement and electrons move through energy bands rather than orbiting individual nuclei. Attosecond spectroscopy has begun to tackle this more complex environment as well. In a landmark experiment with silicon, researchers used few-cycle laser pulses to inject electrons into the conduction band and then probed the resulting changes in the material’s extreme-ultraviolet absorption spectrum with attosecond timing. They observed that the absorption changed in sharp steps synchronized to each half-cycle of the laser’s electric field oscillations, with the step rise time measured at roughly 450 attoseconds. That number provides an upper limit for both the time it takes for injected carriers to alter the band gap and the rate at which electrons scatter off one another inside the conduction band.13PubMed. Attosecond band-gap dynamics in silicon
This matters for technology. The speed at which electrons respond to electric fields inside semiconductors sets a fundamental ceiling on how fast electronic devices can switch. Conventional transistors operate on the nanosecond to picosecond scale. Attosecond measurements in silicon suggest that the intrinsic response of the material is many orders of magnitude faster than any current switching technology exploits. Whether that headroom can ever be harnessed for practical electronics remains an open question, but the ability to measure these processes in real time is a necessary first step.
Controlling Quantum Processes With Attosecond Precision
Measuring fast processes is one thing; steering them is another. Attosecond coherent control aims to manipulate the outcome of quantum events by precisely timing the phase relationship between different light fields. Using a seeded free-electron laser with a novel attosecond timing tool, researchers demonstrated the first attosecond coherent control of photoionization in a two-color field, manipulating the phase of high-order near-infrared transitions to direct how electrons are ejected.14Nature Photonics. Attosecond coherent control of electronic wave packets in two-colour photoionization using a novel timing tool for seeded free-electron laser In practical terms, this means the experimenters could shift the attosecond timing of the light fields and watch how the pattern of emitted electrons changed in response, proving that the quantum outcome was being actively steered rather than merely observed.
Coherent control at this timescale is still in its infancy, but the implications are tantalizing. If you can control where charge goes in the first few hundred attoseconds after a molecule absorbs light, you could in principle influence which chemical bonds break, which products form, or how efficiently energy is transferred. The iodoacetylene charge-migration experiments mentioned earlier already demonstrated a rudimentary version of this: by tuning the laser field, the researchers altered the path of the migrating electron hole. Scaling that level of control to larger, more chemically relevant molecules is a major goal of the next decade of attosecond research.
Shorter Than Attoseconds
As remarkable as attosecond pulses are, some physical processes are faster still. The motion of protons inside atomic nuclei occurs on the zeptosecond timescale, a thousandth of an attosecond, and the dynamics of quarks and gluons play out on yoctosecond timescales, a millionth of an attosecond. Theoretical proposals have outlined pathways toward generating electromagnetic pulses on these scales, which would allow experiments on quantum electrodynamic processes, nuclear dynamics, and quark-gluon plasma physics.15arXiv. Beyond Attoseconds These ideas remain speculative and would require light sources with energies and frequencies far beyond what current laser technology delivers. But the trajectory is clear: each time a new timescale becomes accessible, an entirely new class of physical phenomena comes into view.
For now, the attosecond frontier is where the action is, and Lund remains one of its most active centers. The laboratory there continues to push repetition rates higher, pulse durations shorter, and the range of target materials broader, while training a new generation of physicists in a field that barely existed when L’Huillier first noticed those unexpected harmonics coming out of a gas jet more than three decades ago.