The research was carried out by the Attosecond Microscopy group at the University of Oldenburg’s Institute of Physics, led by Dr Jan Vogelsang. The team worked with scientists from a research group headed by Nobel Prize-winning physicist Professor Anne L’Huillier at Lund University. Their results, published in Applied Physics B – Lasers and Optics, describe the detailed characterization of a laser system based on a 200-kilohertz optical parametric amplifier. The system generates pulses with a central wavelength of approximately two micrometres, placing them in the infrared region of the electromagnetic spectrum, beyond the range visible to human eyes.

What makes the achievement significant is not only the high repetition rate but also the extraordinary similarity between successive pulses. Each laser pulse lasts for an extremely short time, containing only a few oscillations of the electromagnetic field. In conventional light sources, small fluctuations can alter the precise shape and timing of those oscillations from one pulse to the next. For experiments that attempt to influence electron motion on attosecond timescales—billionths of a billionth of a second—even tiny variations can obscure the result. The Oldenburg team has now shown that the system can preserve the phase of the light field with exceptional reliability across timescales ranging from microseconds to several hours.

The crucial parameter is known as the carrier-envelope phase, or CEP. A laser pulse can be imagined as a short envelope containing a rapidly oscillating electromagnetic wave. The envelope describes the overall flash, while the carrier is the wave moving inside it. The CEP specifies the exact position of the wave’s peaks and troughs within the envelope. If the CEP changes, the electric field reaches its maximum strength at a slightly different moment, even when the pulse arrives at the same time overall. That difference may appear negligible, but it can determine whether an electron is accelerated, redirected, released from a material or left almost unaffected.

Katrin Meier, a physics doctoral candidate in the Oldenburg research group and lead author of the study, compares the effect to a precisely timed push on a swing. A small change in timing can determine the direction of the swing’s motion. In a similar way, a minimal shift in the carrier-envelope phase can change how an electron responds to an intense laser pulse. This sensitivity is central to strong-field physics, attosecond science and ultrafast spectroscopy, where researchers seek to observe and control the movement of electrons before the surrounding atoms and molecules have time to respond.

Maintaining a stable CEP is one of the most demanding challenges in ultrafast laser science. The phase can be disturbed by temperature changes, air currents, mechanical vibrations and other environmental effects that might be imperceptible in everyday life. Even the expansion of a component caused by a small temperature fluctuation can alter the optical path traveled by the laser beam. To overcome these influences, the researchers designed the laser setup with careful attention to mechanical stability, thermal control and optical alignment. The exceptionally stable conditions in the University of Oldenburg’s attosecond laboratory also played a critical role in keeping the pulses synchronized and nearly identical.

The team was surprised by how stable the CEP remained in the completed experimental arrangement. The researchers examined the laser’s behavior over a broad range of time intervals, allowing them to identify both rapid fluctuations and slower changes that would become visible only after many minutes or hours. This multiscale analysis is important because a laser can appear stable during a brief measurement while drifting over longer periods. By characterizing the system across these different timescales, the researchers demonstrated that the pulse-to-pulse consistency was not a short-lived effect but a durable property of the setup.

The ability to generate such a continuous sequence of stable pulses could transform experiments designed to manipulate electrons directly with light. In many previous measurements, changes in the CEP introduced uncertainty large enough to conceal the signal scientists were trying to detect. If every pulse has a different electric-field waveform, the resulting electron motion becomes an average of many different responses. That makes it difficult to determine whether an observed effect comes from the physical process under investigation or from uncontrolled variations in the laser. With the new system, researchers can repeat the same optical experiment thousands of times while preserving the field conditions that govern the interaction.

This capability could support future investigations into how electrons move through atoms, molecules and solid materials. It may help scientists explore the earliest stages of chemical reactions, examine charge transport in nanoscale systems and improve their understanding of light-driven processes in matter. In the longer term, precise control over electron motion could contribute to technologies such as ultrafast electronic switches and transistors that operate at speeds approaching the natural timescale of light-driven interactions. Such applications remain a future possibility rather than an immediate outcome, but the researchers describe the laser system as an important foundation for experiments that require unprecedented precision.

The study demonstrates that progress in ultrafast science depends not only on producing shorter and more powerful laser pulses, but also on making every pulse reproducible. By stabilizing the carrier-envelope phase of 200,000 infrared pulses per second and tracking its behavior from microseconds to hours, the Oldenburg and Lund teams have created a platform for probing electron dynamics with greater control. Their work turns the subtle waveform of light into an experimental tool, offering scientists a way to deliver repeated, precisely timed pushes to electrons and to study their response at the fastest scales accessible in physics.

Subject of Research: Not applicable

Article Title: Multiscale carrier-envelope phase characterization of 2-µm pulses delivered by a 200-kHz optical parametric amplifier

News Publication Date: 1 August 2026

Web References: https://doi.org/10.1007/s00340-026-08699-w

References: Applied Physics B

Image Credits: University of Oldenburg / Marcus Windus

Keywords

Ultrafast lasers, carrier-envelope phase, electron control, attosecond physics, infrared light, optical parametric amplifier, laser stability, University of Oldenburg, Lund University, strong-field physics

The breakthrough builds on high-harmonic generation, a process that converts laser light into radiation with frequencies hundreds or even thousands of times higher. The technique was central to the development of attosecond physics, the study of events occurring on timescales shorter than a billionth of a billionth of a second. In the 1990s, Nobel laureate Ferenc Krausz used related methods to produce some of the first controlled attosecond light pulses while working at TU Wien. The latest experiment returns to the same physical foundation but introduces a crucial difference: instead of allowing one electron to do all the work, it exploits the coordinated motion of two.

In the standard picture of high-harmonic generation, a strong laser field first pulls an electron away from its parent atom. Once free, the electron is driven back and forth by the oscillating electric field of the laser. During its return, it can collide with the ion from which it originated and recombine with it. The electron then releases the energy it gained in the laser field as a single high-energy photon. Because the electron’s maximum excursion and return energy are determined by the laser’s intensity and wavelength, the emitted spectrum follows a characteristic pattern with a sharp upper boundary known as the cutoff.

Before the cutoff, many harmonics—integer multiples of the original laser frequency—are emitted with approximately comparable strength. Beyond the cutoff, the signal drops rapidly, reflecting the maximum energy available from a single electron’s excursion. This limit is not simply an experimental inconvenience; it is built into the classical and quantum-mechanical description of the recollision process. Increasing the laser intensity or changing its wavelength can move the cutoff, but for a given driving field, conventional theory predicts that higher-energy X-ray emission should become extremely weak.

The researchers found a way around that restriction by using helium, whose two electrons are strongly connected through their mutual Coulomb interaction. Their experiment began with intense ultraviolet driving pulses capable of removing the electrons sequentially. The first electron was liberated and accelerated by the laser field. Under carefully controlled conditions, a second electron followed, and the timing of the laser pulse was adjusted so that both electrons returned to the helium ion at nearly the same instant. Rather than behaving as two independent particles, they formed a correlated quantum system.

That synchronized return changes the energy budget of the process. When one electron recombines, the photon it emits carries away the energy accumulated during its own journey in the laser field. When two electrons participate coherently, energy associated with both particles can be released during the same recollision event. The result is not merely two ordinary photons emitted side by side. The correlated process can produce one photon with substantially higher energy, extending the spectrum into a range that lies above the single-electron cutoff.

The experimental signature was a second, weaker plateau in the coherent X-ray spectrum. The first plateau matched the familiar high-harmonic response expected from single-electron dynamics. At higher photon energies, however, the helium spectrum displayed another broad region of emission before eventually declining. This second plateau is the key evidence that an additional mechanism is operating. Its appearance cannot be explained adequately by simply increasing the number of independent electrons; it points to a collective, quantum-mechanical release of energy.

The effect was not observed in the same way in argon and neon, whose outer electrons do not exhibit the required combination of strong correlation and synchronized dynamics under the experimental conditions. That contrast strengthens the conclusion that helium’s unusual response is linked to electron-electron interaction rather than to a generic multielectron effect. In helium, the electrons influence one another strongly enough that the timing and energy of one particle’s motion cannot be treated independently of the other. The atom therefore becomes a controllable laboratory for observing correlated motion on attosecond timescales.

Beyond extending the reach of laboratory X-ray sources, the discovery could turn high-harmonic generation into a sensitive probe of many-electron physics. The position and shape of the second plateau, its energy cutoff, and its dependence on the polarization of the driving laser can all reveal how electrons influence one another during an ultrafast collision. Such information is difficult to obtain with conventional spectroscopy because electron correlation unfolds too quickly and involves several quantum pathways at once. The new method effectively converts the atom’s response into a fingerprint of its internal dynamics.

The researchers believe the principle could eventually be applied to molecules and strongly correlated solids, where coordinated electron motion governs chemical reactions, magnetism, superconductivity, and the behavior of advanced electronic materials. In the longer term, the ability to generate and control correlated X-ray bursts could help scientists study quantum many-body processes with unprecedented temporal resolution. High-harmonic generation, once celebrated primarily as a route to attosecond pulses, may now become something even more powerful: a detector of how multiple electrons move, interact, and exchange energy in real time.

Subject of Research: Correlated-electron dynamics and high-harmonic generation beyond the single-electron energy cutoff

Article Title: Correlated electrons extend X-ray high-harmonic generation beyond the single-electron limit

News Publication Date: 7-Aug-2026

Web References: https://doi.org/10.1038/s41566-026-01976-2

References: Nature Photonics, “Correlated electrons extend X-ray high-harmonic generation beyond the single-electron limit”

Keywords

High-harmonic generation, X-ray pulses, helium atoms, electron correlation, attosecond physics, ultrafast science, quantum dynamics, coherent radiation, laser physics, high-energy photons