physics

08/08/2026

Two attosecond flashes capture electrons in motion

We have developed a table-top technique in which attosecond pulses both initiate and probe electronic motion. Using all-attosecond transient absorption spectroscopy, we resolved the oscillatory motion of an electron vacancy (an electronic “hole”) in xenon ions with a period of about three femtoseconds. The results have now been published in Nature Communications.

Electronic motion sets the stage for virtually every light-induced process in nature, from the first step of a chemical reaction to the flow of charge in a solid. Yet these processes unfold so rapidly that they can only be observed with flashes of light lasting a few hundred attoseconds – billionths of a billionth of a second. Most previous attosecond experiments combined an attosecond pulse in the extreme ultraviolet (XUV) with a longer and often intense near-infrared pulse. Such fields can disturb the system under investigation and obscure its intrinsic electronic response. In the new approach, both the pump and the probe are attosecond extreme-ultraviolet pulses, providing a potentially much cleaner view of the underlying dynamics.

We generated pulses with a duration of approximately 270 attoseconds using a laboratory-based high-harmonic source. A precision split-and-delay system produced two pulse replicas with an independently adjustable delay. The first pulse removed an electron from a xenon atom and created the ion in a coherent superposition of two closely spaced electronic states. The resulting electron vacancy, or “hole”, did not remain stationary but oscillated periodically within the ion.

The second attosecond pulse recorded this motion through changes in the extreme-ultraviolet absorption spectrum. The observed period of about three femtoseconds is consistent with the beating expected from the energy separation of the two spin-orbit states. “Using attosecond pulses in both steps allows us to initiate electronic motion and observe it without applying an additional strong infrared field,” explains Bernd Schütte, who led the study. “This brings us closer to watching electronic processes as they unfold, without significantly perturbing them.”

The experiment marks an important step toward a new generation of attosecond spectroscopy that can be carried out using table-top light sources. By combining extreme temporal resolution with the ability to distinguish individual electronic states, the method could make it possible to follow how charge moves through molecules, how electronic energy is redistributed, and how the first decisive steps of a photochemical reaction emerge from the coupled motion of electrons and nuclei.

The potential impact extends far beyond atoms and molecules. Applied to solids, the technique could reveal how electronic screening, scattering, carrier multiplication, exciton formation and charge transfer across interfaces develop on attosecond to femtosecond timescales. Such measurements could provide a new microscopic view of the processes that determine the performance of semiconductors, quantum materials, two-dimensional systems and future optoelectronic devices. In the long term, all-attosecond spectroscopy may offer a route towards observing – and ultimately controlling – the fastest electronic processes that govern how matter responds to light.

Fig. 1 | Observation of hole motion in Xe⁺ ions. The upper panel shows the delay-dependent change in XUV absorption; the arrow marks an oscillatory feature near 16 eV. The lower panel shows a lineout of this feature, revealing a period of about 3 fs.

04/11/2025

Attosecond plasma lens

Together with our colleagues from DESY we have developed a plasma lens capable of focusing attosecond pulses. This breakthrough substantially increases the attosecond power available for experiments, opening up new opportunities for studying ultrafast electron dynamics. The results have now been published in Nature Photonics.

Attosecond pulses—bursts of light lasting only billionths of a billionth of a second—are essential tools for observing and controlling electronic motion in atoms, molecules, and solids. However, focusing these pulses, which lie in the extreme-ultraviolet (XUV) or X-ray region of the electromagnetic spectrum, has proven highly challenging due to the lack of suitable optics. Mirrors are commonly used, but they offer low reflectivity and degrade quickly. Lenses, though the most straightforward tool for focusing visible light, are not suitable for focusing attosecond pulses, because they absorb the XUV light and stretch the attosecond pulses in time.

Researchers at MBI and DESY have solved this problem by generating a plasma lens. To create it, they send strong electrical pulses through hydrogen gas inside a tiny tube (see Fig. 1). This process strips the hydrogen atoms of their electrons, creating a plasma. The electrons naturally move outward toward the edges of the tube, shaping the plasma like a concave lens. Normally, such a lens would spread light out rather than focus it. But because plasma bends light differently than ordinary materials, it instead focuses the attosecond pulses.

In their Nature Photonics publication, the researchers showed that the plasma lens can focus attosecond pulses across different ranges of XUV light, with a tunable focal length controlled by the plasma density. They also achieved a high transmission rate of more than 80%. Importantly, the team found that the plasma lens serves as an effective filter for the infrared driving pulses, which normally require thin metal filters. This means those filters are no longer necessary, allowing more attosecond power to pass through. With stronger pulses now available, scientists have new opportunities to run attosecond experiments that are often limited by weak light sources.

To better understand how the focused attosecond pulses behave over time, the scientists ran computer simulations. They discovered that the pulses stretch only slightly, from 90 to 96 attoseconds. Under more realistic conditions—where different colors of the attosecond pulse arrive at slightly different times (a phenomenon known as chirp)—the plasma lens actually shortened the pulses. In this case, the pulse duration decreased from 189 to 165 attoseconds.

By experimentally demonstrating an attosecond plasma lens, the researchers have addressed a major limitation in attosecond science. The technique offers simple alignment, high transmission, and the ability to focus light across different colors. These advantages open the door to a wide range of applications, from mapping electron dynamics in complex materials to advancing quantum technologies and enabling the next generation of ultrafast microscopy.

Publication: E. Svirplys, Nat. Photon. 20, 151 (2026)

Fig. 1: a, An attosecond pulse enters a capillary, where a strong electrical pulse generates a hydrogen plasma. As the electrons move toward the capillary walls, they form a concave lens that focuses the attosecond pulse. b, The attosecond pulses are focused in the presence of plasma.