Scientists from the Max Born Institute Berlin (Germany), the Université catholique de Louvain (Belgium), the University of Uppsala (Sweden), Universität Kassel(Germany), and Elettra Sincrotone Trieste (Italy) have demonstrated a new way of controlling electrons using extreme ultraviolet (XUV) light, extending so-called "quantum control" concepts into a portion of the electromagnetic spectrum that has long remained beyond reach. The results are published in Nature Photonics.

Light is one of the most powerful tools scientists have for probing matter. By interacting with electrons, light reveals how atoms, molecules and materials are structured and how they evolve in time. Highly energetic ultraviolet and X-ray photons are particularly attractive because they can access electrons located close to atomic nuclei, where each chemical element carries its own distinctive electronic fingerprint.

While researchers can control electrons with visible and infrared lasers, extending these techniques to the XUV and X-ray domains has proven far more challenging. The international research team has now brought a new tool to that demanding region of the spectrum.

Making an electron go both ways... or not.

One of the strangest features of quantum mechanics is that a process can happen in more than one way at the same time. An electron can reach the same final state through different routes, and these routes can either add up or cancel each other, much like overlapping ripples on the surface of water.

The researchers exploited this phenomenon by creating two competing quantum routes to excite a bound state in He. As quantum routes they used the so-called stimulated Raman excitation with XUV pulses (Figure 1a).

"With the stimulated Raman excitation in the XUV it's a little bit as if we could eject an electron from an atom and then bring it back home with light. Remarkably, when we have two of these transition pathways we can decide whether that happens or not simply by introducing very small delays in the light we shine on it. In other words, by controlling the phase of the light pulses in one of the quantum routes, we can decide whether the two quantum pathways cooperate with each other or work against each other," says Matthieu Génévriez, one of the authors and researcher at UCLouvain. The outcome of the excitation varies with the phase of one of the XUV fields as shown in Fig. 1b.

For decades, scientists have relied on stimulated Raman scattering and related techniques using visible and infrared lasers to follow chemical reactions and electronic motion. "The new experiment demonstrates that these powerful methods can now be brought into the extreme ultraviolet, overcoming a long-standing challenge and opening new possibilities for imaging and manipulating matter with unprecedented spatial resolution", says Ulli Eichmann, the lead author and researcher at the Max Born Institute.

The experiment reported in Nature Photonics was carried out at the FERMI free-electron laser in Trieste, Italy, one of only a handful of facilities worldwide capable of producing the intense and highly controlled extreme-ultraviolet light required for this type of study.

"These facilities produce extremely bright and ultrashort flashes of XUV and X-ray light, allowing us to probe matter in ways that were previously impossible," explains Jan-Erik Rubensson, researcher at Uppsala University. "In some sense, they provide a snapshot of the motion of electrons and atomic nuclei, much like an X-ray image captures structures inside the human body.", completes Ulli Eichmann.

The international team believes that the concepts demonstrated in this work could eventually enable a new generation of element-specific imaging and spectroscopy tools, capable of tracking electronic motion and chemical transformations at the smallest scales.