When certain atoms are irradiated with laser light, they can themselves produce a very different kind of laser light: laser pulses with extremely high frequencies in the X-ray range. These laser pulses, with which record-breaking results were achieved at TU Wien in the 1990s, were the subject of the 2023 Nobel Prize in Physics. However, the theoretical model of this effect makes a clear prediction: there is an upper limit to the energy, depending on the properties of the laser beam directed at the atoms. Above a certain value, known as the energy cutoff, hardly any X-rays are produced.
Now, however, a new experiment jointly performed by teams at TU Wien and University of California San Diego has succeeded in overcoming this textbook cutoff rule: using helium atoms, the researchers reached a much higher energy range than the standard theory would allow. The reason for this lies in the interaction between the two electrons in the helium atom: they can release their energy simultaneously. The effect has now been published in the journal Nature Photonics .
“The mechanism behind short X-ray pulses has been well known for a long time,” says Prof. Tenio Popmintchev from the Institute of Photonics at TU Wien. “The laser tears a single electron away from the atom. The electron is then accelerated in the laser’s electric field until it eventually collides with the atom again. The energy it loses in the process is emitted in the form of light.” This produces laser pulses with frequencies much higher than that of the laser originally directed at the atom.
Nobel laureate Ferenc Krausz used precisely this effect in the 1990s. Today, Tenio Popmintchev and his team conduct research at the same institute where Ferenc Krausz worked at the time—and they are now going significantly beyond what was possible then. “It can be shown that when the electron returns to its atom, it generates a whole range of different frequencies with roughly equal intensity,” says Dimitar Popmintchev, postdoc in Tenio Popmitchev’s team at TU Wien. “But there is an upper limit. Frequencies above this limit are much weaker; it is simply not possible to exceed a certain maximum frequency.”
But what happens when not just one electron is involved, but two? Tenio Popmintchev’s team used intense UV lasers and helium atoms, from which two electrons can be removed, one after the other. The first electron is released and accelerated, followed by the second. The two electrons are not independent of one another, however: they are quantum-mechanically correlated.
“Using UV driving pulses, we can arrange for both electrons to return to the atom at exactly the same time,” says Dimitar Popmintchev. “The energy of two electrons is then released all at once. And when more energy is available, a single higher-energy X-ray photon with higher frequency can also be generated.”
The team discovered these remarkably high X-ray frequencies experimentally. In the measured coherent spectrum, a second, weaker plateau appeared well above the previously known range, at substantially higher photon energies.
This was observed only in helium atoms – an atom with extremely strong electron correlations. The valance electrons of argon and neon atoms do not display this behaviour—further evidence that the special interplay between the two electrons in the helium atom is crucial.
This new “kind” of coherent X-ray radiation is particularly interesting as a new spectroscopic “sensor” of electron-electron correlations. Its shape, its energy limit and its dependence on the polarization of the laser can provide information about how two electrons interact on attosecond timescales. In the long term, the principle could also be transferred to molecules or strongly correlated solids. High-harmonic generation, which once paved the way for attosecond physics, is thus itself becoming a probe of quantum many-body processes, fundamentally important for quantum computing and design of function of advanced nanomaterials.
Nature Photonics
Correlated electrons extend X-ray high-harmonic generation beyond the single-electron limit
7-Aug-2026