Scientists Spot Strange New Quasiparticles Using Powerful X-Ray Technique

A team of scientists from DESY, along with collaborators from Finland and France, has made a groundbreaking discovery by detecting signs of polaritons—strange hybrid particles formed by the interaction of light and matter—at extreme-ultraviolet (EUV) wavelengths for the first time.

Polaritons are usually seen under conditions of strong coupling between light and material excitations. But in this case, the researchers took a different approach. Instead of relying on strong coupling, they used a much weaker process known as nonlinear x-ray scattering, capturing the data with extremely high resolution. This allowed them to uncover the clear signature of an EUV polariton—something never seen before.

The experiments were carried out at the European Synchrotron Radiation Facility, and the results mark the beginning of a new research frontier that connects quantum physics, nonlinear optics, solid-state science, and x-ray technology. The study has been published in Nature Communications.

When photons—the tiny quantum particles that make up light—travel through a material, they can do more than just pass through. They interact with the material’s internal energy states, and sometimes, this interaction leads to something remarkable: a merging of light and matter. For example, a photon might be absorbed by an electron, exciting it. If that electron then releases the energy as another photon, the two together can form a new, hybrid state known as a polariton—part light, part matter.

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Scientists have already observed polaritons in many materials and under a range of conditions. However, these have mostly involved low-energy photons, like those found in infrared and visible light. In those cases, researchers often use resonant cavities to strengthen the interaction and make the polaritons easier to detect.

But creating polaritons using high-energy photons—such as those in the extreme-ultraviolet or x-ray range—has long been a challenge. These high-energy polaritons have remained out of reach.

The team led by DESY scientist and CUI Young Investigator Group Leader Christina Bömer has taken a bold, unconventional route to uncover high-energy polaritons. Instead of relying on traditional methods, they turned to a subtle phenomenon known as nonlinear x-ray scattering—using it both to create and detect a high-energy extreme-ultraviolet (EUV) polariton in a single experiment.

Here’s how it works: When x-ray photons pass through a diamond crystal, they can split into tightly linked pairs of photons. One of these drops in energy to fall within the EUV range, where it interacts so strongly with the crystal’s electrons that it transforms into a hybrid state—a polariton, part light and part matter. The other photon retains its x-ray energy and acts as a kind of messenger, carrying information about the newly formed polariton.

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“The scattered light gives us a distinctive signal that clearly points to the presence of the polariton,” explains Xenia Brockmüller, a co-author of the study.

To detect these faint and fleeting signals, the researchers developed a new momentum-resolved technique at the ID20 beamline of the European Synchrotron Radiation Facility (ESRF). This advanced setup gave them an exceptionally detailed view of the nonlinear scattering process, opening the door to an entirely new way of studying light-matter interactions at high energies.

This first-ever observation opens the door to a range of exciting new research directions. “The EUV polariton has some fascinating properties that we’re eager to explore further, both in experiments and through theoretical work,” says Dietrich Krebs, DESY scientist and lead author of the study.

Understanding how light and matter combine at these high EUV photon energies could lead to the development of powerful new spectroscopy methods for studying solid materials. What’s more, the extremely short wavelength of the EUV polariton—around 10 nanometers—makes it ideally suited for examining semiconductor structures at the same tiny scale used in EUV lithography, a key technology in advanced chip manufacturing.