Splitting of x-rays creates new polaritons

Upon encountering matter, photons begin to interact – often being absorbed and re-emitted by electrons en route through a material. This interaction generates a hybrid particle known as a polariton, a part-light, part-matter excitation that exists only as long as the photon and electron are intertwined. That means polaritons are difficult to detect, because only their faint signatures remain imprinted on the photons once they leave the material behind.

To strengthen the light–matter interaction and thereby enhance these polariton signatures, scientists usually have to craft finely mirrored cavities, which act like photon echo chambers. This works well for visible and infrared photons, but fabricating cavities small enough for ultraviolet or X-ray photons is notoriously tricky.

Now, a collaboration between scientists at the ESRF, the German synchrotron DESY, the University of Hamburg and the University of Helsinki has used a completely different approach to detect the signature of an EUV polariton unambiguously for the first time.  “A few groups have seen indications previously, but this here is the first smoking-gun type of evidence,” says Christoph Sahle, the scientist in charge of the ESRF’s ID20 beamline.

The experimental team opted to generate polaritons from X-ray photons in a bulk piece of pure diamond. This means there was no tailor-made cavity – they didn’t need one. Instead, they relied on another light–matter phenomenon known as parametric down-conversion (PDC), in which an incoming photon splits spontaneously into two new photons of lower energy – here, one in the X-ray regime, and one in the extreme ultraviolet (EUV). The trick of X-ray PDC means that the X-ray photon leaving the diamond acts as a messenger, carrying the information about the EUV polariton which would otherwise have been lost.

Next, the researchers needed to map in high resolution the relationship between the energy and momentum of the emerging X-ray light leaving the diamond. Using ID20’s X-ray Raman spectrometer in a novel detection mode, they were able to resolve a two-fold X-ray modulation mirroring the behaviour of the EUV partner photon – the tell-tale imprint of polaritons at work.

“ID20 was the perfect beamline to detect the EUV polariton,” says DESY’s Christina Boemer, the leader of the experimental team. “It has world-leading capabilities when it comes to instrumentation for high-resolution scattering, and still offered all the flexibility we needed to explore our new detection scheme. It was a team effort, and without the exceptional support from Christoph and his colleagues at ID20, this discovery couldn’t have been made.”

The ESRF experiment brings polariton physics into the EUV domain. Scientists will be keen to explore this new quantum playground, and find out whether light interacts with matter in different ways to how it does at lower energies. There may be practical applications, too. EUV polaritons could act as deep probes of the electronic and structural properties of materials. Further ahead, they could even be used to modify structures at nanometer scales for ultra-precise manufacturing or EUV-lithography.

“In the future, polariton X-ray PDC could become a valuable tool to study the interaction of light and matter in materials that are otherwise opaque or difficult to probe,” says DESY’s Dietrich Krebs, the first author of the study. “To help this, we’ll be developing a more comprehensive theoretical model as well.”

Reference:

Krebs, D., et al. Nat Commun 16, 5383 (2025). https://doi.org/10.1038/s41467-025-60845-8

Text by Jon Cartwright