# X-ray parametric down-conversion reveals EUV-polariton

## TL;DR

The paper shows that X-ray parametric down-conversion can reveal polaritons in the extreme ultraviolet, opening new ways to study light-matter interactions at short wavelengths.

## Contribution

The study experimentally resolves X-ray parametric down-conversion and identifies a polariton in the EUV regime.

## Key findings

- X-ray parametric down-conversion's signal cone was fully resolved.
- Evidence of an EUV-polariton was confirmed through theoretical simulations.
- The anti-crossing of polaritonic dispersion branches was directly imaged.

## Abstract

Spontaneous parametric down-conversion (PDC) of photons is a gateway into the quantum realm – thoroughly studied in nonlinear optics and ubiquitously used to generate non-classical states of light. Extending PDC from the visible regime towards shorter wavelengths further enables microscopic resolution of electronic structure and quantum-enhanced X-ray detection, but remained challenging due to the process’ inherently low conversion rate. Here, we resolve the full signal cone of non-degenerate down-conversion at X-ray wavelengths and identify imprints of a polariton in the extreme ultraviolet (EUV) regime. We confirm our finding of the EUV-polariton with theoretical simulations and establish that our approach directly images the characteristic anti-crossing of polaritonic dispersion branches. This insight could open a pathway to explore strong-coupling phenomena of EUV-light-matter interaction.

Polaritons and spontaneous parametric down conversion represent two hallmarks of the quantum nature of light-matter interaction which are poorly understood in the X-ray regime. Here, the authors experimentally resolve the signal cone of X-ray parametric down conversion, providing evidence of polariton formation at extreme ultraviolet wavelengths.