# Observation of strong nonlinear interactions in parametric down-conversion of x-rays into ultraviolet radiation

## TL;DR

This paper reports the discovery of unexpectedly strong nonlinear x-ray interactions in certain crystals, opening new avenues for material spectroscopy across a broad wavelength range.

## Contribution

It presents the first observation of significantly stronger nonlinearities in x-ray parametric down-conversion than previously recorded, challenging existing theories.

## Key findings

- Nonlinearities in x-ray PDC are about 10,000 times stronger than before.
- Observed nonlinear effects cannot be explained by current theories.
- Potential for developing new orbital and band selective spectroscopy methods.

## Abstract

Nonlinear interactions between x-rays and long wavelengths can be used as a powerful atomic scale probe for light-matter interactions and for properties of valence electrons. This probe can provide novel microscopic information in solids that existing methods cannot reveal, hence to advance the understanding of many phenomena in condensed matter physics. However, thus far, reported x-ray nonlinear effects were very small and their observations required tremendous efforts. Here we report the observation of unexpected strong nonlinearities in parametric down-conversion (PDC) of x-rays to long wavelengths in gallium arsenide (GaAs) and in lithium niobate (LiNbO3) crystals, with efficiencies that are about 4 orders of magnitude stronger than the efficiencies measured in any material studied before. These strong nonlinearities cannot be explained by any known theory and indicate on possibilities for the development of a new spectroscopy method that is orbital and band selective. In this work we demonstrate the ability to use PDC of x-rays to investigate the spectral response of materials in a very broad range of wavelengths from the infrared regime to the soft x-ray regime.

---
Source: https://tomesphere.com/paper/1904.13146