  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.
