Non-linear optics meets X-rays

The recent advent of femtosecond X-ray sources offers unprecedented opportunities for structural and dynamical studies. It requires, however, manipulating spectral properties, as commonly done by non-linear optics at visible/infrared wavelengths. Here we show the first evidence for Self-Phase Modulation, a key non-linear effect in ultrafast laser science, in the soft X-Rays. Building on such effect, we demonstrate how to tune spectral properties in this wavelength region critical for core electrons pump-probe spectroscopy and nanoimaging.