High-order harmonic generation in an x-ray range from laser-induced multivalent ions of noble gas

Tabletop coherent x-ray sources extending to multi-keV or higher photon energies have versatile applications, including in 4D imaging and semiconductor detectors. However, these sources can be realized only via high-order harmonic generation (HHG) with an ∼ 10 µ m laser interacting with neutral atoms. As shown in previous work by Popmintchev et al. [Science 350, 1225 (2015)10.1126/science.aac9755SCIEAS0036-8075], multiply ionized plasmas can efficiently produce hundred-eV harmonics with an ultraviolet laser. Here, we experimentally investigate multi-keV x-ray sources up to ∼ 5.2 k e V , the highest photon energy generated via HHG to date, to our knowledge, using a 1.45-µm driving laser that interacts with multivalent ions. Both the angular distribution and the ellipticity dependence of the signal are strong evidence of the HHG mechanism.