Simultaneous lasing of Ni Kα and Cu Kα lasers from an alloy foil irradiated with an intense X-ray free-electron laser pulse

We report the simultaneous lasing of two distinct Kα emissions at photon energies of 7.48 keV (Ni Kα1) and 8.05 keV (Cu Kα1). This was achieved by a population inversion induced through intense X‐ray free‐electron laser (XFEL) irradiation of a Cu–Ni alloy foil. This demonstration of multi‐color X‐ray lasing using a single XFEL source is expected to contribute significantly to the future development of X‐ray lasers and their applications.