Soft-x-ray amplification of the Lyman-α transition by optical-field-induced ionization

We report the first observation of amplification of spontaneous emission on the Lyman-α transition of hydrogenlike lithium ions by optical-field-induced ionization. Using a subpicosecond KrF laser (0.5 ps, 50 mJ) focused at ${10}^{17}$ W/${\mathrm{cm}}^2$, singly ionized lithium ions have been further ionized to fully stripped states, resulting in the population inversion with respect to the ground state of the hydrogenic ions. A small signal gain coefficient of 20 ${\mathrm{cm}}^{\mathrm{-}1}$ was obtained.