Photoionization-pumped gain at 185 nm in a laser-ablated indium plasma

We report what is to our knowledge the first production of gain by inner-shell photoionization of ions. Photoionization of 4d electrons from In+ ions created a population inversion on a 185-nm In2+ transition. This scheme is an isoelectronic scaling of the 442-nm Cd+ laser. A single-pass gain constant of 0.9 ± 0.3 cm−1 over a 0.4-cm path was pumped by soft x rays generated by less than 50 mJ of 1.06-μm light focused onto a tantalum target. Laser ablation of a liquid-indium target provided a high density (1016 cm−3) of ground-state In+ ions for this scheme. An indium amplified spontaneous emission laser and a synchronously pumped indium laser oscillator are proposed.