Abstract

Inner-shell photoionization of In⁺ ground state ions by a laser-produced-plasma broad-band soft X-ray pump source has created a large population inversion on the 185 nanometer 4d⁹ 5s^{2 2}D_5/2 → 4d¹⁰ 5p ²P_3/2 transition in In⁺⁺. A small signal intensity gain of exp(0.5) was measured in a path length of ~4 millimeters, yielding a gain coefficient of a ≈ 1.25 cm^-1. The initial population of ~10¹⁶ cm^-3 In⁺ ground state ions is prepared by ablating a liquid indium target with a 10 nanosecond, 100 millijoule, 532 nanometer laser pulse yielding a highly ionized indium plasma. The expanding plasma cools and recombines to give the required In⁺ 4d¹⁰ 5s² population, at which time a 100 picosecond, 40 millijoule, 1.06 mm laser pulse, incident on a nearby tantalum target, generates a hot plasma radiating broad-band soft X-rays. The soft X-rays photoionize inner shell 4d electrons from the In⁺ ions, populating the 4d⁹ 5s² In⁺⁺ state, which is inverted with respect to the nearly empty 4d¹⁰ 5p state.

© 1986 Optical Society of America