High-energy stimulated emission from plasma ablation pumped by resonant high-order harmonic generation
In order to explain the recently observed strong intensity enhancement of a single high-order harmonic we analyze high-order harmonic generation (HHG) from low-ionized laser plasma ablation in the presence of a strong radiative transition in plasma ions. If an integer multiple of the laser photon energy is resonant with this transition, a coherent superposition of ground and excited states having different parity is formed by pumping the excited state by HHG radiation. Resonant HHG from this superposition of states produces strong radiation at the plasma ions' transition frequency. The population of the excited state is increased and the stimulated emission becomes significant. The described physical picture of this process is illustrated by numerical examples related to the recent experiments.