Third-harmonic generation in a laser-pre-excited gas: the role of excited-state neutrals

The generation of the third harmonic of 150-fs 4-mJ Ti:sapphire laser pulse s in the atmospheric air preliminary excited by 15-ns 100-mJ pulses of the second harmonic of a Nd:YAG laser is investigated. Preliminary laser excita tion of a gas is shown to considerably increase the efficiency of frequency tripling of femtosecond pulses. Characteristic times corresponding to the maximum intensity of third-harmonic generation are shown to substantially e xceed the characteristic plasma decay times, indicating the important role of excited-state neutrals in the enhancement of THG efficiency. (C) 2000 El sevier Science B.V. All rights reserved.