We investigate the role of excited states in high-order harmonic generation by studying the spectral, spatial, and temporal characteristics of the radiation produced near the ionization threshold of argon by few-cycle laser pulses. We show that the population of excited states can lead either to direct extreme ultraviolet emission through free induction decay or to the generation of high-order harmonics through ionization from these states and recombination to the ground state. By using the attosecond lighthouse technique, we demonstrate that the high-harmonic emission from excited states is temporally delayed by a few femtoseconds compared to the usual harmonics, leading to a strong nonadiabatic spectral redshift.
Publication Source (Journal or Book title)
Physical Review Letters
Beaulieu, S., Camp, S., Descamps, D., Comby, A., Wanie, V., Petit, S., Légaré, F., Schafer, K., Gaarde, M., Catoire, F., & Mairesse, Y. (2016). Role of excited states in high-order harmonic generation. Physical Review Letters, 117 (20) https://doi.org/10.1103/PhysRevLett.117.203001