Quantum interference in attosecond transient absorption of laser-dressed helium atoms

摘要

We calculate the transient absorption of an isolated attosecond pulse (IAP) by helium atoms subject to andelayed infrared (IR) laser pulse.With the central frequency of the broad attosecond spectrum near the ionizationnthreshold, the absorption spectrum is strongly modulated at the sub-IR-cycle level. Given that the absorptionnspectrum results from a time-integrated measurement, we investigate the extent to which the delay dependencenof the absorption yields information about the attosecond dynamics of the atom-field energy exchange. We findntwo configurations in which this is possible. The first involves multiphoton transitions between bound statesnthat result in interference between different excitation pathways. The second involves the modification of thenbound state absorption lines by the IR field, which we find can result in a subcycle time dependence only whennionization limits the duration of the strong-field interaction.