Absorption of Attosecond Pulses by Laser-dressed Atoms

We study the transient absorption of attosecond pulses by IR-laser-dressed He atoms using both single-atom and macroscopic methods [1]. In the case of an attosecond pulse train, we report for the first time a quarter-cycle modulation (mixed with the well-known half-cycle modulation [2-4]) in the absorption as a function of time delay, indicating that high-order couplings between the harmonics can be obtained by modifying parameters of laser and gas medium. We also find that the absorption probability is tied to resonant laser-dressed atomic states, and the timing of absorption is sensitive to laser parameters and reshaping of the attosecond pulses. In the case of a single attoseond pulse, we exhibit the attosecond time-scale evolution of the absorption probability as well as that of the AC Stark shift [5]. We find a light-induced state formed by a resonant two-photon absorption process. We also find electron wavepacket interference between two quantum path ways into the continuum (direct and via bound states) [6].\\[4pt] [1] M.B.Gaarde et al Phys.Rev.A 83 013419\\[0pt] [2] P.Johnsson et al Phys.Rev.Lett. 99 233001\\[0pt] [3] P.Ranitovic et al Phys.Rev.Lett. 106 193008\\[0pt] [4] M.Holler et al Phys.Rev.Lett. 106 123601\\[0pt] [5] A.Wirth et al Science 334 195\\[0pt] [6] J.Mauritsson et al Phys. Rev. Lett. 105 053001.