Phase delays in ω − 2 ω above-threshold ionization

Abstract

Relative phases of atomic above-threshold ionization wave packets were investigated in an experiment [L. J. Zipp, A. Natan, and P. H. Bucksbaum, Optica 1, 361 (2014)] exploiting interferences between different pathways in a weak probe field at half the frequency of the strong ionization pulse. In this work we explore theoretically the extraction of phase delays and time delays of attosecond wave packets formed in strong-field ionization. We perform simulations solving the time-dependent Schrödinger equation and compare these results with the strong-field and Coulomb-Volkov approximations. In order to disentangle short- from long-range effects of the atomic potential, we also perform simulations for atomic model potentials featuring a Yukawa-type short-range potential. We find significant deviations of the ab initio phase delays between different photoelectron pathways from the predictions by the strong-field approximation even at energies well above the ionization threshold. We identify similarities to but also profound differences from the well-known interferometric extraction of phase and time delays in one-photon ionization.