Interferometric measurement of the atomic dipole phase for the two electronic quantum paths generating high-order harmonics

The first direct measurement of the atomic dipole phase in high-order harmonic generation obtained by an interferometric technique in the extreme ultraviolet (XUV) is reported. The effects associated with both the main quantum paths involved in the emission of a given plateau harmonic are investigated and compared with the expectations of the semi-classical model, which offers a very simple physical picture for the understanding of the involved electronic dynamics.