Heterodyne Interferometry Using High-Order Harmonic Generation in Mixed Gases

Abstract

We describe a new and simple method to measure harmonic phases by using the interference of harmonics in mixed gases both experimentally and theoretically. First, we develop a fundamental theory of HHG in mixed gases by combining the propagation equation of high-order harmonics and the Lewenstein model, which describes the single atom (molecule) response. The harmonic phase can be obtained by comparing the harmonic spectra from the two gases and their mixture. Then, we present the first experimental evidence of the destructive and constructive interference in a mixed gas of He and Ne, which prove the validity of the method and facilitate the coherent control of HHG. Finally, we applied this method to the attosecond physics. By measuring the relative phase of high-order harmonics generated in each molecule, attosecond dynamics of nuclear wavepackets in H2 and D2 molecules can be probed. The measured relative phase as well as intensity ratio are successfully reproduced by applying the Feynman’s path integral method fully to the dynamics of the nuclei and electrons in the molecules.