Attosecond wave-packet interferometry using two-color XUV pulses

We generate two attosecond pulse trains ($\mathrm{XU}{\mathrm{V}}_{\ensuremath{\omega}}$ and $\mathrm{XU}{\mathrm{V}}_{2\ensuremath{\omega}}$) by focusing an infrared (IR, $\ensuremath{\omega}$) laser pulse and its temporally advanced second harmonic field ($2\ensuremath{\omega}$) into an argon gas jet. Using the two XUV pulses and another infrared pulse, we perform two types of experiments for electron wave-packet interferences with attosecond time resolution. The delays between three pulses, $\mathrm{XU}{\mathrm{V}}_{2\ensuremath{\omega}}$, $\mathrm{XU}{\mathrm{V}}_{\ensuremath{\omega}}$, and IR, are independently controlled. First, at a fixed delay between the $\mathrm{XU}{\mathrm{V}}_{2\ensuremath{\omega}}$ and the IR pulses, we record the velocity map images of photoelectrons ionized from helium as a function the two XUV pulses. The photoelectron signal intensity is modulated with the period corresponding to the energy separation between the $1s\phantom{\rule{0.28em}{0ex}}\mathrm{and}\phantom{\rule{0.28em}{0ex}}4p$ states of helium, 174 as. This indicates that the $4p$ Rydberg state is populated by harmonic 15 ($15\ensuremath{\omega}$) in both $\mathrm{XU}{\mathrm{V}}_{2\ensuremath{\omega}}$ and $\mathrm{XU}{\mathrm{V}}_{\ensuremath{\omega}}$ pulses. Second, we overlap $\mathrm{XU}{\mathrm{V}}_{2\ensuremath{\omega}}$ with the IR pulse in time and record the VMI images as a function of the two pulses. By analyzing the interference pattern in the VMI images, we find that harmonic 13 (13 $\ensuremath{\omega}$) from $\mathrm{XU}{\mathrm{V}}_{2\ensuremath{\omega}}$ is responsible for the generation of the $g$ wave in the ionization continuum. Our method of using two XUV pulses containing different sets of harmonics can simplify attosecond XUV interferometry experiments.