We generate two attosecond pulse trains ($\mathrm{XU}{\mathrm{V}}_{\omega }$ and $\mathrm{XU}{\mathrm{V}}_{2\omega }$) by focusing an infrared (IR, $\omega$) laser pulse and its temporally advanced second harmonic field ($2\omega$) 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, $\mathrm{XU}{\mathrm{V}}_{2\omega }$, $\mathrm{XU}{\mathrm{V}}_{\omega }$, and IR, are independently controlled. First, at a fixed delay between the $\mathrm{XU}{\mathrm{V}}_{2\omega }$ 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 $1s\mathrm{and}4p$ states of helium, 174 as. This indicates that the $4p$ Rydberg state is populated by harmonic 15 ($15\omega$) in both $\mathrm{XU}{\mathrm{V}}_{2\omega }$ and $\mathrm{XU}{\mathrm{V}}_{\omega }$ pulses. Second, we overlap $\mathrm{XU}{\mathrm{V}}_{2\omega }$ 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 $\omega$) from $\mathrm{XU}{\mathrm{V}}_{2\omega }$ 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.

Close

PrevNext

View figure in article

• Coherent control
• Multiphoton or tunneling ionization & excitation
• Ultrafast phenomena