Figure 1
Differential phase measurement using XUV-XUV interferometry. (a) Experimental scheme. A delay controlled APT (black) and its generating annular IR pulse (red) are focused into a gas target, where a second phase-locked and independent APT (purple) is generated by the same IR pulse. The interference signal between the two APTs as a function of delay is spectrally resolved and recorded in the far-field for a range of IR intensities, controlled by a motorized iris. (b) Fourier amplitudes of an interference signal extracted from a delay scan where argon is used in both sources. Harmonics 11 to 23 oscillate at their fundamental frequency. (c) Schematic description of the phase accumulated during the HHG process. The phase of the emitted XUV photon encodes all the steps of the HHG interaction: strong-field tunneling and acceleration \(\phi ^{\rm {SF}}\) and the dipole transition phase \(\phi ^{\rm {D}}\) at recollision. In addition, the electron accumulates phase \(\phi ^{\rm {C}}\) as it interacts with the ionic Coulomb potential. Changing the IR intensity leads to a variation of the electron trajectory, as indicated by the red and blue lines for higher and lower intensity, respectively. As a result, both \(\phi ^{\rm {SF}}\) and \(\phi ^{\rm {C}}\) are modified, while \(\phi ^{\rm {D}}\) remains unchanged.
