Atomic Two-Color XUV Interferometer

We extend our recently published work which demonstrated the coherent control of population of $2s^{2}\ {{}^{1}S}$ doubly excited state in helium by tuning the interference of $\omega_{1}+\omega_{1}$ and $\omega_{3}-\omega_{1}$ two-photon excitation paths [1]. The maximum yield of electrons from $2s^{2}$ autoionization was observed when the two-color phase difference matched phase difference of the atomic amplitudes describing the two alternative excitation paths. A displacement of position of the maximum yield in the same reference frame therefore signals the presence of an additional phase shifting agent along any of the two paths and also provides a measure of the corresponding phase shift. This constitutes the operational principle of an atomic XUV interferometer which is comparable to the well-known RABBITT method based on using a combination of XUV and IR light pulses [2]. The work was performed at LDM beamline at the free-electron-laser facility FERMI in Trieste (Italy). The phase difference of the two components of the light pulse was set by slightly delaying the $\omega_{3}$ emission from the last three undulators with respect to the $\omega_{1}$ emission produced by the first three undulators and this was achieved by delaying the generating electron bunch by properly adjusted magnetic chicane in between the two undulator sections.