Characterization of the two-photon transition phase in the buildup process of Rabi oscillations

We achieve the complete characterization of the transition phase in the resonant two-photon ionization (TPI) of hydrogen at the high laser intensity where Rabi oscillation occurs. Different from previous studies which focused on the transition phase of resonant TPI in the perturbation region, in our work, we reveal the effect of Rabi oscillations on the transition phase in the high-intensity region. With bichromatic extreme-ultraviolet light, the transition phases of the S and D partial waves are decoded. By solving the time-dependent Schrodinger equation, we show that the transition phase depends on not only the pulse duration but also the laser intensity. This dependence is traced back to the modification of the atomic state structure, rather than the competition between resonant and nonresonant ionization paths at the lower laser intensity. Our work provides a way to manipulate the two-photon transition phase through laser intensity.