Probing nonperturbative characteristics in the retrieval of resonant photoionization time delay by a bicircular two-color interferometry

We theoretically propose a bicircular two-color interferometry to retrieve the resonant photoionization time delay (RPTD) of atomic hydrogen, which can be described by the energy derivative of amplitude phase difference between single- and two-photon ionization pathways. Our results reveal that the RPTD can be well retrieved from the maximal asymmetry of photoelectron momentum distributions in the perturbative and nonperturbative regimes. In the perturbative regime, the RPTD can be characterized by perturbation theory, which demonstrates a decreasing trend with the increasing energy in counter-rotating pulse configurations, while in the co-rotating case, which exhibits pronounced oscillatory characteristics. These oscillations originate from quantum interference between photoelectron wave packets generated through direct single-photon ionization processes and those mediated by resonant excitation pathways caused by the broad spectral bandwidth of ultrashort laser pulses. In the nonperturbative regime, we develop the strong-field nonperturbation theory (SFNPT) by taking the population depletion and Stark energy shift of the ground state into account, which shows a good agreement with the numerical solution of the time-dependent Schrödinger equation. The RPTD shows a smoothly increasing characteristic as photoelectron energy increases for the counter-rotating and co-rotating cases, which differs remarkably from the case in the perturbative regime. Our SFNPT model unveils that this abnormal phenomenon can be attributed to the population depletion of the ground state. Our proposed bicircular two-color interferometry approach provides an avenue for exploring the resonant time delays in atomic photoionization and observing their nonperturbative characteristics.