Phase and time delays of atomic above-threshold ionization are usually experimentally explored by the reconstruction of attosecond harmonic beating by interference of two-photon transitions (RABBIT) technique. Theoretical studies of RABBIT rely on the perturbative treatment of the probe (near infrared or visible) laser pulse with respect to the atomic electric field and the pump composed of a train of attosecond pulses made of several harmonics with frequencies multiple of the probe fundamental frequency [1]. In this talk we present a nonperturbative description of the phase delays for the emission of electrons from rare gas atoms, where more than two photons are involved [2]. Different electron wave packets that produce the interferometric scheme are individualized. We observe different behaviors of the phase delays at different intensities of the probe: Whereas for moderate and intense probe fields the harmonics and sidebands happen to be in phase, when the probe field is sufficiently weak, the well-known rule of thumb for the phase delays developed within the perturbative RABBIT theory is recovered [1]. We show that the intracycle interference of the different paths contributing to the final energy (sideband or high harmonic) handles the different behaviors of the interference pattern [2]. On a different setup, the phases of wave packets ejected from argon by a strong 2ω pulse were experimentally probed as a function of the relative phase of a weaker ω probe pulse exploiting interferences between different pathways in a weak probe field at half the frequency of the strong ionization pulse [3]. In the talk we will introduce a semiclassical strong-field description of the phase delays in the emission of electrons in an ω − 2ω setting including nonperturbative effects in both the 2ω pump and the ω probe field. Significant deviations of the ab initio phase delays among different photoelectron pathways from the predictions by the strong-field approximation even at energies well above the ionization threshold will be shown [4]. The RABBIT-like perturbative description of phase delays breaks down for stronger fields and higher-energy electron emission. In this regime, characterization of the ionization signal requires an entire ensemble of phase delays δi (E) with i = 1, 2,... the difference in photon numbers of the strong 2ω field involved in the interfering paths [,5].
[1] D. Guénot et al., Phys. Rev. A 85, 053424 (2012). [2] S. D. López, M. L. Ocello, and D. G. Arbó, Phys. Rev. A 110, 013104 (2024). [3] L. J. Zipp, A. Natan, and P. H. Bucksbaum, Optica 1, 361 (2014). [4] D. G. Arbó, S. D. López, and J. Burgdörfer, Phys. Rev. A 106, 053101 (2022). [5] S. D. López et al., Phys. Rev. A 104, 043113 (2021).