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Phase and time delays of atomic above-threshold ionization were recently experimentally explored in an $ω-2ω$ setting [Zipp et al, Optica 1, 361 (2014)]. The phases of wavepackets ejected from argon by a strong $2ω$ pulse were probed as a function of the relative phase of a weaker $ω$ probe pulse. Numerical simulations solving the time-dependent Schrödinger equation (TDSE) displayed a sensitive dependence of the doubly differential momentum distribution on the relative phase between the $ω$ and $2ω$ fields. Moreover, a surprisingly strong variation of the extracted phase delays on the intensity of the probe pulse was found. We present a semiclassical strong-field description of the phase delays in the emission of electrons in an $ω-2ω$ setting and apply it to atomic hydrogen. Non-perturbative effects in both the $2ω$ pump and the $ω$ probe field are included. The semiclassical description allows tracing phase delays to path interferences between emission during different points in time of emission within the temporal unit cell of the two-color laser field. We find good agreement between the semiclassical saddle-point approximation, the full strong field approximation (SFA), and previous results applicable in the perturbative limit of probe fields. We show that 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 {$δ_i(E)$} with $i=1,2,\ldots$ the difference in photon numbers of the strong $2ω$ field involved in the interfering paths. Comparison between SFA and TDSE calculations reveals the influence of the Coulomb field even in this strong-field scenario., 32 pages, 12 figures

We analyze interference processes in atomic ionization induced by a two-color laser with fundamental frequency $\omega $ and its second harmonic $2\omega $. The interplay between inter- and intracycle interference processes give rise to multiphoton peaks which can be named as main or ATI peaks and sidebands, in analogy to the well-known RABBIT (reconstruction of attosecond harmonic beating by interference of two-photon transitions). We use the saddle point approximation (SPA) to extract the complex ionization times of the interfering electron trajectories. Changing the relative phase between the two colors, the doubly differential momentum distribution of emitted electrons can be controlled. We study the dependence of the electron emission as a function of the relative phase between the $\omega $ and $% 2\omega $ fields within the strong field approximation (SFA) but beyond the perturbative regime. We focus on the extraction of the phase delays accounting the electron forward emission in the direction of the polarized electric fields. We characterize the time delays in the emission of electrons for visible frequency of the pump and its first harmonic as a probe [Ti:Sapphire laser ($800$ nm) together with the first harmonic ($400$ nm)] for a typical $\omega -2\omega $ configuration for argon ionization. We find excellent agreement between our SPA results and the corresponding SFA (without any further approximation) and also with previous perturbative theories., Comment: 14 pages, 15 figures

We present a theoretical study of atomic laser-assisted photoionization emission (LAPE). We consider an atom driven by a linearly polarized XUV laser in two different scenarios: i) a single attosecond pulse (in both the streaking and sideband regimes) and ii) an attosecond pulse train. The process takes place assisted by a linearly polarized infrared (IR) laser field. In all these cases the energy and angle-resolved photoelectron spectrum (PES) is determined by a leading contribution, related to the intracycle factor [Gramajo et al., J. Phys. B 51, 055603 (2018)], complemented by other ones, derived from the periodicity and symmetry properties of the dipole transition matrix with respect to the IR field. Each of these terms imprint particular features in the PES that can be straightforwardly understood in terms of generalized energy conservation laws. We investigate in detail these PES structures, in particular, for the case of argon initially in the 3s quantum state. Our theoretical scheme, based on the strong-field approximation (SFA), can be applied, however, to other atomic species and field configurations as well., Comment: 10 pages, 8 figures

We report on a combined experimental and theoretical study of XUV ionization of atomic argon in the presence of a near-infrared laser field. The resulting energy- and angle- resolved photoemission spectra have been described in the literature as interferences among different photoionization trajectories. Electron trajectories stemming from different optical laser cycles give rise to \textit{intercycle} interference energy and result in new peaks known as sidebands. These sidebands are modulated by a coarse grained (gross) structure coming from the \textit{intracycle} interference of the electron trajectories born during the same optical cycle. We calculate the photoelectron emission by solving the time-dependent Schr\"{o}dinger equation \textit{ab initio} and within the continuum-distorted wave strong field approximation. In order to compare with the experimental data we average the calculated energy-angle probability distributions over the experimental focal volume. This averaging procedure washes out the intracycle interference pattern, which we experimentally and theoretically (numerically) recover by subtracting two averaged distributions for slightly different near-infrared laser field intensities., Comment: 9 pages, 4 figures, work presented as an invited talk of The International Conference on Many Particle Spectroscopy of Atoms, Molecules, Clusters and Surfaces, 22 August 2018, Budapest, Hungary