We analyze the two-dimensional momentum distribution of electrons ionized by few-cycle laser pulses in the transition regime from multiphoton absorption to tunneling by solving the time-dependent Schrödinger equation and by a classical-trajectory Monte-Carlo simulation with tunneling (CTMC-T). We find a complex two-dimensional interference pattern that resembles above threshold ionization (ATI) rings at higher energies and displays Ramsauer-Townsend-type diffraction oscillations in the angular distribution near threshold. CTMC-T calculations provide a semiclassical explanation for the dominance of selected partial waves. While the present calculation pertains to hydrogen, we find surprising qualitative agreement with recent experimental data for rare gases [A. Rudenko et al., J. Phys. B 37, L407 (2004)].

We present a theoretical analysis of the atomic photoelectron emission spectra produced by a linearly polarized sculpted laser pulse of two colors with frequencies $$\omega${}$ and $2$\omega${}$. The spectrum of the ``direct'' electrons with intermediate energies prominently features both intracycle and intercycle interferences. We derive a simple analytic expression for this spectral range based on a semiclassical approximation of the time-dependent distorted wave strong-field approximation generalized to strong-field ionization by a two-color pulse. We verify its applicability to approximately represent the intricate interference patterns by comparison with the exact solutions of the time-dependent Schr\"odinger equation and with the strong-field approximation. We show that the interference patterns can be tuned and its contrast enhanced by the additional ``knob'' available, the relative phase between the two frequency components. The present results confirm that two-color ionization allows resolving interference structures originating from trajectories launched within a time interval of less than 100 as [X. Xie , Phys. Rev. Lett. 108, 193004 (2012)].

We present a joint experimental and theoretical study of ionization of argon atoms by a linearly polarized two-color laser field (${\lambda }_1=800$ nm, ${\lambda }_2=400$ nm). Changing the relative phase $\phi$ between the two colors, the forward-backward asymmetry of the doubly differential momentum distribution of emitted electrons can be controlled. We find excellent agreement between the measurements and the solution of the time-dependent Schrödinger equation in the single-active electron approximation. Surprisingly we also find good agreement between the quantum and classical calculations of electron momentum distributions generated by lasers at optical wavelengths.

We study the ionization of atomic hydrogen in the direction of polarization due to a linearly polarized XUV pulse in the presence of a strong IR laser. We describe the photoelectron spectra as an interference problem in the time domain. Electron trajectories stemming from different optical laser cycles give rise to intercycle interference energy peaks known as sidebands. These sidebands are modulated by a coarse-grained structure coming from the intracycle interference of the two electron trajectories born during the same optical cycle. We make use of a simple semiclassical model that offers the possibility to establish a connection between emission times and the photoelectron kinetic energy. We analyze such interference pattern as a function of the time delay between the IR and the XUV pulses and also as a function of the laser intensity. We compare the semiclassical predictions with the continuum-distorted-wave strong-field approximation and the ab initio solution of the time-dependent Schrödinger equation.

A theoretical study of the interference pattern imprinted on the doubly differential momentum distribution of the photoelectron due to atomic ionization induced by a short laser pulse is developed from a semiclassical standpoint. We use the semiclassical two-step model of Shvetsov-Shilovski et al. [ Phys. Rev. A 94, 013415 (2016)] to elucidate the nature of the holographic structure. Three different types of trajectories are characterized during the ionization process by a single-cycle pulse with three different types of interferences. We show that the holographic interference arises from the ionization yield only during the first half cycle of the pulse, whereas the coherent superposition of electron trajectories during the first half cycle and the second half cycle gives rise to two other kinds of intracycle interference. Although the picture of interference of a reference beam and a signal beam is adequate, we show that our results for the formation of the holographic pattern agree with the glory rescattering theory of Xia et al. [ Phys. Rev. Lett. 121, 143201 (2018)]. We probe the two-step semiclassical model by comparing it to the numerical results of the time-dependent Schrödinger equation.

We present a theoretical study of atomic laser-assisted photoionization emission. We consider an atom driven by a linearly polarized extreme ultraviolet laser in two scenarios: (i) a single attosecond pulse (in both the streaking and the sideband regimes) and (ii) an attosecond pulse train. The process takes place assisted by a linearly polarized infrared 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 infrared 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, can be applied, however, to other atomic species and field configurations as well.