Atomic photoionization dynamics in ultrashort cycloidal laser fields

Abstract

We present numerical simulations of ultrafast multiphoton ionization dynamics in a twodimensional atomic model driven by co-and counterrotating circularly polarized single-color and bichromatic carrier envelope phase (CEP) stable ultrashort laser pulse sequences. Taking into account phase variations due to CEP fluctuations and the Gouy phase, our results accurately reproduce recently measured photoelectron momentum distributions [1][2][3][4]. The time evolution of the complexvalued electron wave function in coordinate and momentum space is calculated to study the bound state-and the vortex formation dynamics. The non-vanishing azimuthal probability current density proves the vortex nature of electron wave packets with odd-numbered rotational symmetry. Their angular momentum expectation value assumes half-integer values of 3.5 (corotating) and 0.5 (counterrotating). Knowledge of the wave function allows us to analyze the photoionization dynamics and to validate the physical pictures proposed in previous experimental studies. As an outlook, we investigate how electron vortices develop from the multiphoton-to the tunnel regime with increasing laser intensity.