Atomic photoionization dynamics in ultrashort cycloidal laser fields

We present numerical simulations of ultrafast multiphoton ionization dynamics in a two-dimensional atomic model driven by co- and counter-rotating circularly polarized single-color and bichromatic carrier envelope phase (CEP) stable 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 [Phys. Rev. Lett. 118, 053003 (2017); Phys. Rev. A 96, 043426 (2017); Nat. Commun. 10, 658 (2019); Adv. Phys.: X 4, 1672583 (2019)]. The time evolution of the complex-valued electron wave function in coordinate and momentum space is calculated to study the bound-state and vortex formation dynamics. The nonvanishing 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 (counter-rotating). 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 tunneling ionization regime with increasing laser intensity.