Spatial asymmetry of photoelectron emission in strong-field ionization of atoms irradiated by a bichromatic laser field

Abstract: The phenomenon of spatial (polar) asymmetry of produced photoelectron emission due to strong-field multiphoton process of above-threshold ionization (ATI) in atoms irradiated by a linearly polarized two-color (bichromatic) laser field consisting of coherent superposition of two commensurate harmonics (of frequencies and is considered and studied theoretically. The problem is addressed within the conventional strong-field approximation (SFA) in standard velocity gauge (VG) formulation under condition of arbitrary value of the so-called Keldysh parameter $\gamma$ , including both the multiphoton (for $\gamma \gg 1$) and tunneling (for $\gamma \ll 1$) regimes of ionization. According to such VG-SFA consideration, the calculated photoelectron momentum distributions (PMD) demonstrate a clear spatial asymmetry (viz., the photoelectron emission proved to be different along the opposite directions with respect to incident laser field polarization) even for particular bichromatic field(s) with polar symmetry of its total electric field strength. Moreover, a clear correlation between the spatial symmetry/asymmetry of photoelectron emission and that of applied vector potential of bichromatic field is reliably established, so that the symmetry of calculated PMD proved to follow precisely the symmetry of applied vector potential of incident laser bichromatic field, in a close analogy with the well-known Aharonov-Bohm effect. Based on these our findings, the attempt is undertaken to identify the physical mechanism underlying this intriguing phenomenon and provide it with a kind of semiclassical interpretation in terms of the partial contributions from photoelectron momenta in final continuum states to partial strong-field ATI amplidude(s) corresponding to a certain (fixed) number of either of two harmonics absorbed.