  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.
