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The phenomenon of spatial (polar) asymmetry of produced photoelectronemission due to strong-field multiphoton process of above-threshold ionization(ATI) in atoms irradiated by a linearly polarized two-color (bichromatic) laserfield consisting of coherent superposition of two commensurate harmonics (offrequencies and is considered and studied theoretically. The problem isaddressed within the conventional strong-field approximation (SFA) in standardvelocity gauge (VG) formulation under condition of arbitrary value of theso-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 momentumdistributions (PMD) demonstrate a clear spatial asymmetry (viz., thephotoelectron emission proved to be different along the opposite directionswith respect to incident laser field polarization) even for particularbichromatic field(s) with polar symmetry of its total electric field strength.Moreover, a clear correlation between the spatial symmetry/asymmetry ofphotoelectron emission and that of applied vector potential of bichromaticfield is reliably established, so that the symmetry of calculated PMD proved tofollow precisely the symmetry of applied vector potential of incident laserbichromatic field, in a close analogy with the well-known Aharonov-Bohm effect.Based on these our findings, the attempt is undertaken to identify the physicalmechanism underlying this intriguing phenomenon and provide it with a kind ofsemiclassical interpretation in terms of the partial contributions fromphotoelectron momenta in final continuum states to partial strong-field ATIamplidude(s) corresponding to a certain (fixed) number of either of twoharmonics absorbed.

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