  A dielectric material's response to light is macroscopically described by
electric displacement fields due to polarization and susceptibility, but the
atomistic origin is light-cycle-driven motion of electron densities in the
restoring forces of the atomic environment. Here we report how the macroscopic
nonlinear-optical response of a heteronuclear crystal relates to the alignment
and orientation of its chemical bonds. Substantial nonlinear emission is only
observed if the electric field of an optical single-cycle pulse points from the
less electronegative to the more electronegative element and not vice versa.
This asymmetry is a consequence of the unbalanced real-space motion of valence
charges along the direction of the bonds. These results connect a material's
chemical structure to the optical response and may facilitate the comprehension
and design of novel materials for applications in optics and lasers on basis of
the atoms and how they connect.
