Asymmetric nonlinear optics of a polar chemical bond

TL;DR

This study links the macroscopic nonlinear optical response of heteronuclear crystals to the orientation of chemical bonds, revealing an asymmetry driven by electron charge motion that depends on the electric field direction.

Contribution

It demonstrates how the nonlinear optical response is asymmetric and depends on bond orientation, connecting chemical structure to optical behavior at the atomic level.

Findings

Nonlinear emission occurs only when the electric field points from less to more electronegative atoms.

The asymmetry arises from unbalanced real-space valence charge motion along bonds.

Results enable design of new optical materials based on atomic structure.

Abstract

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.