The effect of electron interactions on the universal properties of systems with optimized intrinsic off-resonant hyperpolarizability

TL;DR

This paper investigates how electron interactions influence the universal properties of systems with optimized off-resonant hyperpolarizability, extending previous single-electron models to two-electron systems and revealing common features near the fundamental limit.

Contribution

It generalizes prior single-electron models by including electron interactions, uncovering universal properties in two-electron systems with high hyperpolarizability.

Findings

Universal properties are observed across different systems near the hyperpolarizability limit.

Electron interactions do not alter the fundamental universal behavior.

Two-electron models show similar optimization trends as single-electron models.

Abstract

Because of the potentially large number of important applications of nonlinear optics, researchers have expended a great deal of effort to optimize the second-order molecular nonlinear-optical response, called the hyperpolarizability. The focus of our present studies is the {\em intrinsic} hyperpolarizability, which is a scale-invariant quantity that removes the effects of simple scaling, thus being the relevant quantity for comparing molecules of varying sizes. Past theoretical studies have focused on structural properties that optimize the intrinsic hyperpolarizability, which have characterized the structure of the quantum system based on the potential energy function, placement of nuclei, geometry, and the effects of external electric and magnetic fields. Those previous studies focused on single-electron models under the influence of an average potential. In the present studies, we generalize our calculations to two-electron systems and include electron interactions. As with the single-electron studies, universal properties are found that are common to all systems -- be they molecules, nanoparticles, or quantum gases -- when the hyperpolarizability is near the fundamental limit.