A quantum mechanical model of the upper bounds of the cascading contribution to the second hyperpolarizability

TL;DR

This paper presents a quantum mechanical analysis demonstrating that cascading between two molecules cannot surpass the fundamental nonlinear-optical limit of a single molecule with the same electron count, clarifying the role of intermolecular interactions.

Contribution

It provides a quantum model showing the upper bounds of cascading contributions to second hyperpolarizability and clarifies that cascading cannot exceed single-molecule limits.

Findings

Cascading contribution vanishes when intermolecular interactions are properly considered.

Cascading cannot produce larger nonlinear response than a single molecule with the same electrons.

Designs for large third-order susceptibilities should focus on molecular structure rather than cascading.

Abstract

Microscopic cascading of second-order nonlinearities between two molecules has been proposed to yield an enhanced third-order molecular nonlinear-optical response. In this contribution, we investigate the two-molecule cascaded second hyperpolarizability and show that it will never exceed the fundamental limit of a single molecule with the same number of electrons as the two-molecule system. We show the apparent divergence behavior of the cascading contribution to the second hyperpolarizability vanishes when properly taking into account the intermolecular interactions. Although cascading can never lead to a larger nonlinear-optical response than a single molecule, it provides alternative molecular design configurations for creating materials with large third-order susceptibilities that may be difficult to design into a single molecule.