Three-state interactions determine the second-order nonlinear optical response

TL;DR

This paper analyzes how three-state interactions influence the second-order nonlinear optical response, demonstrating that tuning molecular spectra can control resonant contributions and that three-level models are broadly applicable.

Contribution

It provides a theoretical framework showing the significance of three-state interactions and validates the generality of three-level models for nonlinear optical responses.

Findings

Resonant contributions can be tuned via molecular spectrum adjustments.

Three-state interaction terms behave similarly to three-level models off-resonance.

Most three-level model results are applicable to complex molecules with multiple levels.

Abstract

Using the sum-rules, the sum-over-states expression for the diagonal term of first hyperpolarizability can be expressed as the sum of three-state interaction terms. We study the behavior of a generic three-state term to show that is possible to tune the contribution of resonant terms by tuning the spectrum of the molecule. When extrapolated to the off-resonance regime, the three-state interaction terms are shown to behave in a similar manner as the three-level model used to derive the fundamental limits. We finally show that most results derived using the three-level ansatz are general, and apply to molecules where more than three levels contribute to the second-order nonlinear response or/and far from optimization.