A New Imaginary Term in the 2nd Order Nonlinear Susceptibility from Charged Interfaces

TL;DR

This paper introduces a novel imaginary term in the second-order nonlinear susceptibility, chi(3)X, observed at charged interfaces, which improves modeling of nonlinear optical responses and reveals ion-specific interfacial effects.

Contribution

It identifies and characterizes a new imaginary component in the second-order nonlinear susceptibility at charged interfaces, enhancing understanding of interfacial nonlinear optics.

Findings

The chi(3)X term causes a 90° phase shift in SHG measurements.

Including chi(3)X improves agreement between models and experimental data.

The model distinguishes ion-specific effects in interfacial nonlinear responses.

Abstract

Non-resonant second harmonic generation phase and amplitude measurements obtained from the silica:water interface at varying pH and 0.5 M ionic strength point to the existence of a nonlinear susceptibility term, which we call chi(3)X, that is associated with a 90 deg phase shift. Including this contribution in a model for the total effective second-order nonlinear susceptibility produces reasonable point estimates for interfacial potentials and second-order nonlinear susceptibilities when chi(3)Xis about 1.5 times chi(3)water. A model without this term and containing only traditional chi(2) and chi(3) terms cannot recapitulate the experimental data. The new model also provides a demonstrated utility for distinguishing apparent differences in the second-order nonlinear susceptibility when the electrolyte is NaCl vs MgSO4, pointing to the possibility of using HD-SHG to investigate ion-specificity in interfacial processes.