Beyond the Gouy-Chapman Model with Heterodyne-Detected Second Harmonic Generation

TL;DR

This paper introduces a phase-sensitive SHG technique that distinguishes between different interfacial contributions, enabling more accurate testing of electrostatic models at charged interfaces.

Contribution

It demonstrates a novel heterodyne-detected SHG method that separates chi(2) and chi(3) terms using phase information, advancing interface characterization.

Findings

Phase shifts depend on ionic strength, confirming a dispersion model.

Phase analysis separates chi(2) and chi(3) contributions unambiguously.

Method applied successfully to lipid bilayers and hyper-Rayleigh scattering detection.

Abstract

We report ionic strength-dependent phase shifts in second harmonic generation (SHG) signals from charged interfaces that verify a recent model in which dispersion between the fundamental and second harmonic beams modulates observed signal intensities. We show how phase information can be used to unambiguously separate the chi(2) and interfacial potential-dependent chi(3) terms that contribute to the total signal and provide a path to test primitive ion models and mean field theories for the electrical double layer with experiments to which theory must conform. Finally, we demonstrate the new method on supported lipid bilayers and comment on the ability of our new instrument to identify hyper-Rayleigh scattering contributions to common homodyne SHG measurements in reflection geometries.