Multimodal Tip-Enhanced Nonlinear Optical Nano-Imaging of Plasmonic Silver Nanocubes

TL;DR

This paper demonstrates that nonlinear optical processes can be spatially and spectrally resolved at plasmonic tip-sample junctions with nanometer precision, revealing resonance-dependent enhancement mechanisms in nano-imaging.

Contribution

It introduces a multimodal nonlinear optical nano-imaging technique that achieves high-resolution spectral imaging of plasmonic nanostructures under ambient conditions.

Findings

Nonlinear optical signals are enhanced at plasmonic junctions with nanometer spatial resolution.

Local nonlinear signal efficiencies depend on sharp tip-sample junction resonances.

Different nonlinear signals are selectively enhanced by plasmon resonances matching their optical frequencies.

Abstract

Optical field localization at plasmonic tip-sample nanojunctions has enabled high spatial resolution chemical analysis through tip-enhanced linear optical spectroscopies, including Raman scattering and photoluminescence. Here, we illustrate that nonlinear optical processes, including parametric four-wave mixing (4WM), second harmonic/sum-frequency generation (SHG and SFG), and two-photon photoluminescence (TPPL), can be enhanced at plasmonic junctions and spatio-spectrally resolved simultaneously with few-nm spatial resolution under ambient conditions. More importantly, through a detailed analysis of our spectral nano-images, we find that the efficiencies of the local nonlinear signals are determined by sharp tip-sample junction resonances that vary over the few-nanometer length scale because of the corrugated nature of the probe. Namely, plasmon resonances centered at or around the different nonlinear signals are tracked through TPPL, and they are found to selectively enhance nonlinear signals with closely matched optical resonances.