Origin of third harmonic generation in plasmonic nanoantennas

TL;DR

This paper investigates the origin of third harmonic generation in plasmonic nanoantennas, revealing that the dominant nonlinear source depends on nanoantenna design, particularly width, with monopole antennas performing better than dipoles.

Contribution

It clarifies how nanoantenna design influences the nonlinear emission origin, resolving previous ambiguities and guiding better design choices for enhanced third harmonic generation.

Findings

Gold dominates in thin nanoantennas

Gap material dominates in wider nanoantennas

Monopole nanoantennas outperform dipoles of same width

Abstract

Plasmonic nanoantennas have been recently proposed to boost nonlinear optical processes. In a metal dipole nanoantenna with a dielectric nanoparticle placed in the gap, the linear field enhancement can be exploited to enhance third harmonic emission. Since both metals and dielectrics exhibit nonlinearity, the nonlinear far-field contains contributions from each, and the impossibility of measuring these contributions separately has led to seemingly contradictory interpretations about the origin of the nonlinear emission. We determine that the origin of the third harmonic from metal-dielectric dipole nanoantennas depends on nanoantenna design, and in particular, the width. We find that the emission from gold dominates in thin threadlike nanoantennas, whereas the emission from the gap material dominates in wider nanoantennas. We also find that monopole nanoantennas perform better than dipoles having the same width, and due to their simplicity should be preferred in many applications.