Tuning the second harmonic generation of self-generated metallic islands

Annika Konzelmann,Audrey Berrier,Martin Dressel,Hemant Charaya,Bruno Gompf

doi:10.1063/1.5027715

Abstract

When crossing the percolation threshold the optical behavior of thin metal films dramatically change within a narrow range. Recording the optical properties in the vicinity of the percolation provides insight into the interplay of those contributions that determine the intensity of the second harmonic signal: the metallic islands and the dielectric gap between the islands. Here we demonstrate the active tuning of the linear and nonlinear optical properties of a thin gold film on a stretchable PDMS substrate. Ellipsometry was combined with nonlinear studies (second-harmonic generation) to describe the optical properties of thin gold films around the percolation. We monitor the metal-insulator transition, characterize the evolution of the permittivity of the layer, and explain the strength variations of the second harmonic generation with respect to the spectral difference between fundamental wavelength and plasmonic resonance, as well as the enhancement of the field in the dielectric gaps as a function of the particle-particle distance. A model reproduces the experimental observations by taking into account both effects at the fundamental and at the generated wavelengths.

Highlights

The investigation of the linear and nonlinear optical properties of percolating metal films has attracted considerable attention since several years.[1,2] Percolation is the state where single metal nanoparticles or clusters form a conductive path through the sample
One expects that an active control of the inter-cluster distance as well as the geometry of the clusters themselves in conjunction with exciting the material at the appropriate wavelength would lead to an optimization of the strength of the second harmonic generation (SHG)
The models are composed of one Drude component, which accounts for the overall degree of metallicity, and several Gaussian oscillators, describing the interband transition as well as the spectral position and strength of the plasmon mode, the latter being crucial for the prediction of SHG rates

Summary

Introduction

The investigation of the linear and nonlinear optical properties of percolating metal films has attracted considerable attention since several years.[1,2] Percolation is the state where single metal nanoparticles or clusters form a conductive path through the sample. The real part of the permittivity, ε1, is known to diverge at the percolation threshold due to the vanishing distance between the metallic islands, which leads to a divergence of the capacitive area.[3,4] Strong second harmonic generation around the percolation threshold is related to the vanishing inter-cluster distance, and the strong correlation between the evolution of the SHG and the percolation threshold has already been reported.[5] There the field enhancement due to the excitation of plasmonic resonances supported by the clusters plays an important role.

Results

Conclusion