Space- and time-resolved second harmonic spectroscopy of coupled plasmonic nanocavities

Adi Salomon,Heiko Kollmann,Manfred Mascheck,Slawa Schmidt,Martin Silies,Yehiam Prior,Christoph Lienau

doi:10.1515/nanoph-2021-0049

Abstract

Abstract Localized surface plasmon resonances of individual sub-wavelength cavities milled in metallic films can couple to each other to form a collective behavior. This coupling leads to a delocalization of the plasmon field at the film surface and drastically alters both the linear and nonlinear optical properties of the sample. In periodic arrays of nanocavities, the coupling results in the formation of propagating surface plasmon polaritons (SPP), eigenmodes extending across the array. When artificially introducing dislocations, defects and imperfections, multiple scattering of these SPP modes can lead to hot-spot formation, intense and spatially confined fluctuations of the local plasmonic field within the array. Here, we study the underlying coupling effects by probing plasmonic modes in well-defined individual triangular dimer cavities and in arrays of triangular cavities with and without artificial defects. Nonlinear confocal spectro-microscopy is employed to map the second harmonic (SH) radiation from these systems. Pronounced spatial localization of the SPP field and significant enhancements of the SH intensity in certain, randomly distributed hot spots by more than an order of magnitude are observed from the triangular arrays as compared to a bare silver film by introducing a finite degree of disorder into the array structure. Hot-spot formation and the resulting enhancement of the nonlinear efficiency are correlated with an increase in the lifetime of the localized SPP modes. By using interferometric SH autocorrelation measurements, we reveal lifetimes of hot-spot resonances in disordered arrays that are much longer than the few-femtosecond lifetimes of the localized surface plasmon resonances of individual nanocavity dimers. This suggests that hot spot lifetime engineering provides a path for manipulating the linear and nonlinear optical properties of nanosystems by jointly exploiting coherent couplings and tailored disorder.

Highlights

The interaction of light with individual metallic nanostructures leads to the excitation of localized surface plasmon (LSPs) modes, i.e., resonant and collective oscillations of the conduction electrons in the metal nanoparticle which are coupled to the surrounding electromagnetic field
We attribute the enhanced fields within these hot spots to the coherent superposition of different randomly scattered surface plasmon polariton modes off-resonantly excited at the nanostructured silver film
Measurements of the hot spot diameters within the samples reveal that the most dominant hot spot is confined to 500–700 nm, approximately the inter-triangle distance d3 of the hexagonal pattern

Summary

Introduction

The interaction of light with individual metallic nanostructures leads to the excitation of localized surface plasmon (LSPs) modes, i.e., resonant and collective oscillations of the conduction electrons in the metal nanoparticle which are coupled to the surrounding electromagnetic field. Their resonance frequencies are determined by the geometrical parameters of the metallic nanostructure and the polarizability of both the metal and the surrounding medium. While adjacent nanoparticles are coupled via their electromagnetic near- and/or far-fields [5], the coupling between adjacent nanocavities generally relies on the exchange of propagating surface plasmon polaritons (SPPs), surface-bound waves at the metal-dielectric

Methods

Results

Conclusion