Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths

TL;DR

This study uses femtosecond laser pulses and photoemission electron microscopy to image and analyze localized surface plasmon field enhancements in silver nanostructures at telecommunication wavelengths, revealing size and shape independence and potential interface effects.

Contribution

It demonstrates a novel imaging approach for plasmonic field enhancement in silver nanostructures at 1.55 micrometers, with detailed spectral and morphological analysis.

Findings

Field enhancement observed in 30% of Ag nanoparticles

No enhancement detected in nanowires

Field enhancement independent of size and shape, possibly due to substrate interface effects

Abstract

We image the field enhancement at Ag nanostructures using femtosecond laser pulses with a center wavelength of 1.55 micrometer. Imaging is based on non-linear photoemission observed in a photoemission electron microscope (PEEM). The images are directly compared to ultra violet PEEM and scanning electron microscopy (SEM) imaging of the same structures. Further, we have carried out atomic scale scanning tunneling microscopy (STM) on the same type of Ag nanostructures and on the Au substrate. Measuring the photoelectron spectrum from individual Ag particles shows a larger contribution from higher order photoemission process above the work function threshold than would be predicted by a fully perturbative model, consistent with recent results using shorter wavelengths. Investigating a wide selection of both Ag nanoparticles and nanowires, field enhancement is observed from 30% of the Ag nanoparticles and from none of the nanowires. No laser-induced damage is observed of the nanostructures neither during the PEEM experiments nor in subsequent SEM analysis. By direct comparison of SEM and PEEM images of the same nanostructures, we can conclude that the field enhancement is independent of the average nanostructure size and shape. Instead, we propose that the variations in observed field enhancement could originate from the wedge interface between the substrate and particles electrically connected to the substrate.