Direct phase mapping of the light scattered by single plasmonic nanoparticles

TL;DR

This paper introduces a new method to directly measure the phase shift of light scattered by single plasmonic nanoparticles using a confocal microscope with interferometry, enhancing understanding of their optical properties.

Contribution

The work presents a novel experimental technique combining confocal microscopy and interferometry to measure phase shifts of individual nanoparticles, supported by a theoretical model.

Findings

Phase shifts vary with plasmon resonance frequency.

Harmonic oscillator model explains phase behavior.

Method improves understanding of nanoparticle scattering.

Abstract

In this work, we present a novel technique to directly measure the phase shift of the optical signal scattered by single plasmonic nanoparticles in a diffraction-limited laser focus. We accomplish this by equipping an inverted confocal microscope with a Michelson interferometer and scanning single nanoparticles through the focal volume while recording interferograms of the scattered and a reference wave for each pixel. For the experiments, lithographically prepared gold nanorods where used, since their plasmon resonances can be controlled via their aspect ratio. We have developed a theoretical model for image formation in confocal scattering microscopy for nanoparticles considerably smaller than the diffraction limited focus We show that the phase shift observed for particles with different longitudinal particle plasmon resonances can be well explained by the harmonic oscillator model. The direct measurement of the phase shift can further improve the understanding of the elastic scattering of individual gold nanoparticles with respect to their plasmonic properties.