Plasmonic nanofocusing spectral interferometry

Martin Esmann,Anke Korte,Abbas Chimeh,Nahid Talebi,Christoph Lienau,Jin-Hui Zhong,Julia Witt,Gunther Wittstock,Sven Stephan

doi:10.1515/nanoph-2019-0397

Abstract

Abstract We describe and demonstrate a novel experimental approach to measure broadband, amplitude- and phase-resolved scattering spectra of single nanoparticles with 10-nm spatial resolution. Nanofocusing of surface plasmon polaritons (SPPs) propagating along the shaft of a conical gold taper is used to create a spatially isolated, spectrally broad nanoscale light source at its very apex. The interference between these incident SPPs and SPPs that are backpropagating from the apex leads to the formation of an inherently phase-stable interferogram, which we detect in the far field by partially scattering SPPs off a small protrusion on the taper shaft. We show that these interferograms allow the reconstruction of both the amplitude and phase of the local optical near fields around individual nanoparticles optically coupled to the taper apex. We extract local light scattering spectra of particles and quantify line broadenings and spectral shifts induced by tip-sample coupling. Our experimental findings are supported by corresponding finite-difference time-domain and coupled dipole simulations and show that, in the limit of weak tip-sample coupling, the measurements directly probe the projected local density of optical states of the plasmonic system. The combination of a highly stable inline interferometer with the inherent optical background suppression through nanofocusing makes it a promising tool for the locally resolved study of the spectral and temporal optical response of coupled hybrid nanosystems.

Highlights

The electronic local density of states, which represents the number of electronic states in a certain volume and energy interval, is one of the most fundamental quantities in nanoscience
When the tip approaches the sample, the nanofocused light spot induces an optical polarization in the sample, and the fields Er that are re-emitted by the sample can couple back to the tip apex
The taper is used to locally excite the sample and to scatter optical near fields into surface plasmon polaritons (SPPs) waves that are backpropagating along the taper shaft

Summary

Introduction

The electronic local density of states, which represents the number of electronic states in a certain volume and energy interval, is one of the most fundamental quantities in nanoscience. In plasmonic and photonic nanostructures, the corresponding quantity is the electromagnetic local density of states (LDOS), i.e. the electromagnetic field – created by the structure after excitation by a point-like isotropic source – at the position of the source [5, 6]. It is of immediate relevance for controlling the spontaneous emission of quantum emitters by their nanostructured environment [7]. Because of its importance for designing optical properties at the nanoscale, experimental LDOS studies have received substantial attention [8,9,10,11,12,13,14,15]

Methods

Results

Discussion

Conclusion