k-space Imaging of the Eigenmodes of Sharp Gold Tapers for Scanning Near-Field Optical Microscopy

TL;DR

This paper explores the radiation patterns of gold tapers used in near-field optical microscopy, demonstrating a method to distinguish eigenmodes to enhance signal quality at the nanoscale.

Contribution

It introduces a novel spatial frequency filtering technique to separate eigenmodes in gold tapers, improving near-field signal detection in SNOM.

Findings

Only the lowest eigenmode reaches the apex, generating strong near-field signals.

Higher order modes are coupled into the far field at finite distances.

Spatial frequency filtering can distinguish and separate eigenmodes.

Abstract

We investigate the radiation patterns of sharp conical gold tapers, designed as adiabatic nanofocusing probes for scanning near-field optical microscopy (SNOM). Field calculations show that only the lowest order eigenmode of such a taper can reach the very apex and thus induce the generation of strongly enhanced near-field signals. Higher order modes are coupled into the far field at finite distances from the apex. Here, we demonstrate experimentally how to distinguish and separate between the lowest and higher order eigenmodes of such a metallic taper by filtering in the spatial frequency domain. Our approach has the potential to considerably improve the signal-to-background ratio in spectroscopic experiments on the nanoscale.