Tuning the resonance frequency of Ag-coated dielectric tips

Xudong Cui,Christian Hafner,Renato Zenobi,Boon-Siang Yeo,Daniel Erni,Weihua Zhang

doi:10.1364/oe.15.008309

Abstract

A finite element model was built to investigate how to optimize localized plasmon resonances of an Ag-coated dielectric tip for tip-enhanced Raman spectroscopy (TERS). The relation between the resonance frequency, the electric field enhancement and the optical constant of the dielectric tip was numerically investigated. The results show that increasing the refractive index of the dielectric tip can significantly red shift the localized plasmon modes excited on the Ag-coated dielectric tip, and consequently alter the field enhancement. Moreover, the influence of the width of the resonance on the Raman enhancement was also considered. When taking all the factors into account, we find that an Ag-coated low-refractive index dielectric tip provides the best Raman enhancement in the blue-green spectral range. This is consistent with our prior experimental results.

Highlights

Tip-enhanced Raman spectroscopy (TERS) is a promising technique for nanoscale analysis, because of its capacity of providing rich chemical information [1, 2], high spatial resolution [3] and high sensitivity [4,5,6]
It is well known that the resonance of localized plasmons (LPs) on a metal nanostructure is sensitive to its shape and the optical properties of its surrounding materials [14,15,16]
The performance of Agcoated atomic force microscope (AFM) tips has been significantly improved by using low refractive index AFM tips

Summary

Introduction

Tip-enhanced Raman spectroscopy (TERS) is a promising technique for nanoscale analysis, because of its capacity of providing rich chemical information [1, 2], high spatial resolution [3] and high sensitivity [4,5,6]. According to mechanistic studies on surface-enhanced Raman spectroscopy (SERS), this field enhancement has two origins: resonant excitation of the localized plasmons (LPs), and the sharp-edge induced ‘lightning rod’ effect which confines the electric field to the tip apex [9] From this point of view, an adequate fabrication technology for properly shaping the sharp metal tip, which enables the presence of strongly localized modal fields at a chosen resonance frequency, holds the key for successful TERS. It is well known that the resonance of LPs on a metal nanostructure is sensitive to its shape and the optical properties of its surrounding materials [14,15,16] From this point of view, the mismatch between the laser source and the resonance frequency of the Ag-coated tip is caused by the improper choice of the tip shape and the underlying material.

Simulation

Results and discussion

Does the conductivity of the AFM influence the tips performance?

Other considerations

Conclusion