Tip-Enhanced Raman Scattering from Bridged Metal Nanocones

Abstract

Tip-enhanced Raman scattering (TERS) is a powerful near-field spectroscopic tool to measure Raman spectra of materials with nanometer spatial resolution and even with sensitivity down to single-molecule level [1]. The optical enhancement in TERS relies on lightning-rod effect and plasmonic coupling of input electric field into sharp metal tips. In general, the presence of strong electric field component along the tip axis (i.e., longitudinal field) is required for efficient coupling and several coupling schemes (e.g., use of radial polarization) have been suggested to enhance the interaction [1, 2].We propose a new approach for coupling light efficiently into the tip structures, which relies on coupling two metal tips together with a metal bridge. Due to the bridge, no strong longitudinal field is anymore needed for efficient coupling. The structures are manufactured using UV-nanoimprint lithography (UV-NIL) combined with electron-beam evaporation, which makes the technique fast, simple and relatively cheap [3]. In addition, large arrays of structures can be manufactured e.g. for lab-on-a-chip or optical tweezing applications.We measured Raman spectra from aqueous solution of crystal violet molecules using arrays of bridged and single gold and silver nanocones [4]. Almost ten-fold increase in the Raman signal for the bridged nanocones was observed, demonstrating the enhanced coupling of light.