Optical antennas with multiple plasmonic nanoparticles for tip-enhanced Raman microscopy

Abstract

Tip-enhanced Raman spectroscopy (TERS) has recently become one of the most important tools for analyzing advanced nano-devices and nano-materials, because it allows strong enhancement of weak Raman signal from the nanometric volume of a sample. However, consistent enhancement in TERS is still an issue and scientists have been struggling to fabricate good tips for reliable, strong and reproducible enhancement. There is a strong need to study the morphology and the arrangement of metal nanostructures near the tip apex for efficient plasmonic enhancement in TERS. Here, we present a study on the metal grains attached to the tip surface for producing higher and much consistent enhancement in TERS. Our study shows that the plasmonic enhancement strongly depends on the number of grains and on the their separations. We found through simulations that multiple grains arranged closely but discretely on a dielectric probe act as an efficient plasmonic antenna and that enhancement in TERS is maximum for an optimized number of grains. The number of grains and the nano-gap between them are crucial for reproducible enhancement. This promising result, which we also demonstrate and prove by experiments, will bring TERS to a new level, where it can be utilized with more confidence of large reproducible enhancement for those nano-sized samples that have extremely weak Raman scattering.