Alignment-free Plasmonic Nanowire Optical Probe for Tip-enhanced Raman Spectroscopy

S. Kim, Y. Zhu, X. Ma, R. Yan
University of California, Riverside,
United States

Keywords: TERS, surface plasmon, adiabatic nano-focusing, alignment-free

Summary:

Tip-ehnahced Raman spectroscopy (TERS) is a vibrational spectroscopic tool that combines surface enhanced Raman spectroscopy with scanning probe microscopy. By placing a sharp metallic tip in the focused laser beam, localized surface plasmon excited in the gap between the metallic tip and substrate, can provide a significantly enhanced vibrational spectroscopic information with sub-nanometer resolution. Even though TERS is an attractive and powerful technique, it still remains esoteric due to its difficult optical alignment, and limitations in sensitivity and resolution resulting from the poor optical focusing at the molecular length scale. To build TERS platform with alignment-free and far-field background-free, concentrating excitation light and collecting Raman signal must be achieved via the TERS probe. There have been attempts to concentrate light through the TERS probe, which are called “adiabatic plasmonic nanofocusing”. However, they still suffer from a low signal to noise ratio due to a low light coupling efficiency which significantly limits the sensitivity and speed of image construction. In this work, we introduce a novel TERS probe using a sharp metal nanowire based on “genuine” adiabatic plasmonic nanofocusing and alignment-free fiber-in-fiber-out platform. This adiabatic plasmonic nanofocusing not only allows for a high transmission, but also Raman signal collection through the same nanowire light path, for the excitation. These excitation and collection configurations do not require optical components used for focusing light nor a very careful tedious optical system alignment. By combining the probe with commercial scanning tunneling microscopy, 1600cps could be obtained, which is the highest Raman signal intensity obtainable to the best of our knowledge. This new probe design also allows for higher device reliability due to a simple probe preparation method, which is another major limitation for conventional adiabatic nanofocusing TERS probes. We believe that this new TERS probe will open exciting avenues in the variety of characterization fields from 2D solids to biological materials.