Tip-Enhanced Sum Frequency Generation for Molecular Vibrational Nanospectroscopy

TL;DR

This paper introduces a tip-enhanced sum frequency generation (TE-SFG) technique using a scanning tunneling microscope to achieve nanoscale vibrational spectroscopy of surface molecules beyond the diffraction limit.

Contribution

The authors developed a novel TE-SFG system that enables vibrational spectroscopy at the nanoscale, overcoming diffraction limitations of traditional far-field methods.

Findings

Successfully detected vibrational TE-SFG signals from molecules on gold.

Phase analysis revealed molecular orientation information.

Signals originate from localized regions within tip-sample gaps.

Abstract

Vibrational sum frequency generation (SFG) is a nonlinear spectroscopic technique widely used to study the molecular structure and dynamics of surface systems. However, the spatial resolution achieved by far-field observations is constrained by the diffraction limit, obscuring molecular details in inhomogeneous structures smaller than the wavelength of light. To overcome this limitation, we developed a system for tip-enhanced SFG (TE-SFG) spectroscopy based on a scanning tunneling microscope. We successfully detected vibrational TE-SFG signals from adsorbed molecules on a gold substrate under ambient conditions. The phase analysis of interferometric SFG spectra provided information on molecular orientation. Furthermore, the observed TE-SFG signal was confirmed to originate from a highly localized region within a gap between the tip apex and the sample substrate. This method offers a novel platform for nonlinear optical nanospectroscopy, paving the way for the investigation of surface molecular systems beyond the diffraction limit.