Substrate matters: Surface-polariton enhanced infrared nanospectroscopy of molecular vibrations

TL;DR

This paper demonstrates that using polariton-resonant tip-substrate coupling significantly enhances nano-FTIR signals, enabling more effective detection of thin molecular layers and single molecules in infrared nanospectroscopy.

Contribution

It introduces a method to enhance nano-FTIR signals via surface polariton resonances, improving detection sensitivity on specific substrates.

Findings

Up to tenfold signal enhancement on quartz substrates.

Two orders of magnitude signal increase compared to standard substrates.

Potential for routine detection of monolayers and single molecules.

Abstract

Infrared nanospectroscopy based on Fourier transform infrared near-field spectroscopy (nano-FTIR) is an emerging nanoanalytical tool with large application potential for label-free mapping and identification of organic and inorganic materials with nanoscale spatial resolution. However, the detection of thin molecular layers and nanostructures on standard substrates is still challenged by weak signals. Here, we demonstrate a significant enhancement of nano-FTIR signals of a thin organic layer by exploiting polariton-resonant tip-substrate coupling and surface polariton illumination of the probing tip. When the molecular vibration matches the tip-substrate resonance, we achieve up to nearly one order of magnitude signal enhancement on a phonon-polaritonic quartz (c-SiO2) substrate, as compared to nano-FTIR spectra obtained on metal (Au) substrates, and up to two orders of magnitude when compared to the standard infrared spectroscopy substrate CaF2. Our results will be of critical importance for boosting nano-FTIR spectroscopy towards the routine detection of monolayers and single molecules.