Mid-infrared-perturbed Molecular Vibrational Signatures in Plasmonic Nanocavities

TL;DR

This paper introduces a novel MIR sensing method using SERS in nanoparticle-on-foil nanocavities, enabling real-time detection of molecular vibrations with high sensitivity and potential for single-molecule applications.

Contribution

The study develops a combined visible and MIR plasmonic nanocavity platform for modulating and detecting molecular vibrational signals in real-time.

Findings

MIR signals are modulated by visible plasmonic hotspots.

MIR absorption bands of SiO₂ and polystyrene influence SERS signals.

Phonon resonances can trap MIR surface plasmons, affecting visible plasmons.

Abstract

Recent developments in surface-enhanced Raman scattering (SERS) enable observation of single-bond vibrations in real-time at room temperature. By contrast, mid-infrared (MIR) vibrational spectroscopy is limited to inefficient slow detection. Here we develop a new method for MIR sensing using SERS. This method utilizes nanoparticle-on-foil (NPoF) nanocavities supporting both visible and MIR plasmonic hotspots in the same nano-gap formed by a monolayer of molecules. Molecular SERS signals from individual NPoF nanocavities are modulated in the presence of MIR photons. The strength of this modulation depends on the MIR wavelength, and is maximized at the 6-12$\mu$m absorption bands of SiO$_2$ or polystyrene placed under the foil. Using a single-photon lock-in detection scheme we time-resolve the rise and decay of the signal in a few 100ns. Our observations reveal that the phonon resonances of SiO$_2$ can trap intense MIR surface plasmons within the Reststrahlen band, tuning the visible-wavelength localized plasmons by reversibly perturbing the nanostructure crevices. This suggests new ways to couple nano-scale bond vibrations for optomechanics, with potential to push detection limits down to single-photon and single-molecule regimes.