Accelerated Molecular Vibrational Decay and Suppressed Electronic Nonlinearities in Plasmonic Cavities through Coherent Raman Scattering

TL;DR

This study demonstrates that plasmonic nanogaps significantly accelerate molecular vibrational decay and suppress electronic nonlinearities, enabling advanced single-molecule vibrational studies and optomechanics at ultralow powers.

Contribution

It introduces a novel three-colour time-resolved coherent anti-Stokes Raman spectroscopy method to study vibrational dynamics in plasmonic nanogaps, revealing enhanced decay rates and suppressed nonlinearities.

Findings

Vibrational decay rates are accelerated ten-fold in nanogaps.

FWM is strongly suppressed in nm-wide plasmonic gaps.

Ultrafast vibrational decay is driven by local temperature rise.

Abstract

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-colour time-resolved coherent anti-Stokes Raman spectroscopy (trCARS) using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM), while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nm-wide plasmonic gaps compared to plasmonic nanoparticles. The ultrafast vibrational decay rates of biphenyl-4-thiol molecules are accelerated ten-fold by a transient rise in local non-equilibrium temperature excited by enhanced, pulsed optical fields within these plasmonic nanocavities. Separating the contributions of vibrational population decay and dephasing carefully explores the vibrational decay channels of these tightly confined molecules. Such extreme plasmonic enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally-coupled dynamics and diverse molecular optomechanics phenomena.