Electron hopping induced phonon pumping in opto-mechanical molecular nanocavities

TL;DR

This paper demonstrates plasmon-mediated phonon pumping in molecular nanojunctions driven by inelastic electron hopping, leading to enhanced Raman effects at low light intensities and providing a microscopic framework for opto-mechanical coupling.

Contribution

It introduces a microscopic model based on Marcus electron hopping for opto-mechanical electron-phonon interactions in molecular nanojunctions, revealing new regimes of phonon pumping.

Findings

Enhanced Raman nonlinearity at low light intensities

Up to four-fold increase in Raman polarizability

Observation of transition between tunneling and hopping regimes

Abstract

Plasmonic molecular nanojunctions exhibit opto-mechanical coupling at the nanoscale, enabling intertwined optical, vibrational and electronic phenomena. Here, we demonstrate plasmon-mediated phonon pumping, driven by inelastic electron hopping in conductive molecules, which results in strong Raman nonlinearity at the light intensities almost three orders of magnitude lower than in the conventional opto-mechanical systems and up to four-fold enhancement of the effective Raman polarizability due to vibrational electron-phonon coupling, as confirmed by the significant increase in anti-Stokes Raman scattering intensity, indicating enhanced vibrational occupancy. We also developed a microscopic framework of opto-mechanical electron-phonon coupling in molecular nanojunctions based on the Marcus electron hopping. Systematically varying electrical conductance of the molecules in the junction and laser intensity, we observed the transition between a photo-assisted tunneling regime and an electron hopping process. Our findings provide a microscopic description for vibrational, optical, and electronic phenomena in plasmonic nanocavities important for efficient phonon lasing, representing the first attempt to exploit conductive molecules as quantum-mechanical oscillators.