Molecular cavity optomechanics: a theory of plasmon-enhanced Raman scattering

TL;DR

This paper introduces a cavity optomechanics model to explain plasmon-enhanced Raman scattering, revealing a new dynamical backaction mechanism that amplifies molecular vibrations beyond traditional electromagnetic field focusing effects.

Contribution

The study develops a novel theoretical framework mapping plasmon-molecule interactions onto cavity optomechanics, uncovering a dynamical backaction mechanism for Raman enhancement.

Findings

Dynamical backaction can amplify molecular vibrations under specific conditions.

The model quantitatively predicts Raman cross-section enhancements.

It offers a new perspective for designing plasmonic systems with mode-selective enhancement.

Abstract

The conventional explanation of plasmon-enhanced Raman scattering attributes the enhancement to the antenna effect focusing the electromagnetic field into sub-wavelength volumes. Here we introduce a new model that additionally accounts for the dynamical and coherent nature of the plasmon-molecule interaction and thereby reveals an enhancement mechanism not contemplated before: dynamical backaction amplification of molecular vibrations. We first map the problem onto the canonical model of cavity optomechanics, in which the molecular vibration and the plasmon are \textit{parametrically coupled}. The optomechanical coupling rate, from which we derive the Raman cross section, is computed from the molecules Raman activities and the plasmonic field distribution. When the plasmon decay rate is comparable or smaller than the vibrational frequency and the excitation laser is blue-detuned from the plasmon onto the vibrational sideband, the resulting delayed feedback force can lead to efficient parametric amplification of molecular vibrations. The optomechanical theory provides a quantitative framework for the calculation of enhanced cross-sections, recovers known results, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode-selective enhancement. It yields a new understanding of plasmon-enhanced Raman scattering and opens a route to molecular quantum optomechanics.