Nanocavities for Molecular Optomechanics: their fundamental description and applications

TL;DR

This paper reviews the fundamental theory and recent progress in molecular cavity optomechanics within nanocavities, emphasizing key parameters like coupling efficiency and cooperativity, and discusses experimental prospects such as observing optomechanically induced transparency.

Contribution

It clarifies the theoretical framework of molecular cavity optomechanics and connects it with experimental parameters, making it accessible for the nanoplasmonics community.

Findings

Key parameters for nanocavity performance identified: coupling efficiency and cooperativity.

Feasibility analysis for observing optomechanically induced transparency.

Summary of main results and challenges in molecular cavity optomechanics.

Abstract

Vibrational Raman scattering -- a process where light exchanges energy with a molecular vibration through inelastic scattering -- is most fundamentally described in a quantum framework where both light and vibration are quantized. When the Raman scatterer is embedded inside a plasmonic nanocavity, as in some sufficiently controlled implementations of surface-enhanced Raman scattering (SERS), the coupled system realizes an optomechanical cavity, where coherent and parametrically amplified light-vibration interaction becomes a resource for vibrational state engineering and nanoscale nonlinear optics. The purpose of this Perspective is to clarify the connection between the languages and parameters used in the fields of molecular cavity optomechanics (McOM) vs. its conventional, `macroscopic' counterpart, and to summarize the main results achieved so far in McOM and the most pressing experimental and theoretical challenges. We aim to make the theoretical framework of molecular cavity optomechanics practically usable for the SERS and nanoplasmonics community at large. While quality factors ($Q$'s) and mode volumes ($V$'s) essentially describe the performance of a nanocavity in enhancing light-matter interaction, we point to the light-cavity coupling efficiencies ($\eta$'s) and optomechanical cooperativities ($\mathcal{C}$'s) as the key parameters for molecular optomechanics. As an illustration of the significance of these quantities, we investigate the feasibility of observing optomechanically induced transparency with a molecular vibration -- a measurement that would allow for a direct estimate of the optomechanical cooperativity.