Spontaneous Raman Scattering under Vibrational Strong Coupling: The Critical Role of Polariton Spatial Mode Coherence

TL;DR

This paper develops a quantum model to explain how spatial mode coherence affects Raman scattering under vibrational strong coupling, clarifying conflicting experimental results and highlighting the importance of cavity-vibration spatial overlap.

Contribution

It introduces a microscopic quantum framework that accounts for spatial mode coherence, explaining the conditions under which Raman peaks are observed in strongly coupled systems.

Findings

In homogeneous cavities, spatial overlap suppresses Raman peaks.

In quasi-2D layers, lifted selection rules produce Raman peaks at polariton energies.

The model clarifies the role of spatial mode coherence in vibrational strong coupling.

Abstract

Resonant coupling of a vibration to a cavity mode has been reported to dramatically modify spontaneous Raman scattering, but subsequent studies have produced conflicting results. In this Letter, we develop a microscopic quantum framework that captures the spatial structure of polaritonic modes. In a homogeneously filled cavity, spatial overlap between polaritons and cavity resonances enforces selection rules that suppress the initially reported polaritonic Raman peaks, consistent with most experiments. In contrast, for a quasi-two-dimensional (2d) molecular layer, these rules are lifted, yielding Raman peaks at the polariton energies. Our work clarifies that the Raman response under vibrational strong coupling is determined by cavity-vibration spatial mode overlap and offers a framework for Raman studies of strongly coupled quasi-2d systems.