Quantum theory of collective strong coupling of molecular vibrations with a microcavity mode

TL;DR

This paper presents a quantum mechanical model for the strong coupling between molecular vibrations and a microcavity mode, highlighting how different phonon baths influence the dynamics and coherence of the coupled system.

Contribution

It introduces a formalism that accounts for dephasing interactions and distinguishes effects of common versus independent phonon baths on vibrational strong coupling.

Findings

Long-range phonons decouple bright and dark states

Incoherent energy transfer occurs with independent baths

Strong Rabi splitting suppresses bath-induced decoherence

Abstract

We develop a quantum mechanical formalism to treat the strong coupling between an electromagnetic mode and a vibrational excitation of an ensemble of organic molecules. By employing a Bloch-Redfield-Wangsness approach, we show that the influence of dephasing-type interactions, i.e., elastic collisions with a background bath of phonons, critically depends on the nature of the bath modes. In particular, for long-range phonons corresponding to a common bath, the dynamics of the "bright state" (the collective superposition of molecular vibrations coupling to the cavity mode) is effectively decoupled from other system eigenstates. For the case of independent baths (or short-range phonons), incoherent energy transfer occurs between the bright state and the uncoupled dark states. However, these processes are suppressed when the Rabi splitting is larger than the frequency range of the bath modes, as achieved in a recent experiment [Shalabney et al., Nat. Commun. 6, 5981 (2015)]. In both cases, the dynamics can thus be described through a single collective oscillator coupled to a photonic mode, making this system an ideal candidate to explore cavity optomechanics at room temperature.