Local fluctuations of vibrational polaritons monitored by two-dimensional infrared spectroscopy

TL;DR

This paper investigates how local fluctuations affect vibrational polaritons in an IR microcavity, using 2D-IR spectroscopy to reveal dark states and the breakdown of molecular cooperativity.

Contribution

It develops a 2D-IR spectroscopy method to resolve the dynamics of vibrational polaritons and dark states, providing new insights into their local fluctuations and coherence breakdown.

Findings

Dark states are identified via cross peaks in 2D-IR spectra.

Breakdown of cooperativity is linked to vibrational excitation localization.

Polaritons remain extended over multiple molecules despite local fluctuations.

Abstract

We study the collective behavior of molecules placed in an infrared (IR) microcavity, incorporating the local fluctuations, i.e., dynamical disorder. The cooperative feature in vibrational polaritons is shown to be dynamically eroded, due to intermolecule coherence. To further resolve such process, we develop a two-dimensional infrared spectroscopy (2D-IR) for molecules interacting with cavity modes. The cooperative feature in correspondence to the spectroscopic signal is specified. The results reveal the dark states by the cross peaks apart from the ones for polaritons, as a result of the breakdown of cooperativity between molecules. We further show that the breakdown of cooperativity profoundly connects to the localization of the vibrational excitations whereas the polariton modes are extended wave over several molecules. Besides, our work offers new physical insight for understanding the recent 2D-IR experiments where the interaction between dark modes and bright polaritons was evident.