Polariton assisted down-conversion of photons via nonadiabatic molecular dynamics: a molecular dynamical Casimir effect

TL;DR

This study demonstrates that nonadiabatic molecular dynamics in a strongly coupled molecule-cavity system can generate photon pairs during photoisomerization, revealing a new mechanism for photon down-conversion without requiring ultrastrong coupling.

Contribution

It introduces a novel mechanism where polariton dynamics enable photon pair generation through molecular isomerization without counterrotating terms or ultrastrong coupling.

Findings

Photon pairs are generated during molecular isomerization in strong coupling.

Counterrotating terms are not necessary for polariton manifold mixing.

Collective strong coupling conditions are unfeasible for the studied system.

Abstract

Quantum dynamics of the photoisomerization of a single 3,3'-diethyl-2,2'-thiacynine iodide molecule embedded in an optical microcavity was theoretically studied. The molecule was coupled to a single cavity mode via the quantum Rabi Hamiltonian, and the corresponding time-dependent Schr\"odinger equation starting with a purely molecular excitation was solved using the Multiconfigurational Time-Dependent Hartree Method (MCTDH). We show that, for single-molecule strong coupling with the photon mode, nonadiabatic molecular dynamics produces mixing of polariton manifolds with differing number of excitations, without the need of counterrotating light-matter coupling terms. As a consequence, an electronic excitation of the molecule at {\it cis} configuration leads to the generation of photon pairs in the {\it trans} configuration upon isomerization. Conditions for this phenomenon to be operating in the collective strong light-matter coupling regime are discussed and found to be unfeasible for the present system, based on simulations of two molecules inside the microcavity. Yet, our finding suggests a new mechanism that, without ultrastrong coupling, achieves photon down-conversion by exploiting the emergent molecular dynamics arising in polaritonic architectures.