Theory of cavity-polariton self-trapping and optical strain in polymer chains

TL;DR

This paper develops a theoretical model for polariton behavior in polymer chains within a cavity, revealing a phase transition to self-trapping driven by optical excitation-induced lattice strain, with implications for exciton-photon interactions.

Contribution

It introduces a variational approach to analyze polariton-lattice coupling and predicts a self-trapping phase transition in long polymer chains under strong coupling conditions.

Findings

Polaritons induce strain on the polymer lattice.

A phase transition to self-trapping occurs at strong coupling.

The model accounts for exciton spontaneous emission and cavity damping.

Abstract

We consider a semiconductor polymer chain coupled to a single electromagnetic mode in a cavity. The excitations of the chain have a mixed exciton-photon character and are described as polaritons. Polaritons are coupled to the lattice by the deformation potential interaction and can propagate in the chain. We find that the presence of optical excitation in the polymer induces strain on the lattice. We use a BCS variational wavefunction to calculate the chemical potential of the polaritons as a function of their density. We analyze first the case of a short chain with only two unit cells in order to check the validity of our variational approach. In the case of a long chain and for a strong coupling with the lattice, the system undergoes a phase transition corresponding to the self-trapping of polaritons. The role of the exciton spontaneous emission and cavity damping are discussed in the case of homogeneous optical lattice strain.