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

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.