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

Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824(Dated: February 6, 2008)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 propagatein the chain. We find that the presence of optical excitation in the polymer induces strain on thelattice. We use a BCS variational wavefunction to calculate the chemical potential of the polaritonsas a function of their density. We analyze first the case of a short chain with only two unit cells inorder to check the validity of our variational approach. In the case of a long chain and for a strongcoupling with the lattice, the system undergoes a phase transition corresponding to the self-trappingof polaritons. The role of the exciton spontaneous emission and cavity damping are discussed in thecase of homogeneous optical lattice strain.I. INTRODUCTION