Ortho-exciton polariton relaxation by phonon scattering in Cu2O

The Boltzmann relaxation kinetics of ortho-excitons by scattering with thermal acoustic phonons is treated, taking into account the weak polariton effect due to the quadrupole-allowed optical transitions. The relatively small polariton splitting is shown to be sufficient to allow a decay of the upper-branch polaritons into the states on the lower branch. Because of this decay, the Bose–Einstein condensation in the k=0 state of the upper-branch polaritons is, even without other finite lifetime effects, only a transient phenomenon.