Polariton laser and polariton superfluidity in microcavities

Excitons coupled to light in microcavities form exciton-polaritons (polaritons) having very light effective masses. These 2-dimensional quasiparticles exhibit local condensation or Kosterlitz-Thouless phase transition towards superfluidity, allowing for polariton lasing at temperatures that could be higher than 300 K. Polariton lasers are devices producing monochromatic and coherent light spontaneously emitted by Bose-condensed exciton-polaritons. They do not require inversion of population and, theoretically, have no threshold. We show great advantages of cavity polaritons with respect to excitons to achieve bosonic phase transition. We review experimental and theoretical activity that has been developed in this research field since the discovery of strong exciton-light coupling in microcavities 10 years ago. We present basic thermodynamic properties of polariton systems modeled as a weakly interacting Bose gas at equilibrium. We present an analysis of the relaxation kinetics of cavity polaritons in the framework of a semiclassical Boltzmann equation. The main obstacle that prevents polariton condensation is their finite lifetime: polaritons disappear before they have time to condense. We discuss the ways to improve their relaxation kinetics in real structures