Physics and applications of exciton-polariton lasers

Soon after the demonstration of Bose-Einstein condensation of atomic gases [1, 2], it was predicted that a novel coherent light source with an extremely low threshold carrier density can be realized by exploiting the quantum condensation in exciton-polariton systems [3]. Exciton-polaritons are quasiparticles arising from the strong coupling between bound electron–hole pairs (excitons) and photons trapped inside a high-finesse optical resonator. Due to their bosonic character, these half-light, half-matter quasiparticles can form condensates, in close analogy to atomic Bose–Einstein condensates. As they exhibit very low effective mass, polariton condensates can be formed at low density and high temperature. Following the theoretical proposal on polariton lasers, the condensation of exciton-polaritons has been experimentally demonstrated for various material systems including inorganic semiconductors (such as GaAs [4], CdTe [5], GaN [6] and ZnO [7]), and organic materials [8]. The exciton-polariton condensate, which is sometimes referred to as the exciton-polariton laser, is distinct from the conventional superfluid and the Bose-Einstein condensates (BEC) [9] due to its inherent open-dissipative nature and two-dimensional confinement. In this commentary, we review some interesting physics and demonstrations reported in the last 20 years, and highlight other potential applications of this unique system.