NONEQUILIBRIUM CONDENSATES AND LASERS WITHOUT INVERSION - EXCITON-POLARITON LASERS

We analyze elementary properties of exciton and polariton lasers - dev ices that generate coherent optical and matter waves using final-state stimulation of exciton-phonon scattering. First we discuss the relati on between the conditions for the onset of equilibrium and nonequilibr ium excitonic condensates. Provided that the thermal de Broglie wavele ngth lambda(T) exceeds the exciton Bohr radius a(B), an exciton laser operates without electronic population inversion. In contrast to previ ous proposals, this is a different type of laser without inversion whi ch utilizes many-body coherences. When the excitonic character of the polariton branch vanishes, a polariton laser becomes indistinguishable from a photon laser.