Nonequilibrium condensates and lasers without inversion: Exciton-polariton lasers

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