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 ${\lambda }_{\mathit{T}}$ exceeds the exciton Bohr radius ${\mathit{a}}_{\mathit{B}}$, 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. © 1996 The American Physical Society.