Light Emission and Lasing of a Partially Ionized Electron-Hole Plasma

Light emission of semiconductors is discussed for stationary excitation of arbitrary strength ranging from the low density and purely excitonic state up to the fully ionized state of the excited electron-hole plasma. Footing on nonequilibrium Green's functions the quantized electromagnetic field is consistently considered inside the semiconductor and for the surrounding vacuum outside solving the Dyson equation of photons. In contrast to common understanding it appears, that light emission and even lasing goes always spontaneously, however into a photonic density of states which is strongly renormalized compared with the vacuum one due to the presence of the semiconductor gain medium. The many-body effects of the electron-hole plasma are accounted for within a T-matrix approach for quasiequilibrium, i.e. if optical losses in the carrier kinetics can be neglected. Emphasis is laid on a consistent treatment of bound and ionized electron-hole pair states and the correct description of the crossover from absorption to gain at the chemical potential. Basing on this analysis apparently different phenomena as Bose-Einstein condensation, excitonic gain or lasing, and the Mott-transition will be discussed and their intimate connection qualitatively demonstrated.