  Time-resolved photoluminescence spectra after nonresonant excitation show a
distinct 1s resonance, independent of the existence of bound excitons. A
microscopic analysis identifies excitonic and electron-hole plasma
contributions. For low temperatures and low densities the excitonic emission is
extremely sensitive to even minute optically active exciton populations making
it possible to extract a phase diagram for incoherent excitonic populations.
