  We observe ground-state polariton condensation in a two dimensional GaAs/AlAs
semiconductor microcavity under non resonant pulsed optical excitation. We
resolve the formation of a polariton condensate by studying the spatial,
angular, coherence, energy and transient dynamics of polariton
photoluminescence. For high excitation densities we also observe a transition
from the weak- to the strong-coupling regime in the time-domain and resolve the
build-up of a coherent polariton state.
