  We study the loading of a nonequilibrium, dissipative system of composite
bosons -- exciton polaritons -- into a one dimensional periodic lattice
potential. Utilizing momentum resolved photoluminescence spectroscopy, we
observe a transition between an incoherent Bose gas and a polariton condensate,
which undergoes further transitions between different energy states in the
band-gap spectrum of the periodic potential with increasing pumping power. We
demonstrate controlled loading into distinct energy bands by modifying the size
and shape of the excitation beam. The observed effects are comprehensively
described in the framework of a nonequilibrium model of polariton condensation.
In particular, we implement a stochastic treatment of quantum and thermal
fluctuations in the system and confirm that polariton-phonon scattering is a
key energy relaxation mechanism enabling transitions from the highly
nonequilibrium polariton condensate in the gap to the ground band condensation
for large pump powers.
