  We study the photoluminescence dynamics of ultra-cold indirect excitons
optically created in a double quantum well heterostructure. Above a threshold
laser excitation, our experiments reveal the apparition of the so-called inner
photoluminescence ring. It is characterized by a ring shaped photoluminescence
which suddenly collapses once the laser excitation is terminated. We show that
the spectrally resolved dynamics is in agreement with an excitonic origin for
the inner-ring which is formed due to a local heating of indirect excitons by
the laser excitation. To confirm this interpretation and exclude the ionization
of indirect excitons, we evaluate the excitonic density that is extracted from
the energy of the photoluminescence emission. It is shown that optically
injected carriers play a crucial role in that context as these are trapped in
our field-effect device and then vary the electrostatic potential controlling
the confinement of indirect excitons. This disruptive effect blurs the
estimation of the exciton concentration. However, it suppressed by smoothing
the electrostatic environment of the double quantum well by placing the latter
behind a super-lattice. In this improved geometry, we then estimate that the
exciton density remains one order of magnitude smaller than the critical
density for the ionization of indirect excitons (or Mott transition) in the
regime where the inner-ring is formed.
