  Nanometer-thick quantum-well structures are quantum model systems offering a
few discrete unoccupied energy states that can be impulsively filled and that
relax back to equilibrium predominantly via spontaneous emission of light. Here
we report on the response of an indirectly excited quantum-well
heterostructure, probed by means of time and frequency resolved
photoluminescence spectroscopy. This experiment provides access to the
sub-nanosecond evolution of the free electron density, indirectly injected in
the quantum-wells. In particular, the modelling of the time-dependent
photoluminescence spectra unveils the time evolution of the temperature and of
the chemical potentials for electrons and holes, from which the sub-nanosecond
time-dependent electron density is determined. This information allows to prove
that the recombination of excited carriers is mainly radiative and bimolecular
at early delays after excitation, while, as the carrier density decreases, a
monomolecular and non-radiative recombination channel becomes relevant. Access
to the sub-nanosecond chronology of the mechanisms responsible for the
relaxation of charge carriers provides a wealth of information for designing
novel luminescent devices with engineered spectral and temporal behavior.
