  Time resolved photoluminescence is a powerful technique to study the
collective dynamics of excitons and polaritons in semiconductor nanostructures.
We present a two excitation pulses technique to induce the ultrafast and
controlled quenching of the exciton emission in a quantum well. The depth of
the dip is given by the magnitude of the warming of the carriers induced by the
arrival of a laser pulse when an exciton population is already present in the
sample. We use this technique to study the relaxation mechanisms of polaritons
in semiconductor microcavities, which are of great importance to enhance the
conditions for their condensation under non-resonant excitation. We also
explore the dynamics of polariton fluids resonantly created in the lower
polariton branch in a triggered optical parametric oscillator configuration,
showing evidence of polariton superfluidity, and opening up the way to the
real-time study of quantum fluids.
