  Dark excitons are of fundamental importance for a wide variety of processes
in semiconductors, but are difficult to investigate using optical techniques
due to their weak interaction with light fields. We reveal and characterize
dark excitons injected by non-resonant pulsed excitation in a semiconductor
microcavity structure containing InGaAs/GaAs quantum wells by monitoring their
interactions with the bright lower polariton mode. We find a surprisingly long
dark exciton lifetime of more than 20 ns which is longer than the time delay
between two consecutive pulses. This creates a memory effect that we clearly
observe through the variation of the time-resolved transmission signal. We
propose a rate equation model that provides a quantitative agreement with the
experimental data.
