  The effects of high optical depth phenomena, such as superradiance, are
investigated in potential quantum memory materials. The results may have
relevance for several schemes, including CRIB, AFC and EIT-based quantum
memories, which are based on using ensembles as storage media. It is shown that
strong superradiant effects, manifested as decay rates larger than 1/T2*, are
present even for moderate values of alphaL < 5, and increases as a function of
alphaL. For even higher alphaL, effects like off-resonant slow light is
demonstrated and discussed, and finally, the efficiency of time-reversed
optimized input pulses are tested. A maximum retrieval efficiency of ~20% is
reached, and agreement with the theoretically expected result is discussed.
