Slow and stored light and optical depth
  We present a preliminary experimental study of the dependence on optical depth of slow and stored light pulses in Rb vapor. In particular, we characterize the efficiency of slow and stored light as a function of Rb density; pulse duration, delay and storage time; and control field intensity. Experimental results are in good qualitative agreement with theoretical calculations based on a simplified three-level model at moderate densities. 
M. Klein, Y. Xiao, A.V. Gorshkov, M. Hohensee, C.D. Leung, M.R.   Browning, D.F. Phillips, I. Novikova, R.L. Walsworth