Optical precursors with finite rise and fall time

We report results of both theoretical and experimental studies of optical precursors generated from a square-modulated probe laser pulse having a finite rise and fall time and propagating through a cold atomic ensemble, under the conditions of either a two-level Lorentz absorber system or a three-level system with electromagnetically induced transparency (EIT). Because of the finiteness of the rise (and fall) time, the precursor signal is observed to decrease with increasing optical depth (alpha L-0). We find that the precursor can experience little absorption even at high optical depth if the rise (and fall) time is sufficiently short. At an optical depth of alpha L-0 = 42, the normalized precursor peak intensity is observed to increase from 9% to 27% when the rise (and fall) time is shortened from 7 to 3 ns. Meanwhile, we reaffirm that there is no violation of Einstein's causality principle in light propagation through both slow and fast light media. In the EIT system with high optical depth, the main field propagates with a subluminal group velocity and it is separated from the precursor. In the two-level system, the effect of negative group velocity in the anomalous dispersion regime is observed, but we detect no advancement in the rising edge of the precursors. In both cases, the leading edges of the precursors show no detectable delay to that through vacuum.