Superluminal to subluminal transition in the pulse propagation in a resonantly absorbing medium

We have examined the propagation of femtosecond laser pulses in an absorbin g dye solution through a short to a long range of propagation distance. The transmitted pulses show strong spectral shift and a superluminal to sublum inal transition in the propagation velocity keeping its initial shape almos t intact. It is verified that the peak velocity is well described by a modi fied group velocity vs defined within the framework of the saddle-point met hod as well as by a recent prediction of the net group delay of surviving f requency.