Coherent spectroscopic effects in the propagation of ultrashort pulses through a two-level system

A theoretical and experimental investigation is made of the spectrum of a near-resonant femtosecond laser pulse propagating through a two-level atomic system. Measurements made using a high-resolution Fabry-Perot microcavity show that at the transition frequency the spectrum of the transmitted pulse acquires a feature whose structure depends sensitively on the pulse area, the pulse detuning, and the absorption path length. Our observations are in excellent agreement with the predictions of the Maxwell-Bloch equations. In the thin-sample limit, these results show that from the shape of the spectral feature it is possible to infer the quantum state of the atomic system excited by the pulse. [S1050-2947(98)50801-3].