COHERENT SPECTROSCOPIC EFFECTS IN THE PROPAGATION OF ULTRASHORT PULSES THROUGH A 2-LEVEL SYSTEM

A theoretical and experimental investigation is made of the spectrum o f a near-resonant femtosecond laser pulse propagating through a two-le vel atomic system. Measurements made using a high-resolution Fabry-Per ot microcavity show that at the transition frequency the spectrum of t he transmitted pulse acquires a feature whose structure depends sensit ively on the pulse area, the pulse detuning, and the absorption path l ength. Our observations are in excellent agreement with the prediction s of the Maxwell-Bloch equations. In the thin-sample limit, these resu lts 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. [S 1050-2947(98)50801-3].