Observation of coherent transients in ultrashort chirped excitation of an undamped two-level system - art. no. 033001

The effects of coherent excitation of a two-level system with a linearly ch irped pulse are studied theoretically and experimentally [in Rb (5s - 5p)] in the low field regime. The coherent transients are measured directly on t he excited state population on an ultrashort time scale. A sharp step corre sponds to the passage through resonance. It is followed by oscillations res ulting from interferences between off-resonant and-resonant contributions. We finally show the equivalence between this experiment and Fresnel diffrac tion by a sharp edge.