The effects of coherent excitation of a two-level system with a linearly chirped pulse are studied theoretically and experimentally [in Rb ( 5s-5p)] in the low field regime. The coherent transients are measured directly on the excited state population on an ultrashort time scale. A sharp step corresponds to the passage through resonance. It is followed by oscillations resulting from interferences between off-resonant and resonant contributions. We finally show the equivalence between this experiment and Fresnel diffraction by a sharp edge.
DOI: 10.1103/PhysRevLett.87.033001
View online : Phys. Rev. Lett. 87, 033001 (2001)