Switching from a sequential transition to quantum beating in atomic rubidium pumped by a femtosecond laser

We demonstrate temporal coherent control in two-photon transitions. A pair of broadband (Δλ ̃ 20 nm), ultrashort (Δt ̃ 100 fs), collinear pulses with a variable time delay excites the rubidium atoms into the 5 D state from the 5 S ground state where the two-photon transitions are enhanced by the intermediate level 5 P. The excited atoms radiate 5 μm (5 D-6 P) and 420 nm (6 P-5 S) light. As a result of tuning the wavelength of the input laser, a superfluorescence at 420 nm exhibits different temporal behaviors. A switching from a beating at the frequency given by the difference between the sequential atomic transitions that involve the 5P3=2 intermediate level, to a quantum beating due to two different two-photon excitation paths, 5S → 5P → 5D and 5S → 5P → 5D is observed. Based on the simple atom-field interaction theory, an analytic solution, which qualitatively elucidates experimental results, is obtained. © 2011 Optical Society of America.