INTERFERENCE IN CLIMBING A QUANTUM LADDER SYSTEM WITH FREQUENCY-CHIRPED LASER-PULSES

Excitation Of a quantum ladder system by frequency-swept (-chirped) la ser pulses has been investigated by studying an atomic model system, t he 5s-5p-5d ladder of rubidium. The population transfer is significant ly enhanced when the frequency is swept such that it follows the spaci ng of the ladder. At intermediate intensities we observe oscillations in the population transfer to the upper level (5d), which can be attri buted to interference between two routes of excitation: sequential tra nsfer and direct two-photon excitation via a virtual intermediate stat e. We determine simultaneously the population transfer to the upper le vel(5d) and the direct three-photon ionization, the latter correspondi ng to a three-step !adder with the final state being in the continuum: 5s-5p-5d-epsilon p/epsilon f. Experiments were performed, using 100 f s pulses at similar to 780 nm from a Self-mode-locked Ti:Al2O3 oscilla tor amplified at 10 Hz.