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[en] An ultrashort weak pulse that propagates resonantly on the a>→b> transition of a three-level system (a>,b>, vertical bar c> vertical bar experiences reshaping effects on a time scale longer than its duration. When a strong field is applied (nonresonantly) on the b>→c> transition, important light shifts are induced. We have theoretically studied the novel behavior of both the medium and weak propagating laser pulses in such conditions. For the transmitted field profile, drastic changes occur: at long times, efficient reduction of the distortion is obtained, while strong oscillations appear for short times, mapping out the dynamical light shift. These shaping effects, which are accompanied by new features in the spectrum for the propagating pulse, are interpreted as the result of the interference between incident field and induced dipole radiated field, whose frequency sweeps in time because of the induced light shift
[en] The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale. (author)
[en] We study theoretically the temporal shaping effects experienced by a one-photon wavepacket propagating in a one-dimensional waveguide containing a two-level atom. We show that the transmitted field distorts such that its pulse area (time integral of electric field amplitude) vanishes. As a consequence, specific pulses with initial zero pulse area are shown to be robust with respect to propagation whereas pulses with initial non-vanishing pulse area are shown to be more sensitive, leading to significant distortion after propagation.
[en] Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments