Effective focusing of a diverging atomic beam by a sequence of alternatively chirped few-cycle pulsed laser fields

Abstract

We theoretically show how the effective focusing and defocusing of a diverging atomic beam can be realized using an appropriate sequence of linearly chirped few-cycle pulsed laser fields. First, we investigate and show that the time averaged optical dipole force induced by the sequence of either positively or negatively chirped pulses vanishes due to the periodic oscillations of the phases of in-phase components of atomic dipole moments. Then, we demonstrate that this issue could be overcome by utilizing a sequence of alternatively chirped pulses instead of either positively or negatively chirped pulses. It is shown that for such a sequence of pulses, the phases of in-phase components of atomic dipole moments does not oscillate periodically and in fact remain constant at either 0 or $\ensuremath{\pi}$ depending on the chirping direction of the initial pulse, resulting in either an effective focusing or defocusing optical dipole force. The trajectory of atoms under an optical dipole force is also investigated to actually show the focusing and defocusing. Finally, the role of beam shape on the proposed scheme is investigated and it turns out that a super-Gaussian pulse of order $m=2$ shows more efficacy compared to the normal Gaussian pulse.