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Physical Review A

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

Abdelsalam H. M. Abdelaziz, Parvendra Kumar, Amarendra K. Sarma

Published Feb 6, 2019• 0 citations

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.

DOI: 10.1103/physreva.99.023408

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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.

We report on a study of the optical dipole force in a ladderlike three-level atomic system induced by the trains of few-cycle-pulse laser fields. In the present study, we consider the nonresonant as well as the chirped pulse excitation of three-level atoms. The so-called optical dipole force is calculated by numerically solving the force equation and the density matrix equations self-consistently beyond the rotating wave approximations. By analyzing the center-of-mass motion, it is shown that under nonresonant excitation, the optical dipole force induced by the trains of few-cycle pulses may be used for focusing and defocusing of atoms in an atomic beam. Moreover, we have demonstrated that the chirped nanosecond pulses may be used for both coherent population transfer and focusing or defocusing of atoms simultaneously.

We report a study on light force on a beam of neutral two-level atoms superimposed upon a few-cycle-pulse Gaussian laser field under both resonant and off-resonant conditions. The phenomena of focusing, defocusing, and steering of the neutral atoms in the laser field are analyzed by solving the optical Bloch equation beyond the rotating-wave approximation and the force equation self-consistently. We find that two-level atoms in an atomic beam can be focused and defocused for large, positively and negatively detuned interactions even in the regime of extreme nonlinear optics. The so-called optical potential may be used for stable trapping of the neutral two-level atoms for large positively detuned interaction. This work successfully reproduces some of the features reported in recent experimental and theoretical works.

In this letter we describe a new method of reducing one or two components of atomic velocity using an optical dipole force. The optical dipole force produces a harmonic potential transverse to the Gaussian optical beam. If the initial atomic position is close to the center of the beam, the transverse velocity component becomes almost zero after a quarter period of harmonic oscillation. If the laser is switched off at this moment, a very small transverse velocity component can be obtained. The use of a far-detuned laser reduces the heating due to spontaneous scattering and enables us to obtain a sub-photon recoil velocity component.

The optical trap potential is studied in a four-level double-Λ closed-loop atomic system in the multiphoton resonance condition. We apply four femtosecond Gaussian laser beams to generate a closed-loop configuration. It is shown that the induced optical dipole force on the system dramatically depends on the relative phase of applied fields, and it switches from focusing to defocusing in the transverse direction, simply by properly changing the relative phase. Through special switching of the relative phase, the trapping potential splits into two positions, which has novel applications in trapping and manipulating the particles. Moreover, it is further demonstrated that trap depth can be manipulated by the relative phase. Finally, a maximum coherent population transfer is achieved for a special value of the relative phase of applied fields.

The optical forces on neutral two-level atoms by the subcycle pulsed focused vector field are investigated by numerically solving the density-matrix equations without the rotating-wave approximation. The phenomena of focusing or defocusing of the neutral atom are analyzed. For the transverse optical force, an acceleration of an atom that is 10 orders of magnitude higher than the Earth's gravitational acceleration

g

can be obtained. The optical force can change from a focusing force to a defocusing force depending on the initial state of the atom. The optical potential may be used to trap the neutral two-level atoms in the experiment of cold atoms.