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Japanese Journal of Applied Physics

Atomic Collimation with a Single Laser Pulse

Masatoshi Kajita, Takao Morikawa

Published Nov 1, 1999• 1 citation
Publisher

Abstract

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.

DOI
10.1143/jjap.38.l1281

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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 need to collimate a beam of particles arises whenever one has to increase their phase space density (the goal is to compress a spatial and a velocity distributions of the atoms) and to increase a spatial coherence of atomic beam. The method of increasing the spatial coherence by using special scheme of collimation is only one so far known for atomic beam. There is optimal laser detuning when the atomic beam collimation is maximum. As the detuning is decreased, the degree of collimation drops drastically. Near zero detuning the broadening of the atomic beam can be clearly seen, and its intensity is decreased. The collimation of atomic beam by the retarding force was first observed with a cesium than with sodium atoms. The collimation of atomic beams by polarization gradient force was demonstrated as in one-dimensional and two-dimensional cases.
The transverse velocity distribution of a sodium atomic beam, laser cooled in one dimension, has been measured with sub-recoil resolution. A velocity-selective detection system was employed to separate the longitudinal and transverse velocity components to provide a direct measure of the transverse velocity distribution. The effects of varying the cooling laser intensity and detuning on the derived transverse temperature are presented, and show a remarkable insensitivity to the laser intensity. Atomic velocities within a factor of two of the recoil velocity are
A promising technique of atom slowing is proposed. It is based upon the dispersive interaction of atoms with optical potential pulses generated by a far-off-resonance standing wave modulated in time. Each pulse reduces the velocity by a small amount. By repeating the process thousands of times, the velocity can be lowered from several hundreds of meters per second down to almost zero, over a path as short as 20cm20\phantom{\rule{0.28em}{0ex}}\mathrm{c}\mathrm{m}20cm. In the absence of any random recoil process, the initial characteristics of the beam are preserved.
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=2m=2m=2 shows more efficacy compared to the normal Gaussian pulse.
We present the results of an experimental investigation of the control of atomic beam motion by the light pressure of laser radiation. Collimation, focusing, and reflection of the atomic beam are considered. Collimation of the atomic beam is achieved by the interaction of laser radiation with atoms, when the light pressure force depends only on the atom’s velocity. A similar regime of atomic beam interaction with radiation was performed with transversal irradiation of a beam by the axis-symmetrical field. The axis-symmetrical field was formed by laser radiation reflected from the conical mirror surface of a reflecting axicon. The axis of the atomic beam coincided with that of the axicon. The collimation regime was reached under negative detuning of the laser radiation frequency from the atomic transition frequency by a value equal to several homogeneous widths. With positive detuning by the same value the regime of beam decollimation was observed.