We develop an approach to the focusing of neutral atomic beams by 2D laser fields. The approach is based on the use of the light pressure force on atoms in the presence of a nonuniform magnetic field. Such an approach was proposed for increasing the phase density of atomic ensembles. In our scheme, the 2D MOT is formed by two mutually orthogonal circularly polarized standing laser waves propagating along the axes of the reference frame and a spatially nonuniform magnetic field.

A feature of the focusing of the atomic beam by means of the 2D MOT is its dissipative character, arising due to a decrease in the kinetic energy of atoms by reemitted photons. In view of this circumstance, the transverse velocity of atoms leaving the 2D MOT in the focusing mode is a function of only the transverse coordinate of the atom and the parameters of the trap field [1]. The focal length, defined as the distance from the 2D MOT to the intersection point of the beam axis by atoms, depends only on the longitudinal velocity V_|| of the atom and the trap parameters.

Figure shows the experimental profiles of the transverse spatial distribution of the atoms in the beam with a longitudinal velocity of 100 m/s when the 2D MOT is switched off and on.

As is seen in next figure, the action of the 2D MOT on the atomic beam reduces its transverse size in the detection region from 2.5 mm to 270 µm.

Profiles of the transverse spatial distribution of the atoms in the initial thermal beam before and after its interactionwith the 2D MOT. The longitudinal velocity of the atoms is equal to 100 m/s.

Also we inclined the 2D MOT by 2° to the atomic beam axis. The probing region of the velocity distribution was located on the axis of the 2D MOT. In this case, atoms are focused at a spatial point outside the atomic beam. For this reason, atoms are absent in the detection region when the 2D MOT is switched off. Figure shows the velocity selection of atoms in this case. The solid line is the initial distribution and the points show the distribution after the focusing of the atomic beam. The velocity distributions shown in this figure indicate that fast atoms are efficiently blocked in this configuration so that the most probable atomic velocity in the beam decreases from 340 to 100 m/s. The FWHM of the distributionis equal to 60 m/s, which is much less than the corresponding value for the thermal distribution of the atoms (~350 m/s) [2].

Schema of velocity monochromatization of the atoms that are focused by the 2D MOT which deviates by 2° from the axis of the atomic beam.

Velocity monochromatization of the atoms that are focused by the 2D MOT which deviates by 2° from the axis of the atomic beam.

References:

1. V. I. Balykin, "Pulsed magneto-optical compression of cold atoms", JETP Lett., Vol. 66, 349 (1997).
2. P. N. Melentiev, P. A. Borisov, S. N. Rudnev, A. E. Afanasiev, and V. I. Balykin, "Focusing of an Atomic Beam by a Two-Dimensional Magneto-Optical Trap", JETP Letters, Vol. 83, No. 1, pp. 14–18 (2006).