Abstract

In this report we present the experimental observation of rings of sodium atoms in a cold vapor cell and a theoretical model based in the existence of vortex forces leading to stable circular orbits. The apparatus for this experiment consists of a Zeeman-shift spontaneous-force trap⁽¹⁾, similar to the one used for Cs atoms⁽²⁾. The magnetic field is produced by a pair of anti-Helmholtz coils and the three circularly polarized retro-reflected laser beams are tuned to the 3S_1/2 (F=2) → 3P_3/2 (F=3) transition of sodium. All laser beams carry a 1712 Mz sideband frequency working as a repumper for atoms in the 3S_1/2 (F=1) ground state. When all beams are well aligned with respect to the retro-reflection and magnetic field and are detuned 10 MHz below resonance, we observe a bright ball with 1 to 2 mm at the trap center. If one of the beams is misaligned by a small rotation of the retro-reflector, the initially spheric ball deforms to a dwell shape. When a second beam is also misaligned, the formation of a ring is observed. The ring increases its radius and keeps the same width as the laser is scanned to larger values of red detunings, as shown in Fig. 1. For detunings larger than Δ = 25 MHz the ring disappears. By adding a small misalignment to the third orthogonal beam we observed a change of the plane where the ring is formed.

© 1991 Optical Society of America