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Japanese Journal of Applied Physics

Accumulative Atom Detection by Magneto-Optical Trap for Near-Field Optical Funnel

Akifumi Takamizawa, Haruhiko Ito, Shungo Yamada, Motoichi Ohtsu

Published Jun 15, 2003• 4 citations
Publisher

Abstract

We use a magneto-optical trap as an integrating counter for descending cold atoms. In the case of cold 87 Rb atoms with an initial mean temperature of 9 µK, 60% of the total number of 2.6 × 10 7 are captured after a 26-cm free-fall and trapped for a long time of 270 s. From the capture efficiency and the decay time, the minimum detectable flux intensity of a cold 87 Rb atomic beam is estimated to be 2 × 10 5 atom/cm 2 ·s. The atom-counting system is used for detection and evaluation of a cold atomic beam produced out of a near-field optical funnel.

DOI
10.1143/jjap.42.3658

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We use a magneto-optical trap as an integrating counter for descending cold atoms. In the case of cold 87 Rb atoms with an initial mean temperature of 9 µK, 60% of the total number of 2.6 × 10 7 are captured after a 26-cm free-fall and trapped for a long time of 270 s. From the capture efficiency and the decay time, the minimum detectable flux intensity of a cold 87 Rb atomic beam is estimated to be 2 × 10 5 atom/cm 2 ·s. The atom-counting system is used for detection and evaluation of a cold atomic beam produced out of a near-field optical funnel.
We demonstrated an atom funnel using blue-detuned near-field light. Cold Rb atoms generated by a magneto-optical trap (MOT) were introduced into the funnel and the output atoms were counted by an accumulative MOT detection system. The number of cold Rb atoms outputted from a 200-/spl mu/m exit hole was maximal at a blue detuning of 1.2 GHz under Sisyphus cooling. The flux intensity was estimated to be 10/sup 5/ atoms/cm/sup 2/s. To increase the flux intensity up to 10/sup 13/ atoms/cm/sup 2/s, we made an atom funnel with a 2-/spl mu/m exit hole.
Multireflection of cold Rb atoms occurs inside a reversed triangular hollow prism with a blue-detuned optical near field. We examine it with a probe light beam, using the frequency detuning and the diameter of an excitation light beam as parameters. In the first bounce, the number density of the reflected atoms reaches a maximum of 10 6 atom/cm 3 at a blue detuning of 0.7 GHz when the beam diameter is 4 mm. The Rb atoms repeat reflection with a period of about 15 ms and an efficiency of 83% per bounce. A cold Rb atomic beam with a high flux of 4 × 10 11 atom/(cm 2 ·s) can be generated through a near-field optical funnel composed of the prism.
Based on the real sense of the time-of-flight , we demonstrate an alternative method of measuring the temperature of cold atoms in magneto-optical traps (MOTs) using a high-finesse optical microcavity, which acts as a pointlike single-atom counter. A cloud of atoms trapped in magneto-optical traps is positioned about 5 mm above the cavity, and the atoms fall freely down through the cavity. The temperature of the cold atoms in the MOT is determined by counting the exact arrival times of the single atoms. A theoretical model based on a ballistic expansion of a cloud of trapped atoms falling in the earth’s gravitational field is used to fit the probability distribution of atom arrivals, and the fittings agree very well with the experimental results. This method could be used for systems with little room, where an extra probe beam is hard to involve, or with fewer atoms initially.