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Physical Review A

Observation of three-level rectified dipole forces acting on trapped atoms

Timothy T. Grove, B. C. Duncan, V. Sánchez-Villicaña, P. L. Gould

Published Jun 1, 1995• 29 citations
Publisher

Abstract

We have observed rectified dipole forces acting on three-level atoms in the cascade configuration. Laser cooled and trapped rubidium atoms are illuminated with an intense bichromatic standing wave (780 and 776 nm) tuned near resonance with the 5${\mathit{S}}_{1/2}$\ensuremath{\rightarrow}5${\mathit{P}}_{3/2}$\ensuremath{\rightarrow}5${\mathit{D}}_{5/2}$ transitions. The resulting rectified forces produce periodic potential wells (71-\ensuremath{\mu}m period), which localize the cold atoms. Experimental results are in reasonable agreement with theoretical predictions. These forces may be useful in atom optics and laser traps.

DOI
10.1103/physreva.51.r4325

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We have observed rectified dipole forces acting on three-level atoms in the cascade configuration. Laser cooled and trapped rubidium atoms are illuminated with an intense bichromatic standing wave (780 and 776 nm) tuned near resonance with the 5S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\rightarrow}5P3/2{\mathit{P}}_{3/2}P3/2​\ensuremath{\rightarrow}5D5/2{\mathit{D}}_{5/2}D5/2​ transitions. The resulting rectified forces produce periodic potential wells (71-\ensuremath{\mu}m period), which localize the cold atoms. Experimental results are in reasonable agreement with theoretical predictions. These forces may be useful in atom optics and laser traps.
Rectified dipole forces1 offer the possibility of providing deep potential wells for neutral atoms. In the intense standing wave, a two-level atom can experience a large dipole force due to the large intensity gradient. However, this force changes sign on the optical wavelength scale, preventing the formation of deep potential wells. If a second, nearly commensurate, standing wave is added, its ac Stark shift causes the effective detuning of the first field to vary on the optical wavelength scale. The resulting change in sign of the detuning can counteract the change in sign of the intensity gradient, causing the force to maintain its sign over many wavelengths (i.e., the force becomes rectified), thereby forming a deep potential well. This rectified force is periodic with the beat wavelength between the two laser fields. In a three-level atom the same basic mechanism applies, but with the advantage that the two frequencies of the bichromatic standing wave can now be tuned dose to resonance with the two transitions of the atom.
We show that a three-level atom in the cascade configuration can be stably trapped and cooled in one dimension by an intense bichromatic standing wave. At the two-photon resonance, rectified dipole forces result in a deep potential well which can be used to localize the atoms in space. In the vicinity of the rectified potential minimum, the spatial dependence of the dressed state energies can lead to a velocity dependence of the force which produces damping of the atomic motion. Consideration of the heating effects of momentum diffusion indicates that cooling and stable trapping at low temperatures is possible in such a bichromatic field.
We show that an imperfection of velocity-selective coherent population trapping (VSCPT) in three-level atoms excited by standing light waves causes a rectified force on cooled atoms. The rectified force as well as the cooling force are calculated both analytically and numerically for and cascade three-level systems. Combination of these forces with the VSCPT mechanism can lead to localization of very cold atoms in potential wells created by the rectified force. This effect should be taken into account in experiments with VSCPT in standing waves, and can be used for realizing superlattices of cold atoms, in particular, cold Rydberg atoms.
A sodium atomic beam is deflected by two optical standing-wave fields which excite near resonance Raman transitions. Observed deflections are consistent with theoretical predictions of a long-range dc component of the force on a three-level atom in the \ensuremath{\Lambda} configuration. The low laser intensities (and small detunings) used, combined with observed evidence of damping in the three-level \ensuremath{\Lambda} system, may ultimately lead to the design of high-density, all optical traps wherein the atoms would be mostly in the dark state.
We have examined the mechanical effects of light on a three level atom. A single excited state decays to two long-lived ground states. One of the transitions is driven by an intense standing wave, while the other is driven by a traveling wave. We examine this system using analytical and computational techniques, and compare with experimental observations of {sup 7}Li. In the absence of the second ground state and laser field, the atom can be cooled by the familiar two level dipole force. In the three level system, the dipole force cooling mechanism is still present, and can be considerably enhanced. As in the two level system, the cold three level atoms are channeled, or localized in space with respect to the standing wave. In the three level case, the atoms can be channeled in the second ground state, even though that state interacts only with the traveling wave laser, which has no spatial structure. We will discuss applications of the three level cooling force to atomic beam slowing experiments, and the attainment of ultra-cold final temperatures using adiabatic cooling.
Cold atoms are traditionally trapped by the dipole force in periodically spaced potential wells induced by the standing laser field. We derive here a theory beyond the conventional dipole approximation which provides field/atom coupling potential terms that so far have not been taken into consideration in theoretical or experimental studies. We show that for some atoms for specific laser parameters despite the absence of dipole transition laser trapping is still possible due to the quadrupole force. Illustrative numerical calculations for Ca and Na trapped by the quadrupole force are given.