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Physical Review A

Effective stimulated laser cooling of atoms with a three-level Λ configuration by two negatively detuned standing light waves

Michael Drewsen

Published Feb 1, 1995• 8 citations
Publisher

Abstract

A laser cooling scheme for atoms with an effective three-level \ensuremath{\Lambda} configuration is presented. The cooling force is obtained by using two standing light waves, each interacting only with one of the two atomic transitions and both detuned red with respect to the transition. For a given laser power, the force exceeds the largest force on a two-level atom by a factor of 2 or more and the atomic velocity capture range is at the same time significantly increased. The spatially averaged force as well as the spatially averaged momentum diffusion coefficient, as a function of the atomic velocity, is calculated. The semiclassical steady-state momentum distributions are presented for some specific cases and an experimental realization of the cooling scheme is proposed for the case of metastable helium.

DOI
10.1103/physreva.51.1407

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A laser cooling scheme for atoms with an effective three-level \ensuremath{\Lambda} configuration is presented. The cooling force is obtained by using two standing light waves, each interacting only with one of the two atomic transitions and both detuned red with respect to the transition. For a given laser power, the force exceeds the largest force on a two-level atom by a factor of 2 or more and the atomic velocity capture range is at the same time significantly increased. The spatially averaged force as well as the spatially averaged momentum diffusion coefficient, as a function of the atomic velocity, is calculated. The semiclassical steady-state momentum distributions are presented for some specific cases and an experimental realization of the cooling scheme is proposed for the case of metastable helium.
A simple, versatile and effective scheme is presented for intensifying atomic beams by transverse laser cooling with two separated, intense standing waves. The first standing wave is negatively detuned while the second is positively detuned with respect to the cooling transition. A realization of the scheme, involving only one laser and simple optics, is proposed. Semiclassical calculations suggest that with a laser power of 0.1-1 W and an interaction length of a few cm, a flux enhancement of approximately 10 2 -10 3 should be obtainable by a two-dimensional set-up. Specific results are presented for the case of metastable argon.
We present a general theoretical description of laser cooling of three-level atoms in a Λ configuration in two standing waves. Detunings, intensity, and spatial phase difference of the two waves are arbitrary. We demonstrate that, for a zero spatial phase shift, sub-Doppler cooling requires equal detunings. The basic mechanism is then coherent population trapping. For different detunings sub-Doppler cooling occurs only when the spatial phases of the two standing waves are different. This effect can be traced back to a mechanism of the Sisyphus type. These schemes of sub-Doppler cooling require neither polarization gradients nor magnetic fields.
We examine the laser cooling of three-level (Lambda) -atoms in the field of two standing waves when frequency detunings are considerably greater than natural width of the excited state as well as the Rabi frequencies in the system. It is shown that at the difference of frequency detunings is small when compared to the Rabi frequencies sub-Doppler cooling of the (Lambda) -atom is possible. We present the dependence of the light pressure force on the atoms velocity and dependence of cooled atoms temperature on the parameters of the exciting waves.
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.
A model is presented to calculate optical forces, friction, and diffusion for three-level atoms in a ladder configuration. This model is then applied to the ${2}^{3}{S}_{1}{\ensuremath{-}2}^{3}{P}_{2}{\ensuremath{-}3}^{3}{D}_{3}$ cascade in metastable helium. It is demonstrated that metastable helium atoms can be decelerated within a much shorter distance using cascade excitation by overlapping traveling waves. The equilibrium temperature representing the final width of the velocity distribution is calculated as a function of laser detuning and intensity. This equilibrium temperature is also calculated for the situation of overlapping standing waves (one-dimension), and is found to be well above the Doppler limit for two-level ${2}^{3}{S}_{1}{\ensuremath{-}2}^{3}{P}_{2}$ excitation. Numerical results are compared with existing experimental data.
We present a calculation of cooling of Rydberg atoms in bichromatic standing light waves. The atoms in our model are assumed to have a cascade three-level transition scheme, with the second excited level being the Rydberg state. The cooling fields, which also excite the atoms to the Rydberg state, consist of two standing waves whose frequencies are tuned near the two transition frequencies of the cascade atomic system. It is shown that the mechanisms of Sisyphus cooling and velocity-selective coherent population trapping can co-exist in such a configuration, and under certain conditions the majority of the laser-cooled atoms are in the Rydberg state.