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Physical Review Letters

Cooling Atoms with Stimulated Emission

Alain Aspect, Jean Dalibard, A. Heidmann, Ch Salomon, Claude N. Cohen-Tannoudji

Published Oct 6, 1986• 187 citations
Publisher

Abstract

We have observed an efficient collimation of a cesium atomic beam crossing at right angles an intense laser standing wave. This new cooling scheme is mainly based on a stimulated redistribution of photons between the two counterpropagating waves by the moving atoms. By contrast with usual radiation pressure cooling, this "stimulated molasses" works for blue detuning and does not saturate at high intensity.

DOI
10.1103/physrevlett.57.1688

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We have observed an efficient collimation of a cesium atomic beam crossing at right angles an intense laser standing wave. This new cooling scheme is mainly based on a stimulated redistribution of photons between the two counterpropagating waves by the moving atoms. By contrast with usual radiation pressure cooling, this "stimulated molasses" works for blue detuning and does not saturate at high intensity.
We have observed a new type of laser cooling1mainly based on stimulated emission, which can be much more efficient than the usual laser cooling, i.e., the so-called radiation pressure optical molasses. This cooling occurs in a strong laser standing wave, detuned blue with respect to the atomic resonance. The force exerted on the atom results from a coherent redistribution of photons between the two running waves forming the standing wave. The cooling process can also be seen in the dressed atom picture as a microscopic realization of the Sisyphus myth: the atoms in this diagram see more uphill parts than downhill parts.
The process of laser cooling in a quasi resonant standing laser wave is one of the principal techniques of laser cooling of free atoms. It will be shown that the nonlinear interaction between the counter propagating waves in a standing wave have a profound affect on the mechanical manifestations of the light on the atoms. This can be understood simply by noting that in the Two Wave Mixing (TWM) process, a momentum kick of 2ℏk is transferred to the atom, as a photon is absorbed from one wave and emitted into the counter propagating wave. This process is in fact responsible for the so called "Dipole force" Or "Stimulated Molasses" and has been verified experimentally!. However, since the TWM process requires high laser intensity under normal conditions it does not usually lead to lower equilibrium temperatures. Nevertheless, It has been shown that the TWM process can be modified when the normal decay rates of the atom are altered. This modification can give rise to the appearance of the TWM at lower intensity and can even change its lineshape. This in turn have an important implications on the laser cooling process in a standing wave. Among the important effects: (i) The appearance of the stimulated force at lower intensity when the dipole decay rate is increased by dephasing events.(ii) The appearance of a narrow resonance characterized by the ground state decay in an open system. (iii) The modification of the stimulated force when the quantum fluctuations of the vacuum field are modified by quadrature squeezing3. Processes (ii) and (iii) can change the sign of the stimulated force from a heating force to a cooling force at the red side of the atomic resonance and give additional cooling force much larger than the radiation pressure force.
A new method is proposed for laser cooling atoms with Doppler shifts an order of magnitude larger than the homogeneous linewidth of the atomic transition. Two traveling waves cross at a small angle and form standing waves whose wavelength is much longer than that of either traveling wave. This allows one to extend the region in which the laser cooling force is proportional to velocity. The resulting broadband stimulated cooling forces are predicted to stop 0.1% of the flux of a 500-K atomic sodium beam in a distance of 300 μm and a time of a few microseconds. This is more than 100 times faster than current methods. Numerical integration and the method of Gordon and Ashkin have been used to calculate the cooling forces which result when two traveling waves cross at an angle 2a. The stimulated cooling forces are reduced in intensity by sin2a but are proportional to velocity over an interval which is larger by 1/sin a. This allows one to stop a substantial fraction of an atomic beam without chirping the laser or Zeeman tuning the atoms.
An atom moving in a strong standing wave radiation field may be translationally cooled by the induced or dipole force.1 The dipole force is proportional to I1/2, where I is the laser intensity, whereas the Doppler (usual) cooling force saturates with increasing I. Since laser sources can provide intensities which are much greater than the saturation intensities of many atomic transitions, cooling via the dipole force can be much more efficient than Doppler cooling. The dipole force has been used to cool the transverse2,3 and the longitudinal4 velocities of atoms in an atomic beam.
The optical forces exerted on a slowly moving two-level atom in a standing wave are profoundly modified when the normal decay rates of the atom are altered. The theory presented in this paper shows that the stimulated emission force becomes dominant at a very low laser intensity. Furthermore, it can change sign to provide an additional cooling force instead of a heating force at laser detunings below the atomic resonance. These phenomena can lead to equilibrium temperatures below the Doppler limit.