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Journal of the Optical Society of America B

Cooling atoms with stimulated emission

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

Published Feb 1, 1987• 0 citations
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Abstract

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.

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Cooling Atoms with Stimulated Emission

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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.

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Using six optical beams (+ and − directions on three orthogonal axes) tuned slightly to the red of resonance it is possible to cool sodium atoms to a few hundred microkelvin. The cooling occurs because the laser radiation pressure creates a damping force which reduces the atomic velocities to a rms value of ~60 cm/s, determined by random scattering. In this optical molasses, the laser beams propagate in all directions and the light acts as a viscous photon fluid which damps the atomic velocity. The resulting motion of the sodium atoms is diffusive until the atoms reach the edge of the laser beams where they escape. The process of atomic confinement by optical molasses can be modeled as a random walk in a viscous medium with a boundary. For a spherical volume of 0.2 cm3 the resulting confinement time is ~0.1 s for half of the atoms to escape. Direct time-of-flight measurement of the atomic temperature gives ~240 μK, which agrees with predictions of quantum limited cooling by resonance radiation pressure. With confinement times of 0.1 s and temperatures of ~240 μK this method provides a source of atoms sufficiently slow for even the highest resolution optical spectroscopy, collision studies, or optical traps.

Annual Meeting Optical Society of America•Jan 1, 1985•1 citation
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.
Laser cooling in standing waves is related to the study of coherent optical transients. Transients in the u-component of the optical Bloch vector lead to a simple and unified physical picture of cooling at both low saturation parameter (optical molasses) and high saturation parameter (stimulated cooling). Optical molasses is shown to result from a free induction decay transient depending on the coherence relaxation time T2, while stimulated cooling is an incoherent effect of the population relaxation time T1. The sign, tuning, and saturation characteristics of each are attributed to specific transient phenomena. The model offers a bare-state physical picture of laser cooling which complements the dressed-state "Sisyphus" model. Phase-interrupting processes which make T2 < 2T1 reduce the trap potential by a factor of T2/2T1 and lead to a cooling force proportional to T2–2T1 which has not appeared in previous theories. The new term dominates both optical molasses and stimulated cooling when T2 «T1.
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.
The results of a recent experiment on the mechanical interaction of a bichromatic light beam with a single three-level ion serve to redefine laser cooling of ions in a trap and of atoms in standing light waves. Novel variants of light forces are demonstrated. They are caused by stimulated Raman-type two-photon transitions combined with spontaneous Raman scattering and by all-stimulated parametric interaction. These efficient processes parallel the generation of a stimulated light force in a standing wave and the operation of a free-electron laser, respectively. The latter force imposes no natural limit on the residual excitation energy of trapped particles.
Using six optical beams (+ and − directions on three orthogonal axes) tuned slightly to the red of resonance it is possible to cool sodium atoms to a few hundred microkelvin. The cooling occurs because the laser radiation pressure creates a damping force which reduces the atomic velocities to a rms value of ~60 cm/s, determined by random scattering. In this optical molasses, the laser beams propagate in all directions and the light acts as a viscous photon fluid which damps the atomic velocity. The resulting motion of the sodium atoms is diffusive until the atoms reach the edge of the laser beams where they escape. The process of atomic confinement by optical molasses can be modeled as a random walk in a viscous medium with a boundary. For a spherical volume of 0.2 cm3 the resulting confinement time is ~0.1 s for half of the atoms to escape. Direct time-of-flight measurement of the atomic temperature gives ~240 μK, which agrees with predictions of quantum limited cooling by resonance radiation pressure. With confinement times of 0.1 s and temperatures of ~240 μK this method provides a source of atoms sufficiently slow for even the highest resolution optical spectroscopy, collision studies, or optical traps.