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arXiv:0901.1671 · Physical Review A

Laser cooling with a modified optical shaker

L. Marmet

Published Jan 15, 2009• 1 citation

Abstract

Some practical improvements are proposed for the ``optical-shaker'' laser-cooling technique [I. S. Averbukh and Y. Prior, Phys. Rev. Lett. 94, 153002 (2005)]. The improved technique results in an increased cooling rate and decreases the minimum cooling temperature achievable with the optical shaker. The modified shaker requires only one measurement of the force on the atoms before each cooling step, resulting in a simplification of the feedback electronics. The force is inferred from the power variations of the transmitted laser beams and is used to determine the best moment at which the cooling steps are applied. The temperature of the atomic system is automatically monitored, which allows maintaining an optimum cooling rate as the temperature decreases. The improved shaker is simple to build, provides a faster rate of cooling, and can work in the microkelvin regime. Numerical modeling shows a reduction by a factor of 3 in the required number of phase jumps and a lower temperature limit reduced by an order of magnitude compared to the initially proposed shaker. The technique is also extendable to cooling in three dimensions.

DOI: 10.1103/physreva.79.013412

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Some practical improvements are proposed for the ``optical-shaker'' laser-cooling technique [I. S. Averbukh and Y. Prior, Phys. Rev. Lett. 94, 153002 (2005)]. The improved technique results in an increased cooling rate and decreases the minimum cooling temperature achievable with the optical shaker. The modified shaker requires only one measurement of the force on the atoms before each cooling step, resulting in a simplification of the feedback electronics. The force is inferred from the power variations of the transmitted laser beams and is used to determine the best moment at which the cooling steps are applied. The temperature of the atomic system is automatically monitored, which allows maintaining an optimum cooling rate as the temperature decreases. The improved shaker is simple to build, provides a faster rate of cooling, and can work in the microkelvin regime. Numerical modeling shows a reduction by a factor of 3 in the required number of phase jumps and a lower temperature limit reduced by an order of magnitude compared to the initially proposed shaker. The technique is also extendable to cooling in three dimensions.

We propose a novel generic approach to laser cooling based on the nonresonant interactions of atoms and molecules with optical standing waves experiencing sudden phase jumps. The technique, termed "optical shaking," combines the elements of stochastic cooling and Sisyphus cooling. An optical signal that measures the instantaneous force applied by the standing wave on the ensemble of particles is used as feedback to determine the phase jumps. This guarantees a drift towards lower energies and higher phase-space density without the loss of particles typical of evaporative cooling.

We explore the prospects of optical shaking, a recently suggested generic approach to laser cooling of neutral atoms and molecules. Optical shaking combines elements of Sisyphus cooling and of stochastic cooling techniques and is based on feedback-controlled interaction of particles with strong nonresonant laser fields. The feedback loop guarantees a monotonous energy decrease without a loss of particles. We discuss two types of feedback algorithms and provide an analytical estimation of their cooling rate. We study the robustness of optical shaking against noise and establish minimal stability requirements for the lasers. The analytical predictions are in a good agreement with the results of detailed numerical simulations.

The bichromatic force allows laser cooling using stimulated processes only. We provide details of our simulations of the bichromatic force when the effects of spontaneous emission are suppressed by restricting the atom-light interaction time to be short enough that there cannot be a significant number of spontaneous emission events. This short interaction time requires that the simulation include dynamics of the atomic motion through the light field that is coupled to the internal state dynamics that determine the force on the atom. The simulation is first tested with several standard optical field configurations to confirm its accuracy. Then the results, using conditions that match our experiment, are presented and compared with our measurements. The simulation shows that the resolution of the experiment greatly obscures the degree of cooling. Our simulation predicts an observed velocity distribution reduced by up to a factor of 4 in width over a time comparable to the excited state lifetime. This technique can allow the direct laser cooling of atoms and molecules without closed cycling transitions.

We demonstrate laser cooling of 85Rb atoms in a two-dimensional optical lattice. We follow the two-step degenerate Raman sideband cooling scheme, Kerman el al., Phys. Rev. Lett. 84, 439 (2000), where a fast cooling of atoms to an auxiliary state followed by a slow cooling to a dark state. This method has the advantage of independent control of the heating rate and cooling rate from the optical pumping beam. We operate the lattice at Lamb-Dicke parameter eta=0.45 and show the cooling of spin-polarized 85Rb atoms to the recoil temperature in both dimension within 2.4 ms with 90% of initial atom numbers.

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.